Modernize codebase

- Use ES modules
- Remove vendored libs and build them from npm using esbuild
- Switch from JSZip to zip.js

14 files changed, 4429 insertions, 0 deletions

diff --git a/dev/lib/handlebars/src/spec/.upstream_git_hash b/dev/lib/handlebars/src/spec/.upstream_git_hash
new file mode 100644
index 00000000..5a6b1831
--- /dev/null
+++ b/dev/lib/handlebars/src/spec/.upstream_git_hash
@@ -0,0 +1 @@
+c65c6cce3f626e4896a9d59250f0908be695adae
\ No newline at end of file
diff --git a/dev/lib/handlebars/src/spec/index.basic.test.ts b/dev/lib/handlebars/src/spec/index.basic.test.ts
new file mode 100644
index 00000000..6acf3ae9
--- /dev/null
+++ b/dev/lib/handlebars/src/spec/index.basic.test.ts
@@ -0,0 +1,481 @@
+/*
+ * This file is forked from the handlebars project (https://github.com/handlebars-lang/handlebars.js),
+ * and may include modifications made by Elasticsearch B.V.
+ * Elasticsearch B.V. licenses this file to you under the MIT License.
+ * See `packages/kbn-handlebars/LICENSE` for more information.
+ */
+
+import Handlebars from '../..';
+import { expectTemplate } from '../__jest__/test_bench';
+
+describe('basic context', () => {
+  it('most basic', () => {
+    expectTemplate('{{foo}}').withInput({ foo: 'foo' }).toCompileTo('foo');
+  });
+
+  it('escaping', () => {
+    expectTemplate('\\{{foo}}').withInput({ foo: 'food' }).toCompileTo('{{foo}}');
+    expectTemplate('content \\{{foo}}').withInput({ foo: 'food' }).toCompileTo('content {{foo}}');
+    expectTemplate('\\\\{{foo}}').withInput({ foo: 'food' }).toCompileTo('\\food');
+    expectTemplate('content \\\\{{foo}}').withInput({ foo: 'food' }).toCompileTo('content \\food');
+    expectTemplate('\\\\ {{foo}}').withInput({ foo: 'food' }).toCompileTo('\\\\ food');
+  });
+
+  it('compiling with a basic context', () => {
+    expectTemplate('Goodbye\n{{cruel}}\n{{world}}!')
+      .withInput({
+        cruel: 'cruel',
+        world: 'world',
+      })
+      .toCompileTo('Goodbye\ncruel\nworld!');
+  });
+
+  it('compiling with a string context', () => {
+    expectTemplate('{{.}}{{length}}').withInput('bye').toCompileTo('bye3');
+  });
+
+  it('compiling with an undefined context', () => {
+    expectTemplate('Goodbye\n{{cruel}}\n{{world.bar}}!')
+      .withInput(undefined)
+      .toCompileTo('Goodbye\n\n!');
+
+    expectTemplate('{{#unless foo}}Goodbye{{../test}}{{test2}}{{/unless}}')
+      .withInput(undefined)
+      .toCompileTo('Goodbye');
+  });
+
+  it('comments', () => {
+    expectTemplate('{{! Goodbye}}Goodbye\n{{cruel}}\n{{world}}!')
+      .withInput({
+        cruel: 'cruel',
+        world: 'world',
+      })
+      .toCompileTo('Goodbye\ncruel\nworld!');
+
+    expectTemplate('    {{~! comment ~}}      blah').toCompileTo('blah');
+    expectTemplate('    {{~!-- long-comment --~}}      blah').toCompileTo('blah');
+    expectTemplate('    {{! comment ~}}      blah').toCompileTo('    blah');
+    expectTemplate('    {{!-- long-comment --~}}      blah').toCompileTo('    blah');
+    expectTemplate('    {{~! comment}}      blah').toCompileTo('      blah');
+    expectTemplate('    {{~!-- long-comment --}}      blah').toCompileTo('      blah');
+  });
+
+  it('boolean', () => {
+    const string = '{{#goodbye}}GOODBYE {{/goodbye}}cruel {{world}}!';
+    expectTemplate(string)
+      .withInput({
+        goodbye: true,
+        world: 'world',
+      })
+      .toCompileTo('GOODBYE cruel world!');
+
+    expectTemplate(string)
+      .withInput({
+        goodbye: false,
+        world: 'world',
+      })
+      .toCompileTo('cruel world!');
+  });
+
+  it('zeros', () => {
+    expectTemplate('num1: {{num1}}, num2: {{num2}}')
+      .withInput({
+        num1: 42,
+        num2: 0,
+      })
+      .toCompileTo('num1: 42, num2: 0');
+
+    expectTemplate('num: {{.}}').withInput(0).toCompileTo('num: 0');
+
+    expectTemplate('num: {{num1/num2}}')
+      .withInput({ num1: { num2: 0 } })
+      .toCompileTo('num: 0');
+  });
+
+  it('false', () => {
+    /* eslint-disable no-new-wrappers */
+    expectTemplate('val1: {{val1}}, val2: {{val2}}')
+      .withInput({
+        val1: false,
+        val2: new Boolean(false),
+      })
+      .toCompileTo('val1: false, val2: false');
+
+    expectTemplate('val: {{.}}').withInput(false).toCompileTo('val: false');
+
+    expectTemplate('val: {{val1/val2}}')
+      .withInput({ val1: { val2: false } })
+      .toCompileTo('val: false');
+
+    expectTemplate('val1: {{{val1}}}, val2: {{{val2}}}')
+      .withInput({
+        val1: false,
+        val2: new Boolean(false),
+      })
+      .toCompileTo('val1: false, val2: false');
+
+    expectTemplate('val: {{{val1/val2}}}')
+      .withInput({ val1: { val2: false } })
+      .toCompileTo('val: false');
+    /* eslint-enable */
+  });
+
+  it('should handle undefined and null', () => {
+    expectTemplate('{{awesome undefined null}}')
+      .withInput({
+        awesome(_undefined: any, _null: any, options: any) {
+          return (_undefined === undefined) + ' ' + (_null === null) + ' ' + typeof options;
+        },
+      })
+      .toCompileTo('true true object');
+
+    expectTemplate('{{undefined}}')
+      .withInput({
+        undefined() {
+          return 'undefined!';
+        },
+      })
+      .toCompileTo('undefined!');
+
+    expectTemplate('{{null}}')
+      .withInput({
+        null() {
+          return 'null!';
+        },
+      })
+      .toCompileTo('null!');
+  });
+
+  it('newlines', () => {
+    expectTemplate("Alan's\nTest").toCompileTo("Alan's\nTest");
+    expectTemplate("Alan's\rTest").toCompileTo("Alan's\rTest");
+  });
+
+  it('escaping text', () => {
+    expectTemplate("Awesome's").toCompileTo("Awesome's");
+    expectTemplate('Awesome\\').toCompileTo('Awesome\\');
+    expectTemplate('Awesome\\\\ foo').toCompileTo('Awesome\\\\ foo');
+    expectTemplate('Awesome {{foo}}').withInput({ foo: '\\' }).toCompileTo('Awesome \\');
+    expectTemplate(" ' ' ").toCompileTo(" ' ' ");
+  });
+
+  it('escaping expressions', () => {
+    expectTemplate('{{{awesome}}}').withInput({ awesome: "&'\\<>" }).toCompileTo("&'\\<>");
+
+    expectTemplate('{{&awesome}}').withInput({ awesome: "&'\\<>" }).toCompileTo("&'\\<>");
+
+    expectTemplate('{{awesome}}')
+      .withInput({ awesome: '&"\'`\\<>' })
+      .toCompileTo('&amp;&quot;&#x27;&#x60;\\&lt;&gt;');
+
+    expectTemplate('{{awesome}}')
+      .withInput({ awesome: 'Escaped, <b> looks like: &lt;b&gt;' })
+      .toCompileTo('Escaped, &lt;b&gt; looks like: &amp;lt;b&amp;gt;');
+  });
+
+  it("functions returning safestrings shouldn't be escaped", () => {
+    expectTemplate('{{awesome}}')
+      .withInput({
+        awesome() {
+          return new Handlebars.SafeString("&'\\<>");
+        },
+      })
+      .toCompileTo("&'\\<>");
+  });
+
+  it('functions', () => {
+    expectTemplate('{{awesome}}')
+      .withInput({
+        awesome() {
+          return 'Awesome';
+        },
+      })
+      .toCompileTo('Awesome');
+
+    expectTemplate('{{awesome}}')
+      .withInput({
+        awesome() {
+          return this.more;
+        },
+        more: 'More awesome',
+      })
+      .toCompileTo('More awesome');
+  });
+
+  it('functions with context argument', () => {
+    expectTemplate('{{awesome frank}}')
+      .withInput({
+        awesome(context: any) {
+          return context;
+        },
+        frank: 'Frank',
+      })
+      .toCompileTo('Frank');
+  });
+
+  it('pathed functions with context argument', () => {
+    expectTemplate('{{bar.awesome frank}}')
+      .withInput({
+        bar: {
+          awesome(context: any) {
+            return context;
+          },
+        },
+        frank: 'Frank',
+      })
+      .toCompileTo('Frank');
+  });
+
+  it('depthed functions with context argument', () => {
+    expectTemplate('{{#with frank}}{{../awesome .}}{{/with}}')
+      .withInput({
+        awesome(context: any) {
+          return context;
+        },
+        frank: 'Frank',
+      })
+      .toCompileTo('Frank');
+  });
+
+  it('block functions with context argument', () => {
+    expectTemplate('{{#awesome 1}}inner {{.}}{{/awesome}}')
+      .withInput({
+        awesome(context: any, options: any) {
+          return options.fn(context);
+        },
+      })
+      .toCompileTo('inner 1');
+  });
+
+  it('depthed block functions with context argument', () => {
+    expectTemplate('{{#with value}}{{#../awesome 1}}inner {{.}}{{/../awesome}}{{/with}}')
+      .withInput({
+        value: true,
+        awesome(context: any, options: any) {
+          return options.fn(context);
+        },
+      })
+      .toCompileTo('inner 1');
+  });
+
+  it('block functions without context argument', () => {
+    expectTemplate('{{#awesome}}inner{{/awesome}}')
+      .withInput({
+        awesome(options: any) {
+          return options.fn(this);
+        },
+      })
+      .toCompileTo('inner');
+  });
+
+  it('pathed block functions without context argument', () => {
+    expectTemplate('{{#foo.awesome}}inner{{/foo.awesome}}')
+      .withInput({
+        foo: {
+          awesome() {
+            return this;
+          },
+        },
+      })
+      .toCompileTo('inner');
+  });
+
+  it('depthed block functions without context argument', () => {
+    expectTemplate('{{#with value}}{{#../awesome}}inner{{/../awesome}}{{/with}}')
+      .withInput({
+        value: true,
+        awesome() {
+          return this;
+        },
+      })
+      .toCompileTo('inner');
+  });
+
+  it('paths with hyphens', () => {
+    expectTemplate('{{foo-bar}}').withInput({ 'foo-bar': 'baz' }).toCompileTo('baz');
+
+    expectTemplate('{{foo.foo-bar}}')
+      .withInput({ foo: { 'foo-bar': 'baz' } })
+      .toCompileTo('baz');
+
+    expectTemplate('{{foo/foo-bar}}')
+      .withInput({ foo: { 'foo-bar': 'baz' } })
+      .toCompileTo('baz');
+  });
+
+  it('nested paths', () => {
+    expectTemplate('Goodbye {{alan/expression}} world!')
+      .withInput({ alan: { expression: 'beautiful' } })
+      .toCompileTo('Goodbye beautiful world!');
+  });
+
+  it('nested paths with empty string value', () => {
+    expectTemplate('Goodbye {{alan/expression}} world!')
+      .withInput({ alan: { expression: '' } })
+      .toCompileTo('Goodbye  world!');
+  });
+
+  it('literal paths', () => {
+    expectTemplate('Goodbye {{[@alan]/expression}} world!')
+      .withInput({ '@alan': { expression: 'beautiful' } })
+      .toCompileTo('Goodbye beautiful world!');
+
+    expectTemplate('Goodbye {{[foo bar]/expression}} world!')
+      .withInput({ 'foo bar': { expression: 'beautiful' } })
+      .toCompileTo('Goodbye beautiful world!');
+  });
+
+  it('literal references', () => {
+    expectTemplate('Goodbye {{[foo bar]}} world!')
+      .withInput({ 'foo bar': 'beautiful' })
+      .toCompileTo('Goodbye beautiful world!');
+
+    expectTemplate('Goodbye {{"foo bar"}} world!')
+      .withInput({ 'foo bar': 'beautiful' })
+      .toCompileTo('Goodbye beautiful world!');
+
+    expectTemplate("Goodbye {{'foo bar'}} world!")
+      .withInput({ 'foo bar': 'beautiful' })
+      .toCompileTo('Goodbye beautiful world!');
+
+    expectTemplate('Goodbye {{"foo[bar"}} world!')
+      .withInput({ 'foo[bar': 'beautiful' })
+      .toCompileTo('Goodbye beautiful world!');
+
+    expectTemplate('Goodbye {{"foo\'bar"}} world!')
+      .withInput({ "foo'bar": 'beautiful' })
+      .toCompileTo('Goodbye beautiful world!');
+
+    expectTemplate("Goodbye {{'foo\"bar'}} world!")
+      .withInput({ 'foo"bar': 'beautiful' })
+      .toCompileTo('Goodbye beautiful world!');
+  });
+
+  it("that current context path ({{.}}) doesn't hit helpers", () => {
+    expectTemplate('test: {{.}}')
+      .withInput(null)
+      // @ts-expect-error Setting the helper to a string instead of a function doesn't make sense normally, but here it doesn't matter
+      .withHelpers({ helper: 'awesome' })
+      .toCompileTo('test: ');
+  });
+
+  it('complex but empty paths', () => {
+    expectTemplate('{{person/name}}')
+      .withInput({ person: { name: null } })
+      .toCompileTo('');
+
+    expectTemplate('{{person/name}}').withInput({ person: {} }).toCompileTo('');
+  });
+
+  it('this keyword in paths', () => {
+    expectTemplate('{{#goodbyes}}{{this}}{{/goodbyes}}')
+      .withInput({ goodbyes: ['goodbye', 'Goodbye', 'GOODBYE'] })
+      .toCompileTo('goodbyeGoodbyeGOODBYE');
+
+    expectTemplate('{{#hellos}}{{this/text}}{{/hellos}}')
+      .withInput({
+        hellos: [{ text: 'hello' }, { text: 'Hello' }, { text: 'HELLO' }],
+      })
+      .toCompileTo('helloHelloHELLO');
+  });
+
+  it('this keyword nested inside path', () => {
+    expectTemplate('{{#hellos}}{{text/this/foo}}{{/hellos}}').toThrow(
+      'Invalid path: text/this - 1:13'
+    );
+
+    expectTemplate('{{[this]}}').withInput({ this: 'bar' }).toCompileTo('bar');
+
+    expectTemplate('{{text/[this]}}')
+      .withInput({ text: { this: 'bar' } })
+      .toCompileTo('bar');
+  });
+
+  it('this keyword in helpers', () => {
+    const helpers = {
+      foo(value: any) {
+        return 'bar ' + value;
+      },
+    };
+
+    expectTemplate('{{#goodbyes}}{{foo this}}{{/goodbyes}}')
+      .withInput({ goodbyes: ['goodbye', 'Goodbye', 'GOODBYE'] })
+      .withHelpers(helpers)
+      .toCompileTo('bar goodbyebar Goodbyebar GOODBYE');
+
+    expectTemplate('{{#hellos}}{{foo this/text}}{{/hellos}}')
+      .withInput({
+        hellos: [{ text: 'hello' }, { text: 'Hello' }, { text: 'HELLO' }],
+      })
+      .withHelpers(helpers)
+      .toCompileTo('bar hellobar Hellobar HELLO');
+  });
+
+  it('this keyword nested inside helpers param', () => {
+    expectTemplate('{{#hellos}}{{foo text/this/foo}}{{/hellos}}').toThrow(
+      'Invalid path: text/this - 1:17'
+    );
+
+    expectTemplate('{{foo [this]}}')
+      .withInput({
+        foo(value: any) {
+          return value;
+        },
+        this: 'bar',
+      })
+      .toCompileTo('bar');
+
+    expectTemplate('{{foo text/[this]}}')
+      .withInput({
+        foo(value: any) {
+          return value;
+        },
+        text: { this: 'bar' },
+      })
+      .toCompileTo('bar');
+  });
+
+  it('pass string literals', () => {
+    expectTemplate('{{"foo"}}').toCompileTo('');
+    expectTemplate('{{"foo"}}').withInput({ foo: 'bar' }).toCompileTo('bar');
+
+    expectTemplate('{{#"foo"}}{{.}}{{/"foo"}}')
+      .withInput({
+        foo: ['bar', 'baz'],
+      })
+      .toCompileTo('barbaz');
+  });
+
+  it('pass number literals', () => {
+    expectTemplate('{{12}}').toCompileTo('');
+    expectTemplate('{{12}}').withInput({ '12': 'bar' }).toCompileTo('bar');
+    expectTemplate('{{12.34}}').toCompileTo('');
+    expectTemplate('{{12.34}}').withInput({ '12.34': 'bar' }).toCompileTo('bar');
+    expectTemplate('{{12.34 1}}')
+      .withInput({
+        '12.34'(arg: any) {
+          return 'bar' + arg;
+        },
+      })
+      .toCompileTo('bar1');
+  });
+
+  it('pass boolean literals', () => {
+    expectTemplate('{{true}}').toCompileTo('');
+    expectTemplate('{{true}}').withInput({ '': 'foo' }).toCompileTo('');
+    expectTemplate('{{false}}').withInput({ false: 'foo' }).toCompileTo('foo');
+  });
+
+  it('should handle literals in subexpression', () => {
+    expectTemplate('{{foo (false)}}')
+      .withInput({
+        false() {
+          return 'bar';
+        },
+      })
+      .withHelper('foo', function (arg) {
+        return arg;
+      })
+      .toCompileTo('bar');
+  });
+});
diff --git a/dev/lib/handlebars/src/spec/index.blocks.test.ts b/dev/lib/handlebars/src/spec/index.blocks.test.ts
new file mode 100644
index 00000000..2d9a8707
--- /dev/null
+++ b/dev/lib/handlebars/src/spec/index.blocks.test.ts
@@ -0,0 +1,366 @@
+/*
+ * This file is forked from the handlebars project (https://github.com/handlebars-lang/handlebars.js),
+ * and may include modifications made by Elasticsearch B.V.
+ * Elasticsearch B.V. licenses this file to you under the MIT License.
+ * See `packages/kbn-handlebars/LICENSE` for more information.
+ */
+
+import Handlebars, { type HelperOptions } from '../..';
+import { expectTemplate } from '../__jest__/test_bench';
+
+describe('blocks', () => {
+  it('array', () => {
+    const string = '{{#goodbyes}}{{text}}! {{/goodbyes}}cruel {{world}}!';
+
+    expectTemplate(string)
+      .withInput({
+        goodbyes: [{ text: 'goodbye' }, { text: 'Goodbye' }, { text: 'GOODBYE' }],
+        world: 'world',
+      })
+      .toCompileTo('goodbye! Goodbye! GOODBYE! cruel world!');
+
+    expectTemplate(string)
+      .withInput({
+        goodbyes: [],
+        world: 'world',
+      })
+      .toCompileTo('cruel world!');
+  });
+
+  it('array without data', () => {
+    expectTemplate('{{#goodbyes}}{{text}}{{/goodbyes}} {{#goodbyes}}{{text}}{{/goodbyes}}')
+      .withInput({
+        goodbyes: [{ text: 'goodbye' }, { text: 'Goodbye' }, { text: 'GOODBYE' }],
+        world: 'world',
+      })
+      .toCompileTo('goodbyeGoodbyeGOODBYE goodbyeGoodbyeGOODBYE');
+  });
+
+  it('array with @index', () => {
+    expectTemplate('{{#goodbyes}}{{@index}}. {{text}}! {{/goodbyes}}cruel {{world}}!')
+      .withInput({
+        goodbyes: [{ text: 'goodbye' }, { text: 'Goodbye' }, { text: 'GOODBYE' }],
+        world: 'world',
+      })
+      .toCompileTo('0. goodbye! 1. Goodbye! 2. GOODBYE! cruel world!');
+  });
+
+  it('empty block', () => {
+    const string = '{{#goodbyes}}{{/goodbyes}}cruel {{world}}!';
+
+    expectTemplate(string)
+      .withInput({
+        goodbyes: [{ text: 'goodbye' }, { text: 'Goodbye' }, { text: 'GOODBYE' }],
+        world: 'world',
+      })
+      .toCompileTo('cruel world!');
+
+    expectTemplate(string)
+      .withInput({
+        goodbyes: [],
+        world: 'world',
+      })
+      .toCompileTo('cruel world!');
+  });
+
+  it('block with complex lookup', () => {
+    expectTemplate('{{#goodbyes}}{{text}} cruel {{../name}}! {{/goodbyes}}')
+      .withInput({
+        name: 'Alan',
+        goodbyes: [{ text: 'goodbye' }, { text: 'Goodbye' }, { text: 'GOODBYE' }],
+      })
+      .toCompileTo('goodbye cruel Alan! Goodbye cruel Alan! GOODBYE cruel Alan! ');
+  });
+
+  it('multiple blocks with complex lookup', () => {
+    expectTemplate('{{#goodbyes}}{{../name}}{{../name}}{{/goodbyes}}')
+      .withInput({
+        name: 'Alan',
+        goodbyes: [{ text: 'goodbye' }, { text: 'Goodbye' }, { text: 'GOODBYE' }],
+      })
+      .toCompileTo('AlanAlanAlanAlanAlanAlan');
+  });
+
+  it('block with complex lookup using nested context', () => {
+    expectTemplate('{{#goodbyes}}{{text}} cruel {{foo/../name}}! {{/goodbyes}}').toThrow(Error);
+  });
+
+  it('block with deep nested complex lookup', () => {
+    expectTemplate(
+      '{{#outer}}Goodbye {{#inner}}cruel {{../sibling}} {{../../omg}}{{/inner}}{{/outer}}'
+    )
+      .withInput({
+        omg: 'OMG!',
+        outer: [{ sibling: 'sad', inner: [{ text: 'goodbye' }] }],
+      })
+      .toCompileTo('Goodbye cruel sad OMG!');
+  });
+
+  it('works with cached blocks', () => {
+    expectTemplate('{{#each person}}{{#with .}}{{first}} {{last}}{{/with}}{{/each}}')
+      .withCompileOptions({ data: false })
+      .withInput({
+        person: [
+          { first: 'Alan', last: 'Johnson' },
+          { first: 'Alan', last: 'Johnson' },
+        ],
+      })
+      .toCompileTo('Alan JohnsonAlan Johnson');
+  });
+
+  describe('inverted sections', () => {
+    it('inverted sections with unset value', () => {
+      expectTemplate(
+        '{{#goodbyes}}{{this}}{{/goodbyes}}{{^goodbyes}}Right On!{{/goodbyes}}'
+      ).toCompileTo('Right On!');
+    });
+
+    it('inverted section with false value', () => {
+      expectTemplate('{{#goodbyes}}{{this}}{{/goodbyes}}{{^goodbyes}}Right On!{{/goodbyes}}')
+        .withInput({ goodbyes: false })
+        .toCompileTo('Right On!');
+    });
+
+    it('inverted section with empty set', () => {
+      expectTemplate('{{#goodbyes}}{{this}}{{/goodbyes}}{{^goodbyes}}Right On!{{/goodbyes}}')
+        .withInput({ goodbyes: [] })
+        .toCompileTo('Right On!');
+    });
+
+    it('block inverted sections', () => {
+      expectTemplate('{{#people}}{{name}}{{^}}{{none}}{{/people}}')
+        .withInput({ none: 'No people' })
+        .toCompileTo('No people');
+    });
+
+    it('chained inverted sections', () => {
+      expectTemplate('{{#people}}{{name}}{{else if none}}{{none}}{{/people}}')
+        .withInput({ none: 'No people' })
+        .toCompileTo('No people');
+
+      expectTemplate(
+        '{{#people}}{{name}}{{else if nothere}}fail{{else unless nothere}}{{none}}{{/people}}'
+      )
+        .withInput({ none: 'No people' })
+        .toCompileTo('No people');
+
+      expectTemplate('{{#people}}{{name}}{{else if none}}{{none}}{{else}}fail{{/people}}')
+        .withInput({ none: 'No people' })
+        .toCompileTo('No people');
+    });
+
+    it('chained inverted sections with mismatch', () => {
+      expectTemplate('{{#people}}{{name}}{{else if none}}{{none}}{{/if}}').toThrow(Error);
+    });
+
+    it('block inverted sections with empty arrays', () => {
+      expectTemplate('{{#people}}{{name}}{{^}}{{none}}{{/people}}')
+        .withInput({
+          none: 'No people',
+          people: [],
+        })
+        .toCompileTo('No people');
+    });
+  });
+
+  describe('standalone sections', () => {
+    it('block standalone else sections', () => {
+      expectTemplate('{{#people}}\n{{name}}\n{{^}}\n{{none}}\n{{/people}}\n')
+        .withInput({ none: 'No people' })
+        .toCompileTo('No people\n');
+
+      expectTemplate('{{#none}}\n{{.}}\n{{^}}\n{{none}}\n{{/none}}\n')
+        .withInput({ none: 'No people' })
+        .toCompileTo('No people\n');
+
+      expectTemplate('{{#people}}\n{{name}}\n{{^}}\n{{none}}\n{{/people}}\n')
+        .withInput({ none: 'No people' })
+        .toCompileTo('No people\n');
+    });
+
+    it('block standalone chained else sections', () => {
+      expectTemplate('{{#people}}\n{{name}}\n{{else if none}}\n{{none}}\n{{/people}}\n')
+        .withInput({ none: 'No people' })
+        .toCompileTo('No people\n');
+
+      expectTemplate('{{#people}}\n{{name}}\n{{else if none}}\n{{none}}\n{{^}}\n{{/people}}\n')
+        .withInput({ none: 'No people' })
+        .toCompileTo('No people\n');
+    });
+
+    it('should handle nesting', () => {
+      expectTemplate('{{#data}}\n{{#if true}}\n{{.}}\n{{/if}}\n{{/data}}\nOK.')
+        .withInput({
+          data: [1, 3, 5],
+        })
+        .toCompileTo('1\n3\n5\nOK.');
+    });
+  });
+
+  describe('decorators', () => {
+    it('should apply mustache decorators', () => {
+      expectTemplate('{{#helper}}{{*decorator}}{{/helper}}')
+        .withHelper('helper', function (options: HelperOptions) {
+          return (options.fn as any).run;
+        })
+        .withDecorator('decorator', function (fn) {
+          (fn as any).run = 'success';
+          return fn;
+        })
+        .toCompileTo('success');
+    });
+
+    it('should apply allow undefined return', () => {
+      expectTemplate('{{#helper}}{{*decorator}}suc{{/helper}}')
+        .withHelper('helper', function (options: HelperOptions) {
+          return options.fn() + (options.fn as any).run;
+        })
+        .withDecorator('decorator', function (fn) {
+          (fn as any).run = 'cess';
+        })
+        .toCompileTo('success');
+    });
+
+    it('should apply block decorators', () => {
+      expectTemplate('{{#helper}}{{#*decorator}}success{{/decorator}}{{/helper}}')
+        .withHelper('helper', function (options: HelperOptions) {
+          return (options.fn as any).run;
+        })
+        .withDecorator('decorator', function (fn, props, container, options) {
+          (fn as any).run = options.fn();
+          return fn;
+        })
+        .toCompileTo('success');
+    });
+
+    it('should support nested decorators', () => {
+      expectTemplate(
+        '{{#helper}}{{#*decorator}}{{#*nested}}suc{{/nested}}cess{{/decorator}}{{/helper}}'
+      )
+        .withHelper('helper', function (options: HelperOptions) {
+          return (options.fn as any).run;
+        })
+        .withDecorators({
+          decorator(fn, props, container, options) {
+            (fn as any).run = options.fn.nested + options.fn();
+            return fn;
+          },
+          nested(fn, props, container, options) {
+            props.nested = options.fn();
+          },
+        })
+        .toCompileTo('success');
+    });
+
+    it('should apply multiple decorators', () => {
+      expectTemplate(
+        '{{#helper}}{{#*decorator}}suc{{/decorator}}{{#*decorator}}cess{{/decorator}}{{/helper}}'
+      )
+        .withHelper('helper', function (options: HelperOptions) {
+          return (options.fn as any).run;
+        })
+        .withDecorator('decorator', function (fn, props, container, options) {
+          (fn as any).run = ((fn as any).run || '') + options.fn();
+          return fn;
+        })
+        .toCompileTo('success');
+    });
+
+    it('should access parent variables', () => {
+      expectTemplate('{{#helper}}{{*decorator foo}}{{/helper}}')
+        .withHelper('helper', function (options: HelperOptions) {
+          return (options.fn as any).run;
+        })
+        .withDecorator('decorator', function (fn, props, container, options) {
+          (fn as any).run = options.args;
+          return fn;
+        })
+        .withInput({ foo: 'success' })
+        .toCompileTo('success');
+    });
+
+    it('should work with root program', () => {
+      let run;
+      expectTemplate('{{*decorator "success"}}')
+        .withDecorator('decorator', function (fn, props, container, options) {
+          expect(options.args[0]).toEqual('success');
+          run = true;
+          return fn;
+        })
+        .withInput({ foo: 'success' })
+        .toCompileTo('');
+      expect(run).toEqual(true);
+    });
+
+    it('should fail when accessing variables from root', () => {
+      let run;
+      expectTemplate('{{*decorator foo}}')
+        .withDecorator('decorator', function (fn, props, container, options) {
+          expect(options.args[0]).toBeUndefined();
+          run = true;
+          return fn;
+        })
+        .withInput({ foo: 'fail' })
+        .toCompileTo('');
+      expect(run).toEqual(true);
+    });
+
+    describe('registration', () => {
+      beforeEach(() => {
+        global.kbnHandlebarsEnv = Handlebars.create();
+      });
+
+      afterEach(() => {
+        global.kbnHandlebarsEnv = null;
+      });
+
+      it('unregisters', () => {
+        // @ts-expect-error: Cannot assign to 'decorators' because it is a read-only property.
+        kbnHandlebarsEnv!.decorators = {};
+
+        kbnHandlebarsEnv!.registerDecorator('foo', function () {
+          return 'fail';
+        });
+
+        expect(!!kbnHandlebarsEnv!.decorators.foo).toEqual(true);
+        kbnHandlebarsEnv!.unregisterDecorator('foo');
+        expect(kbnHandlebarsEnv!.decorators.foo).toBeUndefined();
+      });
+
+      it('allows multiple globals', () => {
+        // @ts-expect-error: Cannot assign to 'decorators' because it is a read-only property.
+        kbnHandlebarsEnv!.decorators = {};
+
+        // @ts-expect-error: Expected 2 arguments, but got 1.
+        kbnHandlebarsEnv!.registerDecorator({
+          foo() {},
+          bar() {},
+        });
+
+        expect(!!kbnHandlebarsEnv!.decorators.foo).toEqual(true);
+        expect(!!kbnHandlebarsEnv!.decorators.bar).toEqual(true);
+        kbnHandlebarsEnv!.unregisterDecorator('foo');
+        kbnHandlebarsEnv!.unregisterDecorator('bar');
+        expect(kbnHandlebarsEnv!.decorators.foo).toBeUndefined();
+        expect(kbnHandlebarsEnv!.decorators.bar).toBeUndefined();
+      });
+
+      it('fails with multiple and args', () => {
+        expect(() => {
+          kbnHandlebarsEnv!.registerDecorator(
+            // @ts-expect-error: Argument of type '{ world(): string; testHelper(): string; }' is not assignable to parameter of type 'string'.
+            {
+              world() {
+                return 'world!';
+              },
+              testHelper() {
+                return 'found it!';
+              },
+            },
+            {}
+          );
+        }).toThrow('Arg not supported with multiple decorators');
+      });
+    });
+  });
+});
diff --git a/dev/lib/handlebars/src/spec/index.builtins.test.ts b/dev/lib/handlebars/src/spec/index.builtins.test.ts
new file mode 100644
index 00000000..c47ec29f
--- /dev/null
+++ b/dev/lib/handlebars/src/spec/index.builtins.test.ts
@@ -0,0 +1,676 @@
+/*
+ * This file is forked from the handlebars project (https://github.com/handlebars-lang/handlebars.js),
+ * and may include modifications made by Elasticsearch B.V.
+ * Elasticsearch B.V. licenses this file to you under the MIT License.
+ * See `packages/kbn-handlebars/LICENSE` for more information.
+ */
+
+/* eslint-disable max-classes-per-file */
+
+import Handlebars from '../..';
+import { expectTemplate } from '../__jest__/test_bench';
+
+describe('builtin helpers', () => {
+  describe('#if', () => {
+    it('if', () => {
+      const string = '{{#if goodbye}}GOODBYE {{/if}}cruel {{world}}!';
+
+      expectTemplate(string)
+        .withInput({
+          goodbye: true,
+          world: 'world',
+        })
+        .toCompileTo('GOODBYE cruel world!');
+
+      expectTemplate(string)
+        .withInput({
+          goodbye: 'dummy',
+          world: 'world',
+        })
+        .toCompileTo('GOODBYE cruel world!');
+
+      expectTemplate(string)
+        .withInput({
+          goodbye: false,
+          world: 'world',
+        })
+        .toCompileTo('cruel world!');
+
+      expectTemplate(string).withInput({ world: 'world' }).toCompileTo('cruel world!');
+
+      expectTemplate(string)
+        .withInput({
+          goodbye: ['foo'],
+          world: 'world',
+        })
+        .toCompileTo('GOODBYE cruel world!');
+
+      expectTemplate(string)
+        .withInput({
+          goodbye: [],
+          world: 'world',
+        })
+        .toCompileTo('cruel world!');
+
+      expectTemplate(string)
+        .withInput({
+          goodbye: 0,
+          world: 'world',
+        })
+        .toCompileTo('cruel world!');
+
+      expectTemplate('{{#if goodbye includeZero=true}}GOODBYE {{/if}}cruel {{world}}!')
+        .withInput({
+          goodbye: 0,
+          world: 'world',
+        })
+        .toCompileTo('GOODBYE cruel world!');
+    });
+
+    it('if with function argument', () => {
+      const string = '{{#if goodbye}}GOODBYE {{/if}}cruel {{world}}!';
+
+      expectTemplate(string)
+        .withInput({
+          goodbye() {
+            return true;
+          },
+          world: 'world',
+        })
+        .toCompileTo('GOODBYE cruel world!');
+
+      expectTemplate(string)
+        .withInput({
+          goodbye() {
+            return this.world;
+          },
+          world: 'world',
+        })
+        .toCompileTo('GOODBYE cruel world!');
+
+      expectTemplate(string)
+        .withInput({
+          goodbye() {
+            return false;
+          },
+          world: 'world',
+        })
+        .toCompileTo('cruel world!');
+
+      expectTemplate(string)
+        .withInput({
+          goodbye() {
+            return this.foo;
+          },
+          world: 'world',
+        })
+        .toCompileTo('cruel world!');
+    });
+
+    it('should not change the depth list', () => {
+      expectTemplate('{{#with foo}}{{#if goodbye}}GOODBYE cruel {{../world}}!{{/if}}{{/with}}')
+        .withInput({
+          foo: { goodbye: true },
+          world: 'world',
+        })
+        .toCompileTo('GOODBYE cruel world!');
+    });
+  });
+
+  describe('#with', () => {
+    it('with', () => {
+      expectTemplate('{{#with person}}{{first}} {{last}}{{/with}}')
+        .withInput({
+          person: {
+            first: 'Alan',
+            last: 'Johnson',
+          },
+        })
+        .toCompileTo('Alan Johnson');
+    });
+
+    it('with with function argument', () => {
+      expectTemplate('{{#with person}}{{first}} {{last}}{{/with}}')
+        .withInput({
+          person() {
+            return {
+              first: 'Alan',
+              last: 'Johnson',
+            };
+          },
+        })
+        .toCompileTo('Alan Johnson');
+    });
+
+    it('with with else', () => {
+      expectTemplate(
+        '{{#with person}}Person is present{{else}}Person is not present{{/with}}'
+      ).toCompileTo('Person is not present');
+    });
+
+    it('with provides block parameter', () => {
+      expectTemplate('{{#with person as |foo|}}{{foo.first}} {{last}}{{/with}}')
+        .withInput({
+          person: {
+            first: 'Alan',
+            last: 'Johnson',
+          },
+        })
+        .toCompileTo('Alan Johnson');
+    });
+
+    it('works when data is disabled', () => {
+      expectTemplate('{{#with person as |foo|}}{{foo.first}} {{last}}{{/with}}')
+        .withInput({ person: { first: 'Alan', last: 'Johnson' } })
+        .withCompileOptions({ data: false })
+        .toCompileTo('Alan Johnson');
+    });
+  });
+
+  describe('#each', () => {
+    it('each', () => {
+      const string = '{{#each goodbyes}}{{text}}! {{/each}}cruel {{world}}!';
+
+      expectTemplate(string)
+        .withInput({
+          goodbyes: [{ text: 'goodbye' }, { text: 'Goodbye' }, { text: 'GOODBYE' }],
+          world: 'world',
+        })
+        .toCompileTo('goodbye! Goodbye! GOODBYE! cruel world!');
+
+      expectTemplate(string)
+        .withInput({
+          goodbyes: [],
+          world: 'world',
+        })
+        .toCompileTo('cruel world!');
+    });
+
+    it('each without data', () => {
+      expectTemplate('{{#each goodbyes}}{{text}}! {{/each}}cruel {{world}}!')
+        .withInput({
+          goodbyes: [{ text: 'goodbye' }, { text: 'Goodbye' }, { text: 'GOODBYE' }],
+          world: 'world',
+        })
+        .withRuntimeOptions({ data: false })
+        .withCompileOptions({ data: false })
+        .toCompileTo('goodbye! Goodbye! GOODBYE! cruel world!');
+
+      expectTemplate('{{#each .}}{{.}}{{/each}}')
+        .withInput({ goodbyes: 'cruel', world: 'world' })
+        .withRuntimeOptions({ data: false })
+        .withCompileOptions({ data: false })
+        .toCompileTo('cruelworld');
+    });
+
+    it('each without context', () => {
+      expectTemplate('{{#each goodbyes}}{{text}}! {{/each}}cruel {{world}}!')
+        .withInput(undefined)
+        .toCompileTo('cruel !');
+    });
+
+    it('each with an object and @key', () => {
+      const string = '{{#each goodbyes}}{{@key}}. {{text}}! {{/each}}cruel {{world}}!';
+
+      function Clazz(this: any) {
+        this['<b>#1</b>'] = { text: 'goodbye' };
+        this[2] = { text: 'GOODBYE' };
+      }
+      Clazz.prototype.foo = 'fail';
+      const hash = { goodbyes: new (Clazz as any)(), world: 'world' };
+
+      // Object property iteration order is undefined according to ECMA spec,
+      // so we need to check both possible orders
+      // @see http://stackoverflow.com/questions/280713/elements-order-in-a-for-in-loop
+      try {
+        expectTemplate(string)
+          .withInput(hash)
+          .toCompileTo('&lt;b&gt;#1&lt;/b&gt;. goodbye! 2. GOODBYE! cruel world!');
+      } catch (e) {
+        expectTemplate(string)
+          .withInput(hash)
+          .toCompileTo('2. GOODBYE! &lt;b&gt;#1&lt;/b&gt;. goodbye! cruel world!');
+      }
+
+      expectTemplate(string)
+        .withInput({
+          goodbyes: {},
+          world: 'world',
+        })
+        .toCompileTo('cruel world!');
+    });
+
+    it('each with @index', () => {
+      expectTemplate('{{#each goodbyes}}{{@index}}. {{text}}! {{/each}}cruel {{world}}!')
+        .withInput({
+          goodbyes: [{ text: 'goodbye' }, { text: 'Goodbye' }, { text: 'GOODBYE' }],
+          world: 'world',
+        })
+        .toCompileTo('0. goodbye! 1. Goodbye! 2. GOODBYE! cruel world!');
+    });
+
+    it('each with nested @index', () => {
+      expectTemplate(
+        '{{#each goodbyes}}{{@index}}. {{text}}! {{#each ../goodbyes}}{{@index}} {{/each}}After {{@index}} {{/each}}{{@index}}cruel {{world}}!'
+      )
+        .withInput({
+          goodbyes: [{ text: 'goodbye' }, { text: 'Goodbye' }, { text: 'GOODBYE' }],
+          world: 'world',
+        })
+        .toCompileTo(
+          '0. goodbye! 0 1 2 After 0 1. Goodbye! 0 1 2 After 1 2. GOODBYE! 0 1 2 After 2 cruel world!'
+        );
+    });
+
+    it('each with block params', () => {
+      expectTemplate(
+        '{{#each goodbyes as |value index|}}{{index}}. {{value.text}}! {{#each ../goodbyes as |childValue childIndex|}} {{index}} {{childIndex}}{{/each}} After {{index}} {{/each}}{{index}}cruel {{world}}!'
+      )
+        .withInput({
+          goodbyes: [{ text: 'goodbye' }, { text: 'Goodbye' }],
+          world: 'world',
+        })
+        .toCompileTo('0. goodbye!  0 0 0 1 After 0 1. Goodbye!  1 0 1 1 After 1 cruel world!');
+    });
+
+    // TODO: This test has been added to the `4.x` branch of the handlebars.js repo along with a code-fix,
+    // but a new version of the handlebars package containing this fix has not yet been published to npm.
+    //
+    // Before enabling this code, a new version of handlebars needs to be released and the corresponding
+    // updates needs to be applied to this implementation.
+    //
+    // See: https://github.com/handlebars-lang/handlebars.js/commit/30dbf0478109ded8f12bb29832135d480c17e367
+    it.skip('each with block params and strict compilation', () => {
+      expectTemplate('{{#each goodbyes as |value index|}}{{index}}. {{value.text}}!{{/each}}')
+        .withCompileOptions({ strict: true })
+        .withInput({ goodbyes: [{ text: 'goodbye' }, { text: 'Goodbye' }] })
+        .toCompileTo('0. goodbye!1. Goodbye!');
+    });
+
+    it('each object with @index', () => {
+      expectTemplate('{{#each goodbyes}}{{@index}}. {{text}}! {{/each}}cruel {{world}}!')
+        .withInput({
+          goodbyes: {
+            a: { text: 'goodbye' },
+            b: { text: 'Goodbye' },
+            c: { text: 'GOODBYE' },
+          },
+          world: 'world',
+        })
+        .toCompileTo('0. goodbye! 1. Goodbye! 2. GOODBYE! cruel world!');
+    });
+
+    it('each with @first', () => {
+      expectTemplate('{{#each goodbyes}}{{#if @first}}{{text}}! {{/if}}{{/each}}cruel {{world}}!')
+        .withInput({
+          goodbyes: [{ text: 'goodbye' }, { text: 'Goodbye' }, { text: 'GOODBYE' }],
+          world: 'world',
+        })
+        .toCompileTo('goodbye! cruel world!');
+    });
+
+    it('each with nested @first', () => {
+      expectTemplate(
+        '{{#each goodbyes}}({{#if @first}}{{text}}! {{/if}}{{#each ../goodbyes}}{{#if @first}}{{text}}!{{/if}}{{/each}}{{#if @first}} {{text}}!{{/if}}) {{/each}}cruel {{world}}!'
+      )
+        .withInput({
+          goodbyes: [{ text: 'goodbye' }, { text: 'Goodbye' }, { text: 'GOODBYE' }],
+          world: 'world',
+        })
+        .toCompileTo('(goodbye! goodbye! goodbye!) (goodbye!) (goodbye!) cruel world!');
+    });
+
+    it('each object with @first', () => {
+      expectTemplate('{{#each goodbyes}}{{#if @first}}{{text}}! {{/if}}{{/each}}cruel {{world}}!')
+        .withInput({
+          goodbyes: { foo: { text: 'goodbye' }, bar: { text: 'Goodbye' } },
+          world: 'world',
+        })
+        .toCompileTo('goodbye! cruel world!');
+    });
+
+    it('each with @last', () => {
+      expectTemplate('{{#each goodbyes}}{{#if @last}}{{text}}! {{/if}}{{/each}}cruel {{world}}!')
+        .withInput({
+          goodbyes: [{ text: 'goodbye' }, { text: 'Goodbye' }, { text: 'GOODBYE' }],
+          world: 'world',
+        })
+        .toCompileTo('GOODBYE! cruel world!');
+    });
+
+    it('each object with @last', () => {
+      expectTemplate('{{#each goodbyes}}{{#if @last}}{{text}}! {{/if}}{{/each}}cruel {{world}}!')
+        .withInput({
+          goodbyes: { foo: { text: 'goodbye' }, bar: { text: 'Goodbye' } },
+          world: 'world',
+        })
+        .toCompileTo('Goodbye! cruel world!');
+    });
+
+    it('each with nested @last', () => {
+      expectTemplate(
+        '{{#each goodbyes}}({{#if @last}}{{text}}! {{/if}}{{#each ../goodbyes}}{{#if @last}}{{text}}!{{/if}}{{/each}}{{#if @last}} {{text}}!{{/if}}) {{/each}}cruel {{world}}!'
+      )
+        .withInput({
+          goodbyes: [{ text: 'goodbye' }, { text: 'Goodbye' }, { text: 'GOODBYE' }],
+          world: 'world',
+        })
+        .toCompileTo('(GOODBYE!) (GOODBYE!) (GOODBYE! GOODBYE! GOODBYE!) cruel world!');
+    });
+
+    it('each with function argument', () => {
+      const string = '{{#each goodbyes}}{{text}}! {{/each}}cruel {{world}}!';
+
+      expectTemplate(string)
+        .withInput({
+          goodbyes() {
+            return [{ text: 'goodbye' }, { text: 'Goodbye' }, { text: 'GOODBYE' }];
+          },
+          world: 'world',
+        })
+        .toCompileTo('goodbye! Goodbye! GOODBYE! cruel world!');
+
+      expectTemplate(string)
+        .withInput({
+          goodbyes: [],
+          world: 'world',
+        })
+        .toCompileTo('cruel world!');
+    });
+
+    it('each object when last key is an empty string', () => {
+      expectTemplate('{{#each goodbyes}}{{@index}}. {{text}}! {{/each}}cruel {{world}}!')
+        .withInput({
+          goodbyes: {
+            a: { text: 'goodbye' },
+            b: { text: 'Goodbye' },
+            '': { text: 'GOODBYE' },
+          },
+          world: 'world',
+        })
+        .toCompileTo('0. goodbye! 1. Goodbye! 2. GOODBYE! cruel world!');
+    });
+
+    it('data passed to helpers', () => {
+      expectTemplate('{{#each letters}}{{this}}{{detectDataInsideEach}}{{/each}}')
+        .withInput({ letters: ['a', 'b', 'c'] })
+        .withHelper('detectDataInsideEach', function (options) {
+          return options.data && options.data.exclaim;
+        })
+        .withRuntimeOptions({
+          data: {
+            exclaim: '!',
+          },
+        })
+        .toCompileTo('a!b!c!');
+    });
+
+    it('each on implicit context', () => {
+      expectTemplate('{{#each}}{{text}}! {{/each}}cruel world!').toThrow(Handlebars.Exception);
+    });
+
+    it('each on iterable', () => {
+      class Iterator {
+        private arr: any[];
+        private index: number = 0;
+
+        constructor(arr: any[]) {
+          this.arr = arr;
+        }
+
+        next() {
+          const value = this.arr[this.index];
+          const done = this.index === this.arr.length;
+          if (!done) {
+            this.index++;
+          }
+          return { value, done };
+        }
+      }
+
+      class Iterable {
+        private arr: any[];
+
+        constructor(arr: any[]) {
+          this.arr = arr;
+        }
+
+        [Symbol.iterator]() {
+          return new Iterator(this.arr);
+        }
+      }
+
+      const string = '{{#each goodbyes}}{{text}}! {{/each}}cruel {{world}}!';
+
+      expectTemplate(string)
+        .withInput({
+          goodbyes: new Iterable([{ text: 'goodbye' }, { text: 'Goodbye' }, { text: 'GOODBYE' }]),
+          world: 'world',
+        })
+        .toCompileTo('goodbye! Goodbye! GOODBYE! cruel world!');
+
+      expectTemplate(string)
+        .withInput({
+          goodbyes: new Iterable([]),
+          world: 'world',
+        })
+        .toCompileTo('cruel world!');
+    });
+  });
+
+  describe('#log', function () {
+    /* eslint-disable no-console */
+    let $log: typeof console.log;
+    let $info: typeof console.info;
+    let $error: typeof console.error;
+
+    beforeEach(function () {
+      $log = console.log;
+      $info = console.info;
+      $error = console.error;
+
+      global.kbnHandlebarsEnv = Handlebars.create();
+    });
+
+    afterEach(function () {
+      console.log = $log;
+      console.info = $info;
+      console.error = $error;
+
+      global.kbnHandlebarsEnv = null;
+    });
+
+    it('should call logger at default level', function () {
+      let levelArg;
+      let logArg;
+      kbnHandlebarsEnv!.log = function (level, arg) {
+        levelArg = level;
+        logArg = arg;
+      };
+
+      expectTemplate('{{log blah}}').withInput({ blah: 'whee' }).toCompileTo('');
+      expect(1).toEqual(levelArg);
+      expect('whee').toEqual(logArg);
+    });
+
+    it('should call logger at data level', function () {
+      let levelArg;
+      let logArg;
+      kbnHandlebarsEnv!.log = function (level, arg) {
+        levelArg = level;
+        logArg = arg;
+      };
+
+      expectTemplate('{{log blah}}')
+        .withInput({ blah: 'whee' })
+        .withRuntimeOptions({ data: { level: '03' } })
+        .withCompileOptions({ data: true })
+        .toCompileTo('');
+      expect('03').toEqual(levelArg);
+      expect('whee').toEqual(logArg);
+    });
+
+    it('should output to info', function () {
+      let calls = 0;
+      const callsExpected = process.env.AST || process.env.EVAL ? 1 : 2;
+
+      console.info = function (info) {
+        expect('whee').toEqual(info);
+        calls++;
+        if (calls === callsExpected) {
+          console.info = $info;
+          console.log = $log;
+        }
+      };
+      console.log = function (log) {
+        expect('whee').toEqual(log);
+        calls++;
+        if (calls === callsExpected) {
+          console.info = $info;
+          console.log = $log;
+        }
+      };
+
+      expectTemplate('{{log blah}}').withInput({ blah: 'whee' }).toCompileTo('');
+      expect(calls).toEqual(callsExpected);
+    });
+
+    it('should log at data level', function () {
+      let calls = 0;
+      const callsExpected = process.env.AST || process.env.EVAL ? 1 : 2;
+
+      console.error = function (log) {
+        expect('whee').toEqual(log);
+        calls++;
+        if (calls === callsExpected) console.error = $error;
+      };
+
+      expectTemplate('{{log blah}}')
+        .withInput({ blah: 'whee' })
+        .withRuntimeOptions({ data: { level: '03' } })
+        .withCompileOptions({ data: true })
+        .toCompileTo('');
+      expect(calls).toEqual(callsExpected);
+    });
+
+    it('should handle missing logger', function () {
+      let calls = 0;
+      const callsExpected = process.env.AST || process.env.EVAL ? 1 : 2;
+
+      // @ts-expect-error
+      console.error = undefined;
+      console.log = function (log) {
+        expect('whee').toEqual(log);
+        calls++;
+        if (calls === callsExpected) console.log = $log;
+      };
+
+      expectTemplate('{{log blah}}')
+        .withInput({ blah: 'whee' })
+        .withRuntimeOptions({ data: { level: '03' } })
+        .withCompileOptions({ data: true })
+        .toCompileTo('');
+      expect(calls).toEqual(callsExpected);
+    });
+
+    it('should handle string log levels', function () {
+      let calls = 0;
+      const callsExpected = process.env.AST || process.env.EVAL ? 1 : 2;
+
+      console.error = function (log) {
+        expect('whee').toEqual(log);
+        calls++;
+      };
+
+      expectTemplate('{{log blah}}')
+        .withInput({ blah: 'whee' })
+        .withRuntimeOptions({ data: { level: 'error' } })
+        .withCompileOptions({ data: true })
+        .toCompileTo('');
+      expect(calls).toEqual(callsExpected);
+
+      calls = 0;
+
+      expectTemplate('{{log blah}}')
+        .withInput({ blah: 'whee' })
+        .withRuntimeOptions({ data: { level: 'ERROR' } })
+        .withCompileOptions({ data: true })
+        .toCompileTo('');
+      expect(calls).toEqual(callsExpected);
+    });
+
+    it('should handle hash log levels [1]', function () {
+      let calls = 0;
+      const callsExpected = process.env.AST || process.env.EVAL ? 1 : 2;
+
+      console.error = function (log) {
+        expect('whee').toEqual(log);
+        calls++;
+      };
+
+      expectTemplate('{{log blah level="error"}}').withInput({ blah: 'whee' }).toCompileTo('');
+      expect(calls).toEqual(callsExpected);
+    });
+
+    it('should handle hash log levels [2]', function () {
+      let called = false;
+
+      console.info =
+        console.log =
+        console.error =
+        console.debug =
+          function () {
+            called = true;
+            console.info = console.log = console.error = console.debug = $log;
+          };
+
+      expectTemplate('{{log blah level="debug"}}').withInput({ blah: 'whee' }).toCompileTo('');
+      expect(false).toEqual(called);
+    });
+
+    it('should pass multiple log arguments', function () {
+      let calls = 0;
+      const callsExpected = process.env.AST || process.env.EVAL ? 1 : 2;
+
+      console.info = console.log = function (log1, log2, log3) {
+        expect('whee').toEqual(log1);
+        expect('foo').toEqual(log2);
+        expect(1).toEqual(log3);
+        calls++;
+        if (calls === callsExpected) console.log = $log;
+      };
+
+      expectTemplate('{{log blah "foo" 1}}').withInput({ blah: 'whee' }).toCompileTo('');
+      expect(calls).toEqual(callsExpected);
+    });
+
+    it('should pass zero log arguments', function () {
+      let calls = 0;
+      const callsExpected = process.env.AST || process.env.EVAL ? 1 : 2;
+
+      console.info = console.log = function () {
+        expect(arguments.length).toEqual(0);
+        calls++;
+        if (calls === callsExpected) console.log = $log;
+      };
+
+      expectTemplate('{{log}}').withInput({ blah: 'whee' }).toCompileTo('');
+      expect(calls).toEqual(callsExpected);
+    });
+    /* eslint-enable no-console */
+  });
+
+  describe('#lookup', () => {
+    it('should lookup arbitrary content', () => {
+      expectTemplate('{{#each goodbyes}}{{lookup ../data .}}{{/each}}')
+        .withInput({ goodbyes: [0, 1], data: ['foo', 'bar'] })
+        .toCompileTo('foobar');
+    });
+
+    it('should not fail on undefined value', () => {
+      expectTemplate('{{#each goodbyes}}{{lookup ../bar .}}{{/each}}')
+        .withInput({ goodbyes: [0, 1], data: ['foo', 'bar'] })
+        .toCompileTo('');
+    });
+  });
+});
diff --git a/dev/lib/handlebars/src/spec/index.compiler.test.ts b/dev/lib/handlebars/src/spec/index.compiler.test.ts
new file mode 100644
index 00000000..ef5c55f2
--- /dev/null
+++ b/dev/lib/handlebars/src/spec/index.compiler.test.ts
@@ -0,0 +1,86 @@
+/*
+ * This file is forked from the handlebars project (https://github.com/handlebars-lang/handlebars.js),
+ * and may include modifications made by Elasticsearch B.V.
+ * Elasticsearch B.V. licenses this file to you under the MIT License.
+ * See `packages/kbn-handlebars/LICENSE` for more information.
+ */
+
+import Handlebars from '../..';
+import { forEachCompileFunctionName } from '../__jest__/test_bench';
+
+describe('compiler', () => {
+  forEachCompileFunctionName((compileName) => {
+    const compile = Handlebars[compileName].bind(Handlebars);
+
+    describe(`#${compileName}`, () => {
+      it('should fail with invalid input', () => {
+        expect(function () {
+          compile(null);
+        }).toThrow(
+          `You must pass a string or Handlebars AST to Handlebars.${compileName}. You passed null`
+        );
+
+        expect(function () {
+          compile({});
+        }).toThrow(
+          `You must pass a string or Handlebars AST to Handlebars.${compileName}. You passed [object Object]`
+        );
+      });
+
+      it('should include the location in the error (row and column)', () => {
+        try {
+          compile(' \n  {{#if}}\n{{/def}}')();
+          expect(true).toEqual(false);
+        } catch (err) {
+          expect(err.message).toEqual("if doesn't match def - 2:5");
+          if (Object.getOwnPropertyDescriptor(err, 'column')!.writable) {
+            // In Safari 8, the column-property is read-only. This means that even if it is set with defineProperty,
+            // its value won't change (https://github.com/jquery/esprima/issues/1290#issuecomment-132455482)
+            // Since this was neither working in Handlebars 3 nor in 4.0.5, we only check the column for other browsers.
+            expect(err.column).toEqual(5);
+          }
+          expect(err.lineNumber).toEqual(2);
+        }
+      });
+
+      it('should include the location as enumerable property', () => {
+        try {
+          compile(' \n  {{#if}}\n{{/def}}')();
+          expect(true).toEqual(false);
+        } catch (err) {
+          expect(Object.prototype.propertyIsEnumerable.call(err, 'column')).toEqual(true);
+        }
+      });
+
+      it('can utilize AST instance', () => {
+        expect(
+          compile({
+            type: 'Program',
+            body: [{ type: 'ContentStatement', value: 'Hello' }],
+          })()
+        ).toEqual('Hello');
+      });
+
+      it('can pass through an empty string', () => {
+        expect(compile('')()).toEqual('');
+      });
+
+      it('should not modify the options.data property(GH-1327)', () => {
+        // The `data` property is supposed to be a boolean, but in this test we want to ignore that
+        const options = { data: [{ a: 'foo' }, { a: 'bar' }] as unknown as boolean };
+        compile('{{#each data}}{{@index}}:{{a}} {{/each}}', options)();
+        expect(JSON.stringify(options, null, 2)).toEqual(
+          JSON.stringify({ data: [{ a: 'foo' }, { a: 'bar' }] }, null, 2)
+        );
+      });
+
+      it('should not modify the options.knownHelpers property(GH-1327)', () => {
+        const options = { knownHelpers: {} };
+        compile('{{#each data}}{{@index}}:{{a}} {{/each}}', options)();
+        expect(JSON.stringify(options, null, 2)).toEqual(
+          JSON.stringify({ knownHelpers: {} }, null, 2)
+        );
+      });
+    });
+  });
+});
diff --git a/dev/lib/handlebars/src/spec/index.data.test.ts b/dev/lib/handlebars/src/spec/index.data.test.ts
new file mode 100644
index 00000000..94d3b51c
--- /dev/null
+++ b/dev/lib/handlebars/src/spec/index.data.test.ts
@@ -0,0 +1,269 @@
+/*
+ * This file is forked from the handlebars project (https://github.com/handlebars-lang/handlebars.js),
+ * and may include modifications made by Elasticsearch B.V.
+ * Elasticsearch B.V. licenses this file to you under the MIT License.
+ * See `packages/kbn-handlebars/LICENSE` for more information.
+ */
+
+import Handlebars, { type HelperOptions } from '../..';
+import { expectTemplate } from '../__jest__/test_bench';
+
+describe('data', () => {
+  it('passing in data to a compiled function that expects data - works with helpers', () => {
+    expectTemplate('{{hello}}')
+      .withCompileOptions({ data: true })
+      .withHelper('hello', function (this: any, options) {
+        return options.data.adjective + ' ' + this.noun;
+      })
+      .withRuntimeOptions({ data: { adjective: 'happy' } })
+      .withInput({ noun: 'cat' })
+      .toCompileTo('happy cat');
+  });
+
+  it('data can be looked up via @foo', () => {
+    expectTemplate('{{@hello}}')
+      .withRuntimeOptions({ data: { hello: 'hello' } })
+      .toCompileTo('hello');
+  });
+
+  it('deep @foo triggers automatic top-level data', () => {
+    global.kbnHandlebarsEnv = Handlebars.create();
+    const helpers = Handlebars.createFrame(kbnHandlebarsEnv!.helpers);
+
+    helpers.let = function (options: HelperOptions) {
+      const frame = Handlebars.createFrame(options.data);
+
+      for (const prop in options.hash) {
+        if (prop in options.hash) {
+          frame[prop] = options.hash[prop];
+        }
+      }
+      return options.fn(this, { data: frame });
+    };
+
+    expectTemplate(
+      '{{#let world="world"}}{{#if foo}}{{#if foo}}Hello {{@world}}{{/if}}{{/if}}{{/let}}'
+    )
+      .withInput({ foo: true })
+      .withHelpers(helpers)
+      .toCompileTo('Hello world');
+
+    global.kbnHandlebarsEnv = null;
+  });
+
+  it('parameter data can be looked up via @foo', () => {
+    expectTemplate('{{hello @world}}')
+      .withRuntimeOptions({ data: { world: 'world' } })
+      .withHelper('hello', function (noun) {
+        return 'Hello ' + noun;
+      })
+      .toCompileTo('Hello world');
+  });
+
+  it('hash values can be looked up via @foo', () => {
+    expectTemplate('{{hello noun=@world}}')
+      .withRuntimeOptions({ data: { world: 'world' } })
+      .withHelper('hello', function (options) {
+        return 'Hello ' + options.hash.noun;
+      })
+      .toCompileTo('Hello world');
+  });
+
+  it('nested parameter data can be looked up via @foo.bar', () => {
+    expectTemplate('{{hello @world.bar}}')
+      .withRuntimeOptions({ data: { world: { bar: 'world' } } })
+      .withHelper('hello', function (noun) {
+        return 'Hello ' + noun;
+      })
+      .toCompileTo('Hello world');
+  });
+
+  it('nested parameter data does not fail with @world.bar', () => {
+    expectTemplate('{{hello @world.bar}}')
+      .withRuntimeOptions({ data: { foo: { bar: 'world' } } })
+      .withHelper('hello', function (noun) {
+        return 'Hello ' + noun;
+      })
+      .toCompileTo('Hello undefined');
+  });
+
+  it('parameter data throws when using complex scope references', () => {
+    expectTemplate('{{#goodbyes}}{{text}} cruel {{@foo/../name}}! {{/goodbyes}}').toThrow(Error);
+  });
+
+  it('data can be functions', () => {
+    expectTemplate('{{@hello}}')
+      .withRuntimeOptions({
+        data: {
+          hello() {
+            return 'hello';
+          },
+        },
+      })
+      .toCompileTo('hello');
+  });
+
+  it('data can be functions with params', () => {
+    expectTemplate('{{@hello "hello"}}')
+      .withRuntimeOptions({
+        data: {
+          hello(arg: any) {
+            return arg;
+          },
+        },
+      })
+      .toCompileTo('hello');
+  });
+
+  it('data is inherited downstream', () => {
+    expectTemplate(
+      '{{#let foo=1 bar=2}}{{#let foo=bar.baz}}{{@bar}}{{@foo}}{{/let}}{{@foo}}{{/let}}'
+    )
+      .withInput({ bar: { baz: 'hello world' } })
+      .withCompileOptions({ data: true })
+      .withHelper('let', function (this: any, options) {
+        const frame = Handlebars.createFrame(options.data);
+        for (const prop in options.hash) {
+          if (prop in options.hash) {
+            frame[prop] = options.hash[prop];
+          }
+        }
+        return options.fn(this, { data: frame });
+      })
+      .withRuntimeOptions({ data: {} })
+      .toCompileTo('2hello world1');
+  });
+
+  it('passing in data to a compiled function that expects data - works with helpers in partials', () => {
+    expectTemplate('{{>myPartial}}')
+      .withCompileOptions({ data: true })
+      .withPartial('myPartial', '{{hello}}')
+      .withHelper('hello', function (this: any, options: HelperOptions) {
+        return options.data.adjective + ' ' + this.noun;
+      })
+      .withInput({ noun: 'cat' })
+      .withRuntimeOptions({ data: { adjective: 'happy' } })
+      .toCompileTo('happy cat');
+  });
+
+  it('passing in data to a compiled function that expects data - works with helpers and parameters', () => {
+    expectTemplate('{{hello world}}')
+      .withCompileOptions({ data: true })
+      .withHelper('hello', function (this: any, noun, options) {
+        return options.data.adjective + ' ' + noun + (this.exclaim ? '!' : '');
+      })
+      .withInput({ exclaim: true, world: 'world' })
+      .withRuntimeOptions({ data: { adjective: 'happy' } })
+      .toCompileTo('happy world!');
+  });
+
+  it('passing in data to a compiled function that expects data - works with block helpers', () => {
+    expectTemplate('{{#hello}}{{world}}{{/hello}}')
+      .withCompileOptions({
+        data: true,
+      })
+      .withHelper('hello', function (this: any, options) {
+        return options.fn(this);
+      })
+      .withHelper('world', function (this: any, options) {
+        return options.data.adjective + ' world' + (this.exclaim ? '!' : '');
+      })
+      .withInput({ exclaim: true })
+      .withRuntimeOptions({ data: { adjective: 'happy' } })
+      .toCompileTo('happy world!');
+  });
+
+  it('passing in data to a compiled function that expects data - works with block helpers that use ..', () => {
+    expectTemplate('{{#hello}}{{world ../zomg}}{{/hello}}')
+      .withCompileOptions({ data: true })
+      .withHelper('hello', function (options) {
+        return options.fn({ exclaim: '?' });
+      })
+      .withHelper('world', function (this: any, thing, options) {
+        return options.data.adjective + ' ' + thing + (this.exclaim || '');
+      })
+      .withInput({ exclaim: true, zomg: 'world' })
+      .withRuntimeOptions({ data: { adjective: 'happy' } })
+      .toCompileTo('happy world?');
+  });
+
+  it('passing in data to a compiled function that expects data - data is passed to with block helpers where children use ..', () => {
+    expectTemplate('{{#hello}}{{world ../zomg}}{{/hello}}')
+      .withCompileOptions({ data: true })
+      .withHelper('hello', function (options) {
+        return options.data.accessData + ' ' + options.fn({ exclaim: '?' });
+      })
+      .withHelper('world', function (this: any, thing, options) {
+        return options.data.adjective + ' ' + thing + (this.exclaim || '');
+      })
+      .withInput({ exclaim: true, zomg: 'world' })
+      .withRuntimeOptions({ data: { adjective: 'happy', accessData: '#win' } })
+      .toCompileTo('#win happy world?');
+  });
+
+  it('you can override inherited data when invoking a helper', () => {
+    expectTemplate('{{#hello}}{{world zomg}}{{/hello}}')
+      .withCompileOptions({ data: true })
+      .withHelper('hello', function (options) {
+        return options.fn({ exclaim: '?', zomg: 'world' }, { data: { adjective: 'sad' } });
+      })
+      .withHelper('world', function (this: any, thing, options) {
+        return options.data.adjective + ' ' + thing + (this.exclaim || '');
+      })
+      .withInput({ exclaim: true, zomg: 'planet' })
+      .withRuntimeOptions({ data: { adjective: 'happy' } })
+      .toCompileTo('sad world?');
+  });
+
+  it('you can override inherited data when invoking a helper with depth', () => {
+    expectTemplate('{{#hello}}{{world ../zomg}}{{/hello}}')
+      .withCompileOptions({ data: true })
+      .withHelper('hello', function (options) {
+        return options.fn({ exclaim: '?' }, { data: { adjective: 'sad' } });
+      })
+      .withHelper('world', function (this: any, thing, options) {
+        return options.data.adjective + ' ' + thing + (this.exclaim || '');
+      })
+      .withInput({ exclaim: true, zomg: 'world' })
+      .withRuntimeOptions({ data: { adjective: 'happy' } })
+      .toCompileTo('sad world?');
+  });
+
+  describe('@root', () => {
+    it('the root context can be looked up via @root', () => {
+      expectTemplate('{{@root.foo}}')
+        .withInput({ foo: 'hello' })
+        .withRuntimeOptions({ data: {} })
+        .toCompileTo('hello');
+
+      expectTemplate('{{@root.foo}}').withInput({ foo: 'hello' }).toCompileTo('hello');
+    });
+
+    it('passed root values take priority', () => {
+      expectTemplate('{{@root.foo}}')
+        .withInput({ foo: 'should not be used' })
+        .withRuntimeOptions({ data: { root: { foo: 'hello' } } })
+        .toCompileTo('hello');
+    });
+  });
+
+  describe('nesting', () => {
+    it('the root context can be looked up via @root', () => {
+      expectTemplate(
+        '{{#helper}}{{#helper}}{{@./depth}} {{@../depth}} {{@../../depth}}{{/helper}}{{/helper}}'
+      )
+        .withInput({ foo: 'hello' })
+        .withHelper('helper', function (this: any, options) {
+          const frame = Handlebars.createFrame(options.data);
+          frame.depth = options.data.depth + 1;
+          return options.fn(this, { data: frame });
+        })
+        .withRuntimeOptions({
+          data: {
+            depth: 0,
+          },
+        })
+        .toCompileTo('2 1 0');
+    });
+  });
+});
diff --git a/dev/lib/handlebars/src/spec/index.helpers.test.ts b/dev/lib/handlebars/src/spec/index.helpers.test.ts
new file mode 100644
index 00000000..4cfa39bb
--- /dev/null
+++ b/dev/lib/handlebars/src/spec/index.helpers.test.ts
@@ -0,0 +1,958 @@
+/*
+ * This file is forked from the handlebars project (https://github.com/handlebars-lang/handlebars.js),
+ * and may include modifications made by Elasticsearch B.V.
+ * Elasticsearch B.V. licenses this file to you under the MIT License.
+ * See `packages/kbn-handlebars/LICENSE` for more information.
+ */
+
+import Handlebars, { type HelperOptions } from '../..';
+import { expectTemplate } from '../__jest__/test_bench';
+
+beforeEach(() => {
+  global.kbnHandlebarsEnv = Handlebars.create();
+});
+
+afterEach(() => {
+  global.kbnHandlebarsEnv = null;
+});
+
+describe('helpers', () => {
+  it('helper with complex lookup$', () => {
+    expectTemplate('{{#goodbyes}}{{{link ../prefix}}}{{/goodbyes}}')
+      .withInput({
+        prefix: '/root',
+        goodbyes: [{ text: 'Goodbye', url: 'goodbye' }],
+      })
+      .withHelper('link', function (this: any, prefix) {
+        return '<a href="' + prefix + '/' + this.url + '">' + this.text + '</a>';
+      })
+      .toCompileTo('<a href="/root/goodbye">Goodbye</a>');
+  });
+
+  it('helper for raw block gets raw content', () => {
+    expectTemplate('{{{{raw}}}} {{test}} {{{{/raw}}}}')
+      .withInput({ test: 'hello' })
+      .withHelper('raw', function (options: HelperOptions) {
+        return options.fn();
+      })
+      .toCompileTo(' {{test}} ');
+  });
+
+  it('helper for raw block gets parameters', () => {
+    expectTemplate('{{{{raw 1 2 3}}}} {{test}} {{{{/raw}}}}')
+      .withInput({ test: 'hello' })
+      .withHelper('raw', function (a, b, c, options: HelperOptions) {
+        const ret = options.fn() + a + b + c;
+        return ret;
+      })
+      .toCompileTo(' {{test}} 123');
+  });
+
+  describe('raw block parsing (with identity helper-function)', () => {
+    function runWithIdentityHelper(template: string, expected: string) {
+      expectTemplate(template)
+        .withHelper('identity', function (options: HelperOptions) {
+          return options.fn();
+        })
+        .toCompileTo(expected);
+    }
+
+    it('helper for nested raw block gets raw content', () => {
+      runWithIdentityHelper(
+        '{{{{identity}}}} {{{{b}}}} {{{{/b}}}} {{{{/identity}}}}',
+        ' {{{{b}}}} {{{{/b}}}} '
+      );
+    });
+
+    it('helper for nested raw block works with empty content', () => {
+      runWithIdentityHelper('{{{{identity}}}}{{{{/identity}}}}', '');
+    });
+
+    it.skip('helper for nested raw block works if nested raw blocks are broken', () => {
+      // This test was introduced in 4.4.4, but it was not the actual problem that lead to the patch release
+      // The test is deactivated, because in 3.x this template cases an exception and it also does not work in 4.4.3
+      // If anyone can make this template work without breaking everything else, then go for it,
+      // but for now, this is just a known bug, that will be documented.
+      runWithIdentityHelper(
+        '{{{{identity}}}} {{{{a}}}} {{{{ {{{{/ }}}} }}}} {{{{/identity}}}}',
+        ' {{{{a}}}} {{{{ {{{{/ }}}} }}}} '
+      );
+    });
+
+    it('helper for nested raw block closes after first matching close', () => {
+      runWithIdentityHelper(
+        '{{{{identity}}}}abc{{{{/identity}}}} {{{{identity}}}}abc{{{{/identity}}}}',
+        'abc abc'
+      );
+    });
+
+    it('helper for nested raw block throw exception when with missing closing braces', () => {
+      const string = '{{{{a}}}} {{{{/a';
+      expectTemplate(string).toThrow();
+    });
+  });
+
+  it('helper block with identical context', () => {
+    expectTemplate('{{#goodbyes}}{{name}}{{/goodbyes}}')
+      .withInput({ name: 'Alan' })
+      .withHelper('goodbyes', function (this: any, options: HelperOptions) {
+        let out = '';
+        const byes = ['Goodbye', 'goodbye', 'GOODBYE'];
+        for (let i = 0, j = byes.length; i < j; i++) {
+          out += byes[i] + ' ' + options.fn(this) + '! ';
+        }
+        return out;
+      })
+      .toCompileTo('Goodbye Alan! goodbye Alan! GOODBYE Alan! ');
+  });
+
+  it('helper block with complex lookup expression', () => {
+    expectTemplate('{{#goodbyes}}{{../name}}{{/goodbyes}}')
+      .withInput({ name: 'Alan' })
+      .withHelper('goodbyes', function (options: HelperOptions) {
+        let out = '';
+        const byes = ['Goodbye', 'goodbye', 'GOODBYE'];
+        for (let i = 0, j = byes.length; i < j; i++) {
+          out += byes[i] + ' ' + options.fn({}) + '! ';
+        }
+        return out;
+      })
+      .toCompileTo('Goodbye Alan! goodbye Alan! GOODBYE Alan! ');
+  });
+
+  it('helper with complex lookup and nested template', () => {
+    expectTemplate('{{#goodbyes}}{{#link ../prefix}}{{text}}{{/link}}{{/goodbyes}}')
+      .withInput({
+        prefix: '/root',
+        goodbyes: [{ text: 'Goodbye', url: 'goodbye' }],
+      })
+      .withHelper('link', function (this: any, prefix, options: HelperOptions) {
+        return '<a href="' + prefix + '/' + this.url + '">' + options.fn(this) + '</a>';
+      })
+      .toCompileTo('<a href="/root/goodbye">Goodbye</a>');
+  });
+
+  it('helper with complex lookup and nested template in VM+Compiler', () => {
+    expectTemplate('{{#goodbyes}}{{#link ../prefix}}{{text}}{{/link}}{{/goodbyes}}')
+      .withInput({
+        prefix: '/root',
+        goodbyes: [{ text: 'Goodbye', url: 'goodbye' }],
+      })
+      .withHelper('link', function (this: any, prefix, options: HelperOptions) {
+        return '<a href="' + prefix + '/' + this.url + '">' + options.fn(this) + '</a>';
+      })
+      .toCompileTo('<a href="/root/goodbye">Goodbye</a>');
+  });
+
+  it('helper returning undefined value', () => {
+    expectTemplate(' {{nothere}}')
+      .withHelpers({
+        nothere() {},
+      })
+      .toCompileTo(' ');
+
+    expectTemplate(' {{#nothere}}{{/nothere}}')
+      .withHelpers({
+        nothere() {},
+      })
+      .toCompileTo(' ');
+  });
+
+  it('block helper', () => {
+    expectTemplate('{{#goodbyes}}{{text}}! {{/goodbyes}}cruel {{world}}!')
+      .withInput({ world: 'world' })
+      .withHelper('goodbyes', function (options: HelperOptions) {
+        return options.fn({ text: 'GOODBYE' });
+      })
+      .toCompileTo('GOODBYE! cruel world!');
+  });
+
+  it('block helper staying in the same context', () => {
+    expectTemplate('{{#form}}<p>{{name}}</p>{{/form}}')
+      .withInput({ name: 'Yehuda' })
+      .withHelper('form', function (this: any, options: HelperOptions) {
+        return '<form>' + options.fn(this) + '</form>';
+      })
+      .toCompileTo('<form><p>Yehuda</p></form>');
+  });
+
+  it('block helper should have context in this', () => {
+    function link(this: any, options: HelperOptions) {
+      return '<a href="/people/' + this.id + '">' + options.fn(this) + '</a>';
+    }
+
+    expectTemplate('<ul>{{#people}}<li>{{#link}}{{name}}{{/link}}</li>{{/people}}</ul>')
+      .withInput({
+        people: [
+          { name: 'Alan', id: 1 },
+          { name: 'Yehuda', id: 2 },
+        ],
+      })
+      .withHelper('link', link)
+      .toCompileTo(
+        '<ul><li><a href="/people/1">Alan</a></li><li><a href="/people/2">Yehuda</a></li></ul>'
+      );
+  });
+
+  it('block helper for undefined value', () => {
+    expectTemplate("{{#empty}}shouldn't render{{/empty}}").toCompileTo('');
+  });
+
+  it('block helper passing a new context', () => {
+    expectTemplate('{{#form yehuda}}<p>{{name}}</p>{{/form}}')
+      .withInput({ yehuda: { name: 'Yehuda' } })
+      .withHelper('form', function (context, options: HelperOptions) {
+        return '<form>' + options.fn(context) + '</form>';
+      })
+      .toCompileTo('<form><p>Yehuda</p></form>');
+  });
+
+  it('block helper passing a complex path context', () => {
+    expectTemplate('{{#form yehuda/cat}}<p>{{name}}</p>{{/form}}')
+      .withInput({ yehuda: { name: 'Yehuda', cat: { name: 'Harold' } } })
+      .withHelper('form', function (context, options: HelperOptions) {
+        return '<form>' + options.fn(context) + '</form>';
+      })
+      .toCompileTo('<form><p>Harold</p></form>');
+  });
+
+  it('nested block helpers', () => {
+    expectTemplate('{{#form yehuda}}<p>{{name}}</p>{{#link}}Hello{{/link}}{{/form}}')
+      .withInput({
+        yehuda: { name: 'Yehuda' },
+      })
+      .withHelper('link', function (this: any, options: HelperOptions) {
+        return '<a href="' + this.name + '">' + options.fn(this) + '</a>';
+      })
+      .withHelper('form', function (context, options: HelperOptions) {
+        return '<form>' + options.fn(context) + '</form>';
+      })
+      .toCompileTo('<form><p>Yehuda</p><a href="Yehuda">Hello</a></form>');
+  });
+
+  it('block helper inverted sections', () => {
+    const string = "{{#list people}}{{name}}{{^}}<em>Nobody's here</em>{{/list}}";
+    function list(this: any, context: any, options: HelperOptions) {
+      if (context.length > 0) {
+        let out = '<ul>';
+        for (let i = 0, j = context.length; i < j; i++) {
+          out += '<li>';
+          out += options.fn(context[i]);
+          out += '</li>';
+        }
+        out += '</ul>';
+        return out;
+      } else {
+        return '<p>' + options.inverse(this) + '</p>';
+      }
+    }
+
+    // the meaning here may be kind of hard to catch, but list.not is always called,
+    // so we should see the output of both
+    expectTemplate(string)
+      .withInput({ people: [{ name: 'Alan' }, { name: 'Yehuda' }] })
+      .withHelpers({ list })
+      .toCompileTo('<ul><li>Alan</li><li>Yehuda</li></ul>');
+
+    expectTemplate(string)
+      .withInput({ people: [] })
+      .withHelpers({ list })
+      .toCompileTo("<p><em>Nobody's here</em></p>");
+
+    expectTemplate('{{#list people}}Hello{{^}}{{message}}{{/list}}')
+      .withInput({
+        people: [],
+        message: "Nobody's here",
+      })
+      .withHelpers({ list })
+      .toCompileTo('<p>Nobody&#x27;s here</p>');
+  });
+
+  it('pathed lambas with parameters', () => {
+    const hash = {
+      helper: () => 'winning',
+    };
+    // @ts-expect-error
+    hash.hash = hash;
+
+    const helpers = {
+      './helper': () => 'fail',
+    };
+
+    expectTemplate('{{./helper 1}}').withInput(hash).withHelpers(helpers).toCompileTo('winning');
+    expectTemplate('{{hash/helper 1}}').withInput(hash).withHelpers(helpers).toCompileTo('winning');
+  });
+
+  describe('helpers hash', () => {
+    it('providing a helpers hash', () => {
+      expectTemplate('Goodbye {{cruel}} {{world}}!')
+        .withInput({ cruel: 'cruel' })
+        .withHelpers({
+          world() {
+            return 'world';
+          },
+        })
+        .toCompileTo('Goodbye cruel world!');
+
+      expectTemplate('Goodbye {{#iter}}{{cruel}} {{world}}{{/iter}}!')
+        .withInput({ iter: [{ cruel: 'cruel' }] })
+        .withHelpers({
+          world() {
+            return 'world';
+          },
+        })
+        .toCompileTo('Goodbye cruel world!');
+    });
+
+    it('in cases of conflict, helpers win', () => {
+      expectTemplate('{{{lookup}}}')
+        .withInput({ lookup: 'Explicit' })
+        .withHelpers({
+          lookup() {
+            return 'helpers';
+          },
+        })
+        .toCompileTo('helpers');
+
+      expectTemplate('{{lookup}}')
+        .withInput({ lookup: 'Explicit' })
+        .withHelpers({
+          lookup() {
+            return 'helpers';
+          },
+        })
+        .toCompileTo('helpers');
+    });
+
+    it('the helpers hash is available is nested contexts', () => {
+      expectTemplate('{{#outer}}{{#inner}}{{helper}}{{/inner}}{{/outer}}')
+        .withInput({ outer: { inner: { unused: [] } } })
+        .withHelpers({
+          helper() {
+            return 'helper';
+          },
+        })
+        .toCompileTo('helper');
+    });
+
+    it('the helper hash should augment the global hash', () => {
+      kbnHandlebarsEnv!.registerHelper('test_helper', function () {
+        return 'found it!';
+      });
+
+      expectTemplate('{{test_helper}} {{#if cruel}}Goodbye {{cruel}} {{world}}!{{/if}}')
+        .withInput({ cruel: 'cruel' })
+        .withHelpers({
+          world() {
+            return 'world!';
+          },
+        })
+        .toCompileTo('found it! Goodbye cruel world!!');
+    });
+  });
+
+  describe('registration', () => {
+    it('unregisters', () => {
+      deleteAllKeys(kbnHandlebarsEnv!.helpers);
+
+      kbnHandlebarsEnv!.registerHelper('foo', function () {
+        return 'fail';
+      });
+      expect(kbnHandlebarsEnv!.helpers.foo).toBeDefined();
+      kbnHandlebarsEnv!.unregisterHelper('foo');
+      expect(kbnHandlebarsEnv!.helpers.foo).toBeUndefined();
+    });
+
+    it('allows multiple globals', () => {
+      const ifHelper = kbnHandlebarsEnv!.helpers.if;
+      deleteAllKeys(kbnHandlebarsEnv!.helpers);
+
+      kbnHandlebarsEnv!.registerHelper({
+        if: ifHelper,
+        world() {
+          return 'world!';
+        },
+        testHelper() {
+          return 'found it!';
+        },
+      });
+
+      expectTemplate('{{testHelper}} {{#if cruel}}Goodbye {{cruel}} {{world}}!{{/if}}')
+        .withInput({ cruel: 'cruel' })
+        .toCompileTo('found it! Goodbye cruel world!!');
+    });
+
+    it('fails with multiple and args', () => {
+      expect(() => {
+        kbnHandlebarsEnv!.registerHelper(
+          // @ts-expect-error TypeScript is complaining about the invalid input just as the thrown error
+          {
+            world() {
+              return 'world!';
+            },
+            testHelper() {
+              return 'found it!';
+            },
+          },
+          {}
+        );
+      }).toThrow('Arg not supported with multiple helpers');
+    });
+  });
+
+  it('decimal number literals work', () => {
+    expectTemplate('Message: {{hello -1.2 1.2}}')
+      .withHelper('hello', function (times, times2) {
+        if (typeof times !== 'number') {
+          times = 'NaN';
+        }
+        if (typeof times2 !== 'number') {
+          times2 = 'NaN';
+        }
+        return 'Hello ' + times + ' ' + times2 + ' times';
+      })
+      .toCompileTo('Message: Hello -1.2 1.2 times');
+  });
+
+  it('negative number literals work', () => {
+    expectTemplate('Message: {{hello -12}}')
+      .withHelper('hello', function (times) {
+        if (typeof times !== 'number') {
+          times = 'NaN';
+        }
+        return 'Hello ' + times + ' times';
+      })
+      .toCompileTo('Message: Hello -12 times');
+  });
+
+  describe('String literal parameters', () => {
+    it('simple literals work', () => {
+      expectTemplate('Message: {{hello "world" 12 true false}}')
+        .withHelper('hello', function (param, times, bool1, bool2) {
+          if (typeof times !== 'number') {
+            times = 'NaN';
+          }
+          if (typeof bool1 !== 'boolean') {
+            bool1 = 'NaB';
+          }
+          if (typeof bool2 !== 'boolean') {
+            bool2 = 'NaB';
+          }
+          return 'Hello ' + param + ' ' + times + ' times: ' + bool1 + ' ' + bool2;
+        })
+        .toCompileTo('Message: Hello world 12 times: true false');
+    });
+
+    it('using a quote in the middle of a parameter raises an error', () => {
+      expectTemplate('Message: {{hello wo"rld"}}').toThrow(Error);
+    });
+
+    it('escaping a String is possible', () => {
+      expectTemplate('Message: {{{hello "\\"world\\""}}}')
+        .withHelper('hello', function (param) {
+          return 'Hello ' + param;
+        })
+        .toCompileTo('Message: Hello "world"');
+    });
+
+    it("it works with ' marks", () => {
+      expectTemplate('Message: {{{hello "Alan\'s world"}}}')
+        .withHelper('hello', function (param) {
+          return 'Hello ' + param;
+        })
+        .toCompileTo("Message: Hello Alan's world");
+    });
+  });
+
+  describe('multiple parameters', () => {
+    it('simple multi-params work', () => {
+      expectTemplate('Message: {{goodbye cruel world}}')
+        .withInput({ cruel: 'cruel', world: 'world' })
+        .withHelper('goodbye', function (cruel, world) {
+          return 'Goodbye ' + cruel + ' ' + world;
+        })
+        .toCompileTo('Message: Goodbye cruel world');
+    });
+
+    it('block multi-params work', () => {
+      expectTemplate('Message: {{#goodbye cruel world}}{{greeting}} {{adj}} {{noun}}{{/goodbye}}')
+        .withInput({ cruel: 'cruel', world: 'world' })
+        .withHelper('goodbye', function (cruel, world, options: HelperOptions) {
+          return options.fn({ greeting: 'Goodbye', adj: cruel, noun: world });
+        })
+        .toCompileTo('Message: Goodbye cruel world');
+    });
+  });
+
+  describe('hash', () => {
+    it('helpers can take an optional hash', () => {
+      expectTemplate('{{goodbye cruel="CRUEL" world="WORLD" times=12}}')
+        .withHelper('goodbye', function (options: HelperOptions) {
+          return (
+            'GOODBYE ' +
+            options.hash.cruel +
+            ' ' +
+            options.hash.world +
+            ' ' +
+            options.hash.times +
+            ' TIMES'
+          );
+        })
+        .toCompileTo('GOODBYE CRUEL WORLD 12 TIMES');
+    });
+
+    it('helpers can take an optional hash with booleans', () => {
+      function goodbye(options: HelperOptions) {
+        if (options.hash.print === true) {
+          return 'GOODBYE ' + options.hash.cruel + ' ' + options.hash.world;
+        } else if (options.hash.print === false) {
+          return 'NOT PRINTING';
+        } else {
+          return 'THIS SHOULD NOT HAPPEN';
+        }
+      }
+
+      expectTemplate('{{goodbye cruel="CRUEL" world="WORLD" print=true}}')
+        .withHelper('goodbye', goodbye)
+        .toCompileTo('GOODBYE CRUEL WORLD');
+
+      expectTemplate('{{goodbye cruel="CRUEL" world="WORLD" print=false}}')
+        .withHelper('goodbye', goodbye)
+        .toCompileTo('NOT PRINTING');
+    });
+
+    it('block helpers can take an optional hash', () => {
+      expectTemplate('{{#goodbye cruel="CRUEL" times=12}}world{{/goodbye}}')
+        .withHelper('goodbye', function (this: any, options: HelperOptions) {
+          return (
+            'GOODBYE ' +
+            options.hash.cruel +
+            ' ' +
+            options.fn(this) +
+            ' ' +
+            options.hash.times +
+            ' TIMES'
+          );
+        })
+        .toCompileTo('GOODBYE CRUEL world 12 TIMES');
+    });
+
+    it('block helpers can take an optional hash with single quoted stings', () => {
+      expectTemplate('{{#goodbye cruel="CRUEL" times=12}}world{{/goodbye}}')
+        .withHelper('goodbye', function (this: any, options: HelperOptions) {
+          return (
+            'GOODBYE ' +
+            options.hash.cruel +
+            ' ' +
+            options.fn(this) +
+            ' ' +
+            options.hash.times +
+            ' TIMES'
+          );
+        })
+        .toCompileTo('GOODBYE CRUEL world 12 TIMES');
+    });
+
+    it('block helpers can take an optional hash with booleans', () => {
+      function goodbye(this: any, options: HelperOptions) {
+        if (options.hash.print === true) {
+          return 'GOODBYE ' + options.hash.cruel + ' ' + options.fn(this);
+        } else if (options.hash.print === false) {
+          return 'NOT PRINTING';
+        } else {
+          return 'THIS SHOULD NOT HAPPEN';
+        }
+      }
+
+      expectTemplate('{{#goodbye cruel="CRUEL" print=true}}world{{/goodbye}}')
+        .withHelper('goodbye', goodbye)
+        .toCompileTo('GOODBYE CRUEL world');
+
+      expectTemplate('{{#goodbye cruel="CRUEL" print=false}}world{{/goodbye}}')
+        .withHelper('goodbye', goodbye)
+        .toCompileTo('NOT PRINTING');
+    });
+  });
+
+  describe('helperMissing', () => {
+    it('if a context is not found, helperMissing is used', () => {
+      expectTemplate('{{hello}} {{link_to world}}').toThrow(/Missing helper: "link_to"/);
+    });
+
+    it('if a context is not found, custom helperMissing is used', () => {
+      expectTemplate('{{hello}} {{link_to world}}')
+        .withInput({ hello: 'Hello', world: 'world' })
+        .withHelper('helperMissing', function (mesg, options: HelperOptions) {
+          if (options.name === 'link_to') {
+            return new Handlebars.SafeString('<a>' + mesg + '</a>');
+          }
+        })
+        .toCompileTo('Hello <a>world</a>');
+    });
+
+    it('if a value is not found, custom helperMissing is used', () => {
+      expectTemplate('{{hello}} {{link_to}}')
+        .withInput({ hello: 'Hello', world: 'world' })
+        .withHelper('helperMissing', function (options: HelperOptions) {
+          if (options.name === 'link_to') {
+            return new Handlebars.SafeString('<a>winning</a>');
+          }
+        })
+        .toCompileTo('Hello <a>winning</a>');
+    });
+  });
+
+  describe('knownHelpers', () => {
+    it('Known helper should render helper', () => {
+      expectTemplate('{{hello}}')
+        .withCompileOptions({
+          knownHelpers: { hello: true },
+        })
+        .withHelper('hello', function () {
+          return 'foo';
+        })
+        .toCompileTo('foo');
+    });
+
+    it('Unknown helper in knownHelpers only mode should be passed as undefined', () => {
+      expectTemplate('{{typeof hello}}')
+        .withCompileOptions({
+          knownHelpers: { typeof: true },
+          knownHelpersOnly: true,
+        })
+        .withHelper('typeof', function (arg) {
+          return typeof arg;
+        })
+        .withHelper('hello', function () {
+          return 'foo';
+        })
+        .toCompileTo('undefined');
+    });
+
+    it('Builtin helpers available in knownHelpers only mode', () => {
+      expectTemplate('{{#unless foo}}bar{{/unless}}')
+        .withCompileOptions({
+          knownHelpersOnly: true,
+        })
+        .toCompileTo('bar');
+    });
+
+    it('Field lookup works in knownHelpers only mode', () => {
+      expectTemplate('{{foo}}')
+        .withCompileOptions({
+          knownHelpersOnly: true,
+        })
+        .withInput({ foo: 'bar' })
+        .toCompileTo('bar');
+    });
+
+    it('Conditional blocks work in knownHelpers only mode', () => {
+      expectTemplate('{{#foo}}bar{{/foo}}')
+        .withCompileOptions({
+          knownHelpersOnly: true,
+        })
+        .withInput({ foo: 'baz' })
+        .toCompileTo('bar');
+    });
+
+    it('Invert blocks work in knownHelpers only mode', () => {
+      expectTemplate('{{^foo}}bar{{/foo}}')
+        .withCompileOptions({
+          knownHelpersOnly: true,
+        })
+        .withInput({ foo: false })
+        .toCompileTo('bar');
+    });
+
+    it('Functions are bound to the context in knownHelpers only mode', () => {
+      expectTemplate('{{foo}}')
+        .withCompileOptions({
+          knownHelpersOnly: true,
+        })
+        .withInput({
+          foo() {
+            return this.bar;
+          },
+          bar: 'bar',
+        })
+        .toCompileTo('bar');
+    });
+
+    it('Unknown helper call in knownHelpers only mode should throw', () => {
+      expectTemplate('{{typeof hello}}')
+        .withCompileOptions({ knownHelpersOnly: true })
+        .toThrow(Error);
+    });
+  });
+
+  describe('blockHelperMissing', () => {
+    it('lambdas are resolved by blockHelperMissing, not handlebars proper', () => {
+      expectTemplate('{{#truthy}}yep{{/truthy}}')
+        .withInput({
+          truthy() {
+            return true;
+          },
+        })
+        .toCompileTo('yep');
+    });
+
+    it('lambdas resolved by blockHelperMissing are bound to the context', () => {
+      expectTemplate('{{#truthy}}yep{{/truthy}}')
+        .withInput({
+          truthy() {
+            return this.truthiness();
+          },
+          truthiness() {
+            return false;
+          },
+        })
+        .toCompileTo('');
+    });
+  });
+
+  describe('name field', () => {
+    const helpers = {
+      blockHelperMissing(...args: any[]) {
+        return 'missing: ' + args[args.length - 1].name;
+      },
+      helperMissing(...args: any[]) {
+        return 'helper missing: ' + args[args.length - 1].name;
+      },
+      helper(...args: any[]) {
+        return 'ran: ' + args[args.length - 1].name;
+      },
+    };
+
+    it('should include in ambiguous mustache calls', () => {
+      expectTemplate('{{helper}}').withHelpers(helpers).toCompileTo('ran: helper');
+    });
+
+    it('should include in helper mustache calls', () => {
+      expectTemplate('{{helper 1}}').withHelpers(helpers).toCompileTo('ran: helper');
+    });
+
+    it('should include in ambiguous block calls', () => {
+      expectTemplate('{{#helper}}{{/helper}}').withHelpers(helpers).toCompileTo('ran: helper');
+    });
+
+    it('should include in simple block calls', () => {
+      expectTemplate('{{#./helper}}{{/./helper}}')
+        .withHelpers(helpers)
+        .toCompileTo('missing: ./helper');
+    });
+
+    it('should include in helper block calls', () => {
+      expectTemplate('{{#helper 1}}{{/helper}}').withHelpers(helpers).toCompileTo('ran: helper');
+    });
+
+    it('should include in known helper calls', () => {
+      expectTemplate('{{helper}}')
+        .withCompileOptions({
+          knownHelpers: { helper: true },
+          knownHelpersOnly: true,
+        })
+        .withHelpers(helpers)
+        .toCompileTo('ran: helper');
+    });
+
+    it('should include full id', () => {
+      expectTemplate('{{#foo.helper}}{{/foo.helper}}')
+        .withInput({ foo: {} })
+        .withHelpers(helpers)
+        .toCompileTo('missing: foo.helper');
+    });
+
+    it('should include full id if a hash is passed', () => {
+      expectTemplate('{{#foo.helper bar=baz}}{{/foo.helper}}')
+        .withInput({ foo: {} })
+        .withHelpers(helpers)
+        .toCompileTo('helper missing: foo.helper');
+    });
+  });
+
+  describe('name conflicts', () => {
+    it('helpers take precedence over same-named context properties', () => {
+      expectTemplate('{{goodbye}} {{cruel world}}')
+        .withHelper('goodbye', function (this: any) {
+          return this.goodbye.toUpperCase();
+        })
+        .withHelper('cruel', function (world) {
+          return 'cruel ' + world.toUpperCase();
+        })
+        .withInput({
+          goodbye: 'goodbye',
+          world: 'world',
+        })
+        .toCompileTo('GOODBYE cruel WORLD');
+    });
+
+    it('helpers take precedence over same-named context properties$', () => {
+      expectTemplate('{{#goodbye}} {{cruel world}}{{/goodbye}}')
+        .withHelper('goodbye', function (this: any, options: HelperOptions) {
+          return this.goodbye.toUpperCase() + options.fn(this);
+        })
+        .withHelper('cruel', function (world) {
+          return 'cruel ' + world.toUpperCase();
+        })
+        .withInput({
+          goodbye: 'goodbye',
+          world: 'world',
+        })
+        .toCompileTo('GOODBYE cruel WORLD');
+    });
+
+    it('Scoped names take precedence over helpers', () => {
+      expectTemplate('{{this.goodbye}} {{cruel world}} {{cruel this.goodbye}}')
+        .withHelper('goodbye', function (this: any) {
+          return this.goodbye.toUpperCase();
+        })
+        .withHelper('cruel', function (world) {
+          return 'cruel ' + world.toUpperCase();
+        })
+        .withInput({
+          goodbye: 'goodbye',
+          world: 'world',
+        })
+        .toCompileTo('goodbye cruel WORLD cruel GOODBYE');
+    });
+
+    it('Scoped names take precedence over block helpers', () => {
+      expectTemplate('{{#goodbye}} {{cruel world}}{{/goodbye}} {{this.goodbye}}')
+        .withHelper('goodbye', function (this: any, options: HelperOptions) {
+          return this.goodbye.toUpperCase() + options.fn(this);
+        })
+        .withHelper('cruel', function (world) {
+          return 'cruel ' + world.toUpperCase();
+        })
+        .withInput({
+          goodbye: 'goodbye',
+          world: 'world',
+        })
+        .toCompileTo('GOODBYE cruel WORLD goodbye');
+    });
+  });
+
+  describe('block params', () => {
+    it('should take presedence over context values', () => {
+      expectTemplate('{{#goodbyes as |value|}}{{value}}{{/goodbyes}}{{value}}')
+        .withInput({ value: 'foo' })
+        .withHelper('goodbyes', function (options: HelperOptions) {
+          expect(options.fn.blockParams).toEqual(1);
+          return options.fn({ value: 'bar' }, { blockParams: [1, 2] });
+        })
+        .toCompileTo('1foo');
+    });
+
+    it('should take presedence over helper values', () => {
+      expectTemplate('{{#goodbyes as |value|}}{{value}}{{/goodbyes}}{{value}}')
+        .withHelper('value', function () {
+          return 'foo';
+        })
+        .withHelper('goodbyes', function (options: HelperOptions) {
+          expect(options.fn.blockParams).toEqual(1);
+          return options.fn({}, { blockParams: [1, 2] });
+        })
+        .toCompileTo('1foo');
+    });
+
+    it('should not take presedence over pathed values', () => {
+      expectTemplate('{{#goodbyes as |value|}}{{./value}}{{/goodbyes}}{{value}}')
+        .withInput({ value: 'bar' })
+        .withHelper('value', function () {
+          return 'foo';
+        })
+        .withHelper('goodbyes', function (this: any, options: HelperOptions) {
+          expect(options.fn.blockParams).toEqual(1);
+          return options.fn(this, { blockParams: [1, 2] });
+        })
+        .toCompileTo('barfoo');
+    });
+
+    it('should take presednece over parent block params', () => {
+      let value: number;
+      expectTemplate(
+        '{{#goodbyes as |value|}}{{#goodbyes}}{{value}}{{#goodbyes as |value|}}{{value}}{{/goodbyes}}{{/goodbyes}}{{/goodbyes}}{{value}}',
+        {
+          beforeEach() {
+            value = 1;
+          },
+        }
+      )
+        .withInput({ value: 'foo' })
+        .withHelper('goodbyes', function (options: HelperOptions) {
+          return options.fn(
+            { value: 'bar' },
+            {
+              blockParams: options.fn.blockParams === 1 ? [value++, value++] : undefined,
+            }
+          );
+        })
+        .toCompileTo('13foo');
+    });
+
+    it('should allow block params on chained helpers', () => {
+      expectTemplate('{{#if bar}}{{else goodbyes as |value|}}{{value}}{{/if}}{{value}}')
+        .withInput({ value: 'foo' })
+        .withHelper('goodbyes', function (options: HelperOptions) {
+          expect(options.fn.blockParams).toEqual(1);
+          return options.fn({ value: 'bar' }, { blockParams: [1, 2] });
+        })
+        .toCompileTo('1foo');
+    });
+  });
+
+  describe('built-in helpers malformed arguments ', () => {
+    it('if helper - too few arguments', () => {
+      expectTemplate('{{#if}}{{/if}}').toThrow(/#if requires exactly one argument/);
+    });
+
+    it('if helper - too many arguments, string', () => {
+      expectTemplate('{{#if test "string"}}{{/if}}').toThrow(/#if requires exactly one argument/);
+    });
+
+    it('if helper - too many arguments, undefined', () => {
+      expectTemplate('{{#if test undefined}}{{/if}}').toThrow(/#if requires exactly one argument/);
+    });
+
+    it('if helper - too many arguments, null', () => {
+      expectTemplate('{{#if test null}}{{/if}}').toThrow(/#if requires exactly one argument/);
+    });
+
+    it('unless helper - too few arguments', () => {
+      expectTemplate('{{#unless}}{{/unless}}').toThrow(/#unless requires exactly one argument/);
+    });
+
+    it('unless helper - too many arguments', () => {
+      expectTemplate('{{#unless test null}}{{/unless}}').toThrow(
+        /#unless requires exactly one argument/
+      );
+    });
+
+    it('with helper - too few arguments', () => {
+      expectTemplate('{{#with}}{{/with}}').toThrow(/#with requires exactly one argument/);
+    });
+
+    it('with helper - too many arguments', () => {
+      expectTemplate('{{#with test "string"}}{{/with}}').toThrow(
+        /#with requires exactly one argument/
+      );
+    });
+  });
+
+  describe('the lookupProperty-option', () => {
+    it('should be passed to custom helpers', () => {
+      expectTemplate('{{testHelper}}')
+        .withHelper('testHelper', function testHelper(this: any, options: HelperOptions) {
+          return options.lookupProperty(this, 'testProperty');
+        })
+        .withInput({ testProperty: 'abc' })
+        .toCompileTo('abc');
+    });
+  });
+});
+
+function deleteAllKeys(obj: { [key: string]: any }) {
+  for (const key of Object.keys(obj)) {
+    delete obj[key];
+  }
+}
diff --git a/dev/lib/handlebars/src/spec/index.partials.test.ts b/dev/lib/handlebars/src/spec/index.partials.test.ts
new file mode 100644
index 00000000..65930d06
--- /dev/null
+++ b/dev/lib/handlebars/src/spec/index.partials.test.ts
@@ -0,0 +1,591 @@
+/*
+ * This file is forked from the handlebars project (https://github.com/handlebars-lang/handlebars.js),
+ * and may include modifications made by Elasticsearch B.V.
+ * Elasticsearch B.V. licenses this file to you under the MIT License.
+ * See `packages/kbn-handlebars/LICENSE` for more information.
+ */
+
+import Handlebars from '../..';
+import { expectTemplate, forEachCompileFunctionName } from '../__jest__/test_bench';
+
+describe('partials', () => {
+  it('basic partials', () => {
+    const string = 'Dudes: {{#dudes}}{{> dude}}{{/dudes}}';
+    const partial = '{{name}} ({{url}}) ';
+    const hash = {
+      dudes: [
+        { name: 'Yehuda', url: 'http://yehuda' },
+        { name: 'Alan', url: 'http://alan' },
+      ],
+    };
+
+    expectTemplate(string)
+      .withInput(hash)
+      .withPartials({ dude: partial })
+      .toCompileTo('Dudes: Yehuda (http://yehuda) Alan (http://alan) ');
+
+    expectTemplate(string)
+      .withInput(hash)
+      .withPartials({ dude: partial })
+      .withRuntimeOptions({ data: false })
+      .withCompileOptions({ data: false })
+      .toCompileTo('Dudes: Yehuda (http://yehuda) Alan (http://alan) ');
+  });
+
+  it('dynamic partials', () => {
+    const string = 'Dudes: {{#dudes}}{{> (partial)}}{{/dudes}}';
+    const partial = '{{name}} ({{url}}) ';
+    const hash = {
+      dudes: [
+        { name: 'Yehuda', url: 'http://yehuda' },
+        { name: 'Alan', url: 'http://alan' },
+      ],
+    };
+    const helpers = {
+      partial: () => 'dude',
+    };
+
+    expectTemplate(string)
+      .withInput(hash)
+      .withHelpers(helpers)
+      .withPartials({ dude: partial })
+      .toCompileTo('Dudes: Yehuda (http://yehuda) Alan (http://alan) ');
+
+    expectTemplate(string)
+      .withInput(hash)
+      .withHelpers(helpers)
+      .withPartials({ dude: partial })
+      .withRuntimeOptions({ data: false })
+      .withCompileOptions({ data: false })
+      .toCompileTo('Dudes: Yehuda (http://yehuda) Alan (http://alan) ');
+  });
+
+  it('failing dynamic partials', () => {
+    expectTemplate('Dudes: {{#dudes}}{{> (partial)}}{{/dudes}}')
+      .withInput({
+        dudes: [
+          { name: 'Yehuda', url: 'http://yehuda' },
+          { name: 'Alan', url: 'http://alan' },
+        ],
+      })
+      .withHelper('partial', () => 'missing')
+      .withPartial('dude', '{{name}} ({{url}}) ')
+      .toThrow('The partial missing could not be found'); // TODO: Is there a way we can test that the error is of type `Handlebars.Exception`?
+  });
+
+  it('partials with context', () => {
+    expectTemplate('Dudes: {{>dude dudes}}')
+      .withInput({
+        dudes: [
+          { name: 'Yehuda', url: 'http://yehuda' },
+          { name: 'Alan', url: 'http://alan' },
+        ],
+      })
+      .withPartial('dude', '{{#this}}{{name}} ({{url}}) {{/this}}')
+      .toCompileTo('Dudes: Yehuda (http://yehuda) Alan (http://alan) ');
+  });
+
+  it('partials with no context', () => {
+    const partial = '{{name}} ({{url}}) ';
+    const hash = {
+      dudes: [
+        { name: 'Yehuda', url: 'http://yehuda' },
+        { name: 'Alan', url: 'http://alan' },
+      ],
+    };
+
+    expectTemplate('Dudes: {{#dudes}}{{>dude}}{{/dudes}}')
+      .withInput(hash)
+      .withPartial('dude', partial)
+      .withCompileOptions({ explicitPartialContext: true })
+      .toCompileTo('Dudes:  ()  () ');
+
+    expectTemplate('Dudes: {{#dudes}}{{>dude name="foo"}}{{/dudes}}')
+      .withInput(hash)
+      .withPartial('dude', partial)
+      .withCompileOptions({ explicitPartialContext: true })
+      .toCompileTo('Dudes: foo () foo () ');
+  });
+
+  it('partials with string context', () => {
+    expectTemplate('Dudes: {{>dude "dudes"}}')
+      .withPartial('dude', '{{.}}')
+      .toCompileTo('Dudes: dudes');
+  });
+
+  it('partials with undefined context', () => {
+    expectTemplate('Dudes: {{>dude dudes}}')
+      .withPartial('dude', '{{foo}} Empty')
+      .toCompileTo('Dudes:  Empty');
+  });
+
+  it('partials with duplicate parameters', () => {
+    expectTemplate('Dudes: {{>dude dudes foo bar=baz}}').toThrow(
+      'Unsupported number of partial arguments: 2 - 1:7'
+    );
+  });
+
+  it('partials with parameters', () => {
+    expectTemplate('Dudes: {{#dudes}}{{> dude others=..}}{{/dudes}}')
+      .withInput({
+        foo: 'bar',
+        dudes: [
+          { name: 'Yehuda', url: 'http://yehuda' },
+          { name: 'Alan', url: 'http://alan' },
+        ],
+      })
+      .withPartial('dude', '{{others.foo}}{{name}} ({{url}}) ')
+      .toCompileTo('Dudes: barYehuda (http://yehuda) barAlan (http://alan) ');
+  });
+
+  it('partial in a partial', () => {
+    expectTemplate('Dudes: {{#dudes}}{{>dude}}{{/dudes}}')
+      .withInput({
+        dudes: [
+          { name: 'Yehuda', url: 'http://yehuda' },
+          { name: 'Alan', url: 'http://alan' },
+        ],
+      })
+      .withPartials({
+        dude: '{{name}} {{> url}} ',
+        url: '<a href="{{url}}">{{url}}</a>',
+      })
+      .toCompileTo(
+        'Dudes: Yehuda <a href="http://yehuda">http://yehuda</a> Alan <a href="http://alan">http://alan</a> '
+      );
+  });
+
+  it('rendering undefined partial throws an exception', () => {
+    expectTemplate('{{> whatever}}').toThrow('The partial whatever could not be found');
+  });
+
+  it('registering undefined partial throws an exception', () => {
+    global.kbnHandlebarsEnv = Handlebars.create();
+
+    expect(() => {
+      kbnHandlebarsEnv!.registerPartial('undefined_test', undefined as any);
+    }).toThrow('Attempting to register a partial called "undefined_test" as undefined');
+
+    global.kbnHandlebarsEnv = null;
+  });
+
+  it('rendering template partial in vm mode throws an exception', () => {
+    expectTemplate('{{> whatever}}').toThrow('The partial whatever could not be found');
+  });
+
+  it('rendering function partial in vm mode', () => {
+    function partial(context: any) {
+      return context.name + ' (' + context.url + ') ';
+    }
+    expectTemplate('Dudes: {{#dudes}}{{> dude}}{{/dudes}}')
+      .withInput({
+        dudes: [
+          { name: 'Yehuda', url: 'http://yehuda' },
+          { name: 'Alan', url: 'http://alan' },
+        ],
+      })
+      .withPartial('dude', partial)
+      .toCompileTo('Dudes: Yehuda (http://yehuda) Alan (http://alan) ');
+  });
+
+  it('GH-14: a partial preceding a selector', () => {
+    expectTemplate('Dudes: {{>dude}} {{anotherDude}}')
+      .withInput({ name: 'Jeepers', anotherDude: 'Creepers' })
+      .withPartial('dude', '{{name}}')
+      .toCompileTo('Dudes: Jeepers Creepers');
+  });
+
+  it('Partials with slash paths', () => {
+    expectTemplate('Dudes: {{> shared/dude}}')
+      .withInput({ name: 'Jeepers', anotherDude: 'Creepers' })
+      .withPartial('shared/dude', '{{name}}')
+      .toCompileTo('Dudes: Jeepers');
+  });
+
+  it('Partials with slash and point paths', () => {
+    expectTemplate('Dudes: {{> shared/dude.thing}}')
+      .withInput({ name: 'Jeepers', anotherDude: 'Creepers' })
+      .withPartial('shared/dude.thing', '{{name}}')
+      .toCompileTo('Dudes: Jeepers');
+  });
+
+  it('Global Partials', () => {
+    global.kbnHandlebarsEnv = Handlebars.create();
+
+    kbnHandlebarsEnv!.registerPartial('globalTest', '{{anotherDude}}');
+
+    expectTemplate('Dudes: {{> shared/dude}} {{> globalTest}}')
+      .withInput({ name: 'Jeepers', anotherDude: 'Creepers' })
+      .withPartial('shared/dude', '{{name}}')
+      .toCompileTo('Dudes: Jeepers Creepers');
+
+    kbnHandlebarsEnv!.unregisterPartial('globalTest');
+    expect(kbnHandlebarsEnv!.partials.globalTest).toBeUndefined();
+
+    global.kbnHandlebarsEnv = null;
+  });
+
+  it('Multiple partial registration', () => {
+    global.kbnHandlebarsEnv = Handlebars.create();
+
+    kbnHandlebarsEnv!.registerPartial({
+      'shared/dude': '{{name}}',
+      globalTest: '{{anotherDude}}',
+    });
+
+    expectTemplate('Dudes: {{> shared/dude}} {{> globalTest}}')
+      .withInput({ name: 'Jeepers', anotherDude: 'Creepers' })
+      .withPartial('notused', 'notused') // trick the test bench into running with partials enabled
+      .toCompileTo('Dudes: Jeepers Creepers');
+
+    global.kbnHandlebarsEnv = null;
+  });
+
+  it('Partials with integer path', () => {
+    expectTemplate('Dudes: {{> 404}}')
+      .withInput({ name: 'Jeepers', anotherDude: 'Creepers' })
+      .withPartial(404, '{{name}}')
+      .toCompileTo('Dudes: Jeepers');
+  });
+
+  it('Partials with complex path', () => {
+    expectTemplate('Dudes: {{> 404/asdf?.bar}}')
+      .withInput({ name: 'Jeepers', anotherDude: 'Creepers' })
+      .withPartial('404/asdf?.bar', '{{name}}')
+      .toCompileTo('Dudes: Jeepers');
+  });
+
+  it('Partials with escaped', () => {
+    expectTemplate('Dudes: {{> [+404/asdf?.bar]}}')
+      .withInput({ name: 'Jeepers', anotherDude: 'Creepers' })
+      .withPartial('+404/asdf?.bar', '{{name}}')
+      .toCompileTo('Dudes: Jeepers');
+  });
+
+  it('Partials with string', () => {
+    expectTemplate("Dudes: {{> '+404/asdf?.bar'}}")
+      .withInput({ name: 'Jeepers', anotherDude: 'Creepers' })
+      .withPartial('+404/asdf?.bar', '{{name}}')
+      .toCompileTo('Dudes: Jeepers');
+  });
+
+  it('should handle empty partial', () => {
+    expectTemplate('Dudes: {{#dudes}}{{> dude}}{{/dudes}}')
+      .withInput({
+        dudes: [
+          { name: 'Yehuda', url: 'http://yehuda' },
+          { name: 'Alan', url: 'http://alan' },
+        ],
+      })
+      .withPartial('dude', '')
+      .toCompileTo('Dudes: ');
+  });
+
+  // Skipping test as this only makes sense when there's no `compile` function (i.e. runtime-only mode).
+  // We do not support that mode with `@kbn/handlebars`, so there's no need to test it
+  it.skip('throw on missing partial', () => {
+    const handlebars = Handlebars.create();
+    (handlebars.compile as any) = undefined;
+    const template = handlebars.precompile('{{> dude}}');
+    const render = handlebars.template(eval('(' + template + ')')); // eslint-disable-line no-eval
+    expect(() => {
+      render(
+        {},
+        {
+          partials: {
+            dude: 'fail',
+          },
+        }
+      );
+    }).toThrow(/The partial dude could not be compiled/);
+  });
+
+  describe('partial blocks', () => {
+    it('should render partial block as default', () => {
+      expectTemplate('{{#> dude}}success{{/dude}}').toCompileTo('success');
+    });
+
+    it('should execute default block with proper context', () => {
+      expectTemplate('{{#> dude context}}{{value}}{{/dude}}')
+        .withInput({ context: { value: 'success' } })
+        .toCompileTo('success');
+    });
+
+    it('should propagate block parameters to default block', () => {
+      expectTemplate('{{#with context as |me|}}{{#> dude}}{{me.value}}{{/dude}}{{/with}}')
+        .withInput({ context: { value: 'success' } })
+        .toCompileTo('success');
+    });
+
+    it('should not use partial block if partial exists', () => {
+      expectTemplate('{{#> dude}}fail{{/dude}}')
+        .withPartials({ dude: 'success' })
+        .toCompileTo('success');
+    });
+
+    it('should render block from partial', () => {
+      expectTemplate('{{#> dude}}success{{/dude}}')
+        .withPartials({ dude: '{{> @partial-block }}' })
+        .toCompileTo('success');
+    });
+
+    it('should be able to render the partial-block twice', () => {
+      expectTemplate('{{#> dude}}success{{/dude}}')
+        .withPartials({ dude: '{{> @partial-block }} {{> @partial-block }}' })
+        .toCompileTo('success success');
+    });
+
+    it('should render block from partial with context', () => {
+      expectTemplate('{{#> dude}}{{value}}{{/dude}}')
+        .withInput({ context: { value: 'success' } })
+        .withPartials({
+          dude: '{{#with context}}{{> @partial-block }}{{/with}}',
+        })
+        .toCompileTo('success');
+    });
+
+    it('should be able to access the @data frame from a partial-block', () => {
+      expectTemplate('{{#> dude}}in-block: {{@root/value}}{{/dude}}')
+        .withInput({ value: 'success' })
+        .withPartials({
+          dude: '<code>before-block: {{@root/value}} {{>   @partial-block }}</code>',
+        })
+        .toCompileTo('<code>before-block: success in-block: success</code>');
+    });
+
+    it('should allow the #each-helper to be used along with partial-blocks', () => {
+      expectTemplate('<template>{{#> list value}}value = {{.}}{{/list}}</template>')
+        .withInput({
+          value: ['a', 'b', 'c'],
+        })
+        .withPartials({
+          list: '<list>{{#each .}}<item>{{> @partial-block}}</item>{{/each}}</list>',
+        })
+        .toCompileTo(
+          '<template><list><item>value = a</item><item>value = b</item><item>value = c</item></list></template>'
+        );
+    });
+
+    it('should render block from partial with context (twice)', () => {
+      expectTemplate('{{#> dude}}{{value}}{{/dude}}')
+        .withInput({ context: { value: 'success' } })
+        .withPartials({
+          dude: '{{#with context}}{{> @partial-block }} {{> @partial-block }}{{/with}}',
+        })
+        .toCompileTo('success success');
+    });
+
+    it('should render block from partial with context [2]', () => {
+      expectTemplate('{{#> dude}}{{../context/value}}{{/dude}}')
+        .withInput({ context: { value: 'success' } })
+        .withPartials({
+          dude: '{{#with context}}{{> @partial-block }}{{/with}}',
+        })
+        .toCompileTo('success');
+    });
+
+    it('should render block from partial with block params', () => {
+      expectTemplate('{{#with context as |me|}}{{#> dude}}{{me.value}}{{/dude}}{{/with}}')
+        .withInput({ context: { value: 'success' } })
+        .withPartials({ dude: '{{> @partial-block }}' })
+        .toCompileTo('success');
+    });
+
+    it('should render nested partial blocks', () => {
+      expectTemplate('<template>{{#> outer}}{{value}}{{/outer}}</template>')
+        .withInput({ value: 'success' })
+        .withPartials({
+          outer:
+            '<outer>{{#> nested}}<outer-block>{{> @partial-block}}</outer-block>{{/nested}}</outer>',
+          nested: '<nested>{{> @partial-block}}</nested>',
+        })
+        .toCompileTo(
+          '<template><outer><nested><outer-block>success</outer-block></nested></outer></template>'
+        );
+    });
+
+    it('should render nested partial blocks at different nesting levels', () => {
+      expectTemplate('<template>{{#> outer}}{{value}}{{/outer}}</template>')
+        .withInput({ value: 'success' })
+        .withPartials({
+          outer:
+            '<outer>{{#> nested}}<outer-block>{{> @partial-block}}</outer-block>{{/nested}}{{> @partial-block}}</outer>',
+          nested: '<nested>{{> @partial-block}}</nested>',
+        })
+        .toCompileTo(
+          '<template><outer><nested><outer-block>success</outer-block></nested>success</outer></template>'
+        );
+    });
+
+    it('should render nested partial blocks at different nesting levels (twice)', () => {
+      expectTemplate('<template>{{#> outer}}{{value}}{{/outer}}</template>')
+        .withInput({ value: 'success' })
+        .withPartials({
+          outer:
+            '<outer>{{#> nested}}<outer-block>{{> @partial-block}} {{> @partial-block}}</outer-block>{{/nested}}{{> @partial-block}}+{{> @partial-block}}</outer>',
+          nested: '<nested>{{> @partial-block}}</nested>',
+        })
+        .toCompileTo(
+          '<template><outer><nested><outer-block>success success</outer-block></nested>success+success</outer></template>'
+        );
+    });
+
+    it('should render nested partial blocks (twice at each level)', () => {
+      expectTemplate('<template>{{#> outer}}{{value}}{{/outer}}</template>')
+        .withInput({ value: 'success' })
+        .withPartials({
+          outer:
+            '<outer>{{#> nested}}<outer-block>{{> @partial-block}} {{> @partial-block}}</outer-block>{{/nested}}</outer>',
+          nested: '<nested>{{> @partial-block}}{{> @partial-block}}</nested>',
+        })
+        .toCompileTo(
+          '<template><outer>' +
+            '<nested><outer-block>success success</outer-block><outer-block>success success</outer-block></nested>' +
+            '</outer></template>'
+        );
+    });
+  });
+
+  describe('inline partials', () => {
+    it('should define inline partials for template', () => {
+      expectTemplate('{{#*inline "myPartial"}}success{{/inline}}{{> myPartial}}').toCompileTo(
+        'success'
+      );
+    });
+
+    it('should overwrite multiple partials in the same template', () => {
+      expectTemplate(
+        '{{#*inline "myPartial"}}fail{{/inline}}{{#*inline "myPartial"}}success{{/inline}}{{> myPartial}}'
+      ).toCompileTo('success');
+    });
+
+    it('should define inline partials for block', () => {
+      expectTemplate(
+        '{{#with .}}{{#*inline "myPartial"}}success{{/inline}}{{> myPartial}}{{/with}}'
+      ).toCompileTo('success');
+
+      expectTemplate(
+        '{{#with .}}{{#*inline "myPartial"}}success{{/inline}}{{/with}}{{> myPartial}}'
+      ).toThrow(/myPartial could not/);
+    });
+
+    it('should override global partials', () => {
+      expectTemplate('{{#*inline "myPartial"}}success{{/inline}}{{> myPartial}}')
+        .withPartials({
+          myPartial: () => 'fail',
+        })
+        .toCompileTo('success');
+    });
+
+    it('should override template partials', () => {
+      expectTemplate(
+        '{{#*inline "myPartial"}}fail{{/inline}}{{#with .}}{{#*inline "myPartial"}}success{{/inline}}{{> myPartial}}{{/with}}'
+      ).toCompileTo('success');
+    });
+
+    it('should override partials down the entire stack', () => {
+      expectTemplate(
+        '{{#with .}}{{#*inline "myPartial"}}success{{/inline}}{{#with .}}{{#with .}}{{> myPartial}}{{/with}}{{/with}}{{/with}}'
+      ).toCompileTo('success');
+    });
+
+    it('should define inline partials for partial call', () => {
+      expectTemplate('{{#*inline "myPartial"}}success{{/inline}}{{> dude}}')
+        .withPartials({ dude: '{{> myPartial }}' })
+        .toCompileTo('success');
+    });
+
+    it('should define inline partials in partial block call', () => {
+      expectTemplate('{{#> dude}}{{#*inline "myPartial"}}success{{/inline}}{{/dude}}')
+        .withPartials({ dude: '{{> myPartial }}' })
+        .toCompileTo('success');
+    });
+
+    it('should render nested inline partials', () => {
+      expectTemplate(
+        '{{#*inline "outer"}}{{#>inner}}<outer-block>{{>@partial-block}}</outer-block>{{/inner}}{{/inline}}' +
+          '{{#*inline "inner"}}<inner>{{>@partial-block}}</inner>{{/inline}}' +
+          '{{#>outer}}{{value}}{{/outer}}'
+      )
+        .withInput({ value: 'success' })
+        .toCompileTo('<inner><outer-block>success</outer-block></inner>');
+    });
+
+    it('should render nested inline partials with partial-blocks on different nesting levels', () => {
+      expectTemplate(
+        '{{#*inline "outer"}}{{#>inner}}<outer-block>{{>@partial-block}}</outer-block>{{/inner}}{{>@partial-block}}{{/inline}}' +
+          '{{#*inline "inner"}}<inner>{{>@partial-block}}</inner>{{/inline}}' +
+          '{{#>outer}}{{value}}{{/outer}}'
+      )
+        .withInput({ value: 'success' })
+        .toCompileTo('<inner><outer-block>success</outer-block></inner>success');
+    });
+
+    it('should render nested inline partials (twice at each level)', () => {
+      expectTemplate(
+        '{{#*inline "outer"}}{{#>inner}}<outer-block>{{>@partial-block}} {{>@partial-block}}</outer-block>{{/inner}}{{/inline}}' +
+          '{{#*inline "inner"}}<inner>{{>@partial-block}}{{>@partial-block}}</inner>{{/inline}}' +
+          '{{#>outer}}{{value}}{{/outer}}'
+      )
+        .withInput({ value: 'success' })
+        .toCompileTo(
+          '<inner><outer-block>success success</outer-block><outer-block>success success</outer-block></inner>'
+        );
+    });
+  });
+
+  forEachCompileFunctionName((compileName) => {
+    it(`should pass compiler flags for ${compileName} function`, () => {
+      const env = Handlebars.create();
+      env.registerPartial('partial', '{{foo}}');
+      const compile = env[compileName].bind(env);
+      const template = compile('{{foo}} {{> partial}}', { noEscape: true });
+      expect(template({ foo: '<' })).toEqual('< <');
+    });
+  });
+
+  describe('standalone partials', () => {
+    it('indented partials', () => {
+      expectTemplate('Dudes:\n{{#dudes}}\n  {{>dude}}\n{{/dudes}}')
+        .withInput({
+          dudes: [
+            { name: 'Yehuda', url: 'http://yehuda' },
+            { name: 'Alan', url: 'http://alan' },
+          ],
+        })
+        .withPartial('dude', '{{name}}\n')
+        .toCompileTo('Dudes:\n  Yehuda\n  Alan\n');
+    });
+
+    it('nested indented partials', () => {
+      expectTemplate('Dudes:\n{{#dudes}}\n  {{>dude}}\n{{/dudes}}')
+        .withInput({
+          dudes: [
+            { name: 'Yehuda', url: 'http://yehuda' },
+            { name: 'Alan', url: 'http://alan' },
+          ],
+        })
+        .withPartials({
+          dude: '{{name}}\n {{> url}}',
+          url: '{{url}}!\n',
+        })
+        .toCompileTo('Dudes:\n  Yehuda\n   http://yehuda!\n  Alan\n   http://alan!\n');
+    });
+
+    it('prevent nested indented partials', () => {
+      expectTemplate('Dudes:\n{{#dudes}}\n  {{>dude}}\n{{/dudes}}')
+        .withInput({
+          dudes: [
+            { name: 'Yehuda', url: 'http://yehuda' },
+            { name: 'Alan', url: 'http://alan' },
+          ],
+        })
+        .withPartials({
+          dude: '{{name}}\n {{> url}}',
+          url: '{{url}}!\n',
+        })
+        .withCompileOptions({ preventIndent: true })
+        .toCompileTo('Dudes:\n  Yehuda\n http://yehuda!\n  Alan\n http://alan!\n');
+    });
+  });
+});
diff --git a/dev/lib/handlebars/src/spec/index.regressions.test.ts b/dev/lib/handlebars/src/spec/index.regressions.test.ts
new file mode 100644
index 00000000..fc2065fe
--- /dev/null
+++ b/dev/lib/handlebars/src/spec/index.regressions.test.ts
@@ -0,0 +1,379 @@
+/*
+ * This file is forked from the handlebars project (https://github.com/handlebars-lang/handlebars.js),
+ * and may include modifications made by Elasticsearch B.V.
+ * Elasticsearch B.V. licenses this file to you under the MIT License.
+ * See `packages/kbn-handlebars/LICENSE` for more information.
+ */
+
+import Handlebars, { type HelperOptions } from '../..';
+import { expectTemplate, forEachCompileFunctionName } from '../__jest__/test_bench';
+
+describe('Regressions', () => {
+  it('GH-94: Cannot read property of undefined', () => {
+    expectTemplate('{{#books}}{{title}}{{author.name}}{{/books}}')
+      .withInput({
+        books: [
+          {
+            title: 'The origin of species',
+            author: {
+              name: 'Charles Darwin',
+            },
+          },
+          {
+            title: 'Lazarillo de Tormes',
+          },
+        ],
+      })
+      .toCompileTo('The origin of speciesCharles DarwinLazarillo de Tormes');
+  });
+
+  it("GH-150: Inverted sections print when they shouldn't", () => {
+    const string = '{{^set}}not set{{/set}} :: {{#set}}set{{/set}}';
+    expectTemplate(string).toCompileTo('not set :: ');
+    expectTemplate(string).withInput({ set: undefined }).toCompileTo('not set :: ');
+    expectTemplate(string).withInput({ set: false }).toCompileTo('not set :: ');
+    expectTemplate(string).withInput({ set: true }).toCompileTo(' :: set');
+  });
+
+  it('GH-158: Using array index twice, breaks the template', () => {
+    expectTemplate('{{arr.[0]}}, {{arr.[1]}}')
+      .withInput({ arr: [1, 2] })
+      .toCompileTo('1, 2');
+  });
+
+  it("bug reported by @fat where lambdas weren't being properly resolved", () => {
+    const string =
+      '<strong>This is a slightly more complicated {{thing}}.</strong>.\n' +
+      '{{! Just ignore this business. }}\n' +
+      'Check this out:\n' +
+      '{{#hasThings}}\n' +
+      '<ul>\n' +
+      '{{#things}}\n' +
+      '<li class={{className}}>{{word}}</li>\n' +
+      '{{/things}}</ul>.\n' +
+      '{{/hasThings}}\n' +
+      '{{^hasThings}}\n' +
+      '\n' +
+      '<small>Nothing to check out...</small>\n' +
+      '{{/hasThings}}';
+
+    const data = {
+      thing() {
+        return 'blah';
+      },
+      things: [
+        { className: 'one', word: '@fat' },
+        { className: 'two', word: '@dhg' },
+        { className: 'three', word: '@sayrer' },
+      ],
+      hasThings() {
+        return true;
+      },
+    };
+
+    const output =
+      '<strong>This is a slightly more complicated blah.</strong>.\n' +
+      'Check this out:\n' +
+      '<ul>\n' +
+      '<li class=one>@fat</li>\n' +
+      '<li class=two>@dhg</li>\n' +
+      '<li class=three>@sayrer</li>\n' +
+      '</ul>.\n';
+
+    expectTemplate(string).withInput(data).toCompileTo(output);
+  });
+
+  it('GH-408: Multiple loops fail', () => {
+    expectTemplate('{{#.}}{{name}}{{/.}}{{#.}}{{name}}{{/.}}{{#.}}{{name}}{{/.}}')
+      .withInput([
+        { name: 'John Doe', location: { city: 'Chicago' } },
+        { name: 'Jane Doe', location: { city: 'New York' } },
+      ])
+      .toCompileTo('John DoeJane DoeJohn DoeJane DoeJohn DoeJane Doe');
+  });
+
+  it('GS-428: Nested if else rendering', () => {
+    const succeedingTemplate =
+      '{{#inverse}} {{#blk}} Unexpected {{/blk}} {{else}}  {{#blk}} Expected {{/blk}} {{/inverse}}';
+    const failingTemplate =
+      '{{#inverse}} {{#blk}} Unexpected {{/blk}} {{else}} {{#blk}} Expected {{/blk}} {{/inverse}}';
+
+    const helpers = {
+      blk(block: HelperOptions) {
+        return block.fn('');
+      },
+      inverse(block: HelperOptions) {
+        return block.inverse('');
+      },
+    };
+
+    expectTemplate(succeedingTemplate).withHelpers(helpers).toCompileTo('   Expected  ');
+    expectTemplate(failingTemplate).withHelpers(helpers).toCompileTo('  Expected  ');
+  });
+
+  it('GH-458: Scoped this identifier', () => {
+    expectTemplate('{{./foo}}').withInput({ foo: 'bar' }).toCompileTo('bar');
+  });
+
+  it('GH-375: Unicode line terminators', () => {
+    expectTemplate('\u2028').toCompileTo('\u2028');
+  });
+
+  it('GH-534: Object prototype aliases', () => {
+    /* eslint-disable no-extend-native */
+    // @ts-expect-error
+    Object.prototype[0xd834] = true;
+
+    expectTemplate('{{foo}}').withInput({ foo: 'bar' }).toCompileTo('bar');
+
+    // @ts-expect-error
+    delete Object.prototype[0xd834];
+    /* eslint-enable no-extend-native */
+  });
+
+  it('GH-437: Matching escaping', () => {
+    expectTemplate('{{{a}}').toThrow(/Parse error on/);
+    expectTemplate('{{a}}}').toThrow(/Parse error on/);
+  });
+
+  it('GH-676: Using array in escaping mustache fails', () => {
+    const data = { arr: [1, 2] };
+    expectTemplate('{{arr}}').withInput(data).toCompileTo(data.arr.toString());
+  });
+
+  it('Mustache man page', () => {
+    expectTemplate(
+      'Hello {{name}}. You have just won ${{value}}!{{#in_ca}} Well, ${{taxed_value}}, after taxes.{{/in_ca}}'
+    )
+      .withInput({
+        name: 'Chris',
+        value: 10000,
+        taxed_value: 10000 - 10000 * 0.4,
+        in_ca: true,
+      })
+      .toCompileTo('Hello Chris. You have just won $10000! Well, $6000, after taxes.');
+  });
+
+  it('GH-731: zero context rendering', () => {
+    expectTemplate('{{#foo}} This is {{bar}} ~ {{/foo}}')
+      .withInput({
+        foo: 0,
+        bar: 'OK',
+      })
+      .toCompileTo(' This is  ~ ');
+  });
+
+  it('GH-820: zero pathed rendering', () => {
+    expectTemplate('{{foo.bar}}').withInput({ foo: 0 }).toCompileTo('');
+  });
+
+  it('GH-837: undefined values for helpers', () => {
+    expectTemplate('{{str bar.baz}}')
+      .withHelpers({
+        str(value) {
+          return value + '';
+        },
+      })
+      .toCompileTo('undefined');
+  });
+
+  it('GH-926: Depths and de-dupe', () => {
+    expectTemplate(
+      '{{#if dater}}{{#each data}}{{../name}}{{/each}}{{else}}{{#each notData}}{{../name}}{{/each}}{{/if}}'
+    )
+      .withInput({
+        name: 'foo',
+        data: [1],
+        notData: [1],
+      })
+      .toCompileTo('foo');
+  });
+
+  it('GH-1021: Each empty string key', () => {
+    expectTemplate('{{#each data}}Key: {{@key}}\n{{/each}}')
+      .withInput({
+        data: {
+          '': 'foo',
+          name: 'Chris',
+          value: 10000,
+        },
+      })
+      .toCompileTo('Key: \nKey: name\nKey: value\n');
+  });
+
+  it('GH-1054: Should handle simple safe string responses', () => {
+    expectTemplate('{{#wrap}}{{>partial}}{{/wrap}}')
+      .withHelpers({
+        wrap(options: HelperOptions) {
+          return new Handlebars.SafeString(options.fn());
+        },
+      })
+      .withPartials({
+        partial: '{{#wrap}}<partial>{{/wrap}}',
+      })
+      .toCompileTo('<partial>');
+  });
+
+  it('GH-1065: Sparse arrays', () => {
+    const array = [];
+    array[1] = 'foo';
+    array[3] = 'bar';
+    expectTemplate('{{#each array}}{{@index}}{{.}}{{/each}}')
+      .withInput({ array })
+      .toCompileTo('1foo3bar');
+  });
+
+  it('GH-1093: Undefined helper context', () => {
+    expectTemplate('{{#each obj}}{{{helper}}}{{.}}{{/each}}')
+      .withInput({ obj: { foo: undefined, bar: 'bat' } })
+      .withHelpers({
+        helper(this: any) {
+          // It's valid to execute a block against an undefined context, but
+          // helpers can not do so, so we expect to have an empty object here;
+          for (const name in this) {
+            if (Object.prototype.hasOwnProperty.call(this, name)) {
+              return 'found';
+            }
+          }
+          // And to make IE happy, check for the known string as length is not enumerated.
+          return this === 'bat' ? 'found' : 'not';
+        },
+      })
+      .toCompileTo('notfoundbat');
+  });
+
+  it('should support multiple levels of inline partials', () => {
+    expectTemplate('{{#> layout}}{{#*inline "subcontent"}}subcontent{{/inline}}{{/layout}}')
+      .withPartials({
+        doctype: 'doctype{{> content}}',
+        layout: '{{#> doctype}}{{#*inline "content"}}layout{{> subcontent}}{{/inline}}{{/doctype}}',
+      })
+      .toCompileTo('doctypelayoutsubcontent');
+  });
+
+  it('GH-1089: should support failover content in multiple levels of inline partials', () => {
+    expectTemplate('{{#> layout}}{{/layout}}')
+      .withPartials({
+        doctype: 'doctype{{> content}}',
+        layout:
+          '{{#> doctype}}{{#*inline "content"}}layout{{#> subcontent}}subcontent{{/subcontent}}{{/inline}}{{/doctype}}',
+      })
+      .toCompileTo('doctypelayoutsubcontent');
+  });
+
+  it('GH-1099: should support greater than 3 nested levels of inline partials', () => {
+    expectTemplate('{{#> layout}}Outer{{/layout}}')
+      .withPartials({
+        layout: '{{#> inner}}Inner{{/inner}}{{> @partial-block }}',
+        inner: '',
+      })
+      .toCompileTo('Outer');
+  });
+
+  it('GH-1135 : Context handling within each iteration', () => {
+    expectTemplate(
+      '{{#each array}}\n' +
+        ' 1. IF: {{#if true}}{{../name}}-{{../../name}}-{{../../../name}}{{/if}}\n' +
+        ' 2. MYIF: {{#myif true}}{{../name}}={{../../name}}={{../../../name}}{{/myif}}\n' +
+        '{{/each}}'
+    )
+      .withInput({ array: [1], name: 'John' })
+      .withHelpers({
+        myif(conditional, options: HelperOptions) {
+          if (conditional) {
+            return options.fn(this);
+          } else {
+            return options.inverse(this);
+          }
+        },
+      })
+      .toCompileTo(' 1. IF: John--\n' + ' 2. MYIF: John==\n');
+  });
+
+  it('GH-1186: Support block params for existing programs', () => {
+    expectTemplate(
+      '{{#*inline "test"}}{{> @partial-block }}{{/inline}}' +
+        '{{#>test }}{{#each listOne as |item|}}{{ item }}{{/each}}{{/test}}' +
+        '{{#>test }}{{#each listTwo as |item|}}{{ item }}{{/each}}{{/test}}'
+    )
+      .withInput({
+        listOne: ['a'],
+        listTwo: ['b'],
+      })
+      .toCompileTo('ab');
+  });
+
+  it('GH-1319: "unless" breaks when "each" value equals "null"', () => {
+    expectTemplate('{{#each list}}{{#unless ./prop}}parent={{../value}} {{/unless}}{{/each}}')
+      .withInput({
+        value: 'parent',
+        list: [null, 'a'],
+      })
+      .toCompileTo('parent=parent parent=parent ');
+  });
+
+  it('GH-1341: 4.0.7 release breaks {{#if @partial-block}} usage', () => {
+    expectTemplate('template {{>partial}} template')
+      .withPartials({
+        partialWithBlock: '{{#if @partial-block}} block {{> @partial-block}} block {{/if}}',
+        partial: '{{#> partialWithBlock}} partial {{/partialWithBlock}}',
+      })
+      .toCompileTo('template  block  partial  block  template');
+  });
+
+  it('should allow hash with protected array names', () => {
+    expectTemplate('{{helpa length="foo"}}')
+      .withInput({ array: [1], name: 'John' })
+      .withHelpers({
+        helpa(options: HelperOptions) {
+          return options.hash.length;
+        },
+      })
+      .toCompileTo('foo');
+  });
+
+  describe('GH-1598: Performance degradation for partials since v4.3.0', () => {
+    let newHandlebarsInstance: typeof Handlebars;
+    let spy: jest.SpyInstance;
+    beforeEach(() => {
+      newHandlebarsInstance = Handlebars.create();
+    });
+    afterEach(() => {
+      spy.mockRestore();
+    });
+
+    forEachCompileFunctionName((compileName) => {
+      it(`should only compile global partials once when calling #${compileName}`, () => {
+        const compile = newHandlebarsInstance[compileName].bind(newHandlebarsInstance);
+        let calls;
+        switch (compileName) {
+          case 'compile':
+            spy = jest.spyOn(newHandlebarsInstance, 'template');
+            calls = 3;
+            break;
+          case 'compileAST':
+            spy = jest.spyOn(newHandlebarsInstance, 'compileAST');
+            calls = 2;
+            break;
+        }
+        newHandlebarsInstance.registerPartial({
+          dude: 'I am a partial',
+        });
+        const string = 'Dudes: {{> dude}} {{> dude}}';
+        compile(string)(); // This should compile template + partial once
+        compile(string)(); // This should only compile template
+        expect(spy).toHaveBeenCalledTimes(calls);
+        spy.mockRestore();
+      });
+    });
+  });
+
+  describe("GH-1639: TypeError: Cannot read property 'apply' of undefined\" when handlebars version > 4.6.0 (undocumented, deprecated usage)", () => {
+    it('should treat undefined helpers like non-existing helpers', () => {
+      expectTemplate('{{foo}}')
+        .withHelper('foo', undefined as any)
+        .withInput({ foo: 'bar' })
+        .toCompileTo('bar');
+    });
+  });
+});
diff --git a/dev/lib/handlebars/src/spec/index.security.test.ts b/dev/lib/handlebars/src/spec/index.security.test.ts
new file mode 100644
index 00000000..878a0931
--- /dev/null
+++ b/dev/lib/handlebars/src/spec/index.security.test.ts
@@ -0,0 +1,132 @@
+/*
+ * This file is forked from the handlebars project (https://github.com/handlebars-lang/handlebars.js),
+ * and may include modifications made by Elasticsearch B.V.
+ * Elasticsearch B.V. licenses this file to you under the MIT License.
+ * See `packages/kbn-handlebars/LICENSE` for more information.
+ */
+
+import Handlebars from '../..';
+import { expectTemplate } from '../__jest__/test_bench';
+
+describe('security issues', () => {
+  describe('GH-1495: Prevent Remote Code Execution via constructor', () => {
+    it('should not allow constructors to be accessed', () => {
+      expectTemplate('{{lookup (lookup this "constructor") "name"}}').withInput({}).toCompileTo('');
+      expectTemplate('{{constructor.name}}').withInput({}).toCompileTo('');
+    });
+
+    it('GH-1603: should not allow constructors to be accessed (lookup via toString)', () => {
+      expectTemplate('{{lookup (lookup this (list "constructor")) "name"}}')
+        .withInput({})
+        .withHelper('list', function (element) {
+          return [element];
+        })
+        .toCompileTo('');
+    });
+
+    it('should allow the "constructor" property to be accessed if it is an "ownProperty"', () => {
+      expectTemplate('{{constructor.name}}')
+        .withInput({ constructor: { name: 'here we go' } })
+        .toCompileTo('here we go');
+
+      expectTemplate('{{lookup (lookup this "constructor") "name"}}')
+        .withInput({ constructor: { name: 'here we go' } })
+        .toCompileTo('here we go');
+    });
+
+    it('should allow the "constructor" property to be accessed if it is an "own property"', () => {
+      expectTemplate('{{lookup (lookup this "constructor") "name"}}')
+        .withInput({ constructor: { name: 'here we go' } })
+        .toCompileTo('here we go');
+    });
+  });
+
+  describe('GH-1558: Prevent explicit call of helperMissing-helpers', () => {
+    describe('without the option "allowExplicitCallOfHelperMissing"', () => {
+      it('should throw an exception when calling  "{{helperMissing}}" ', () => {
+        expectTemplate('{{helperMissing}}').toThrow(Error);
+      });
+
+      it('should throw an exception when calling  "{{#helperMissing}}{{/helperMissing}}" ', () => {
+        expectTemplate('{{#helperMissing}}{{/helperMissing}}').toThrow(Error);
+      });
+
+      it('should throw an exception when calling  "{{blockHelperMissing "abc" .}}" ', () => {
+        const functionCalls = [];
+        expect(() => {
+          const template = Handlebars.compile('{{blockHelperMissing "abc" .}}');
+          template({
+            fn() {
+              functionCalls.push('called');
+            },
+          });
+        }).toThrow(Error);
+        expect(functionCalls.length).toEqual(0);
+      });
+
+      it('should throw an exception when calling  "{{#blockHelperMissing .}}{{/blockHelperMissing}}"', () => {
+        expectTemplate('{{#blockHelperMissing .}}{{/blockHelperMissing}}')
+          .withInput({
+            fn() {
+              return 'functionInData';
+            },
+          })
+          .toThrow(Error);
+      });
+    });
+  });
+
+  describe('GH-1563', () => {
+    it('should not allow to access constructor after overriding via __defineGetter__', () => {
+      // @ts-expect-error
+      if ({}.__defineGetter__ == null || {}.__lookupGetter__ == null) {
+        return; // Browser does not support this exploit anyway
+      }
+      expectTemplate(
+        '{{__defineGetter__ "undefined" valueOf }}' +
+          '{{#with __lookupGetter__ }}' +
+          '{{__defineGetter__ "propertyIsEnumerable" (this.bind (this.bind 1)) }}' +
+          '{{constructor.name}}' +
+          '{{/with}}'
+      )
+        .withInput({})
+        .toThrow(/Missing helper: "__defineGetter__"/);
+    });
+  });
+
+  describe('GH-1595: dangerous properties', () => {
+    const templates = [
+      '{{constructor}}',
+      '{{__defineGetter__}}',
+      '{{__defineSetter__}}',
+      '{{__lookupGetter__}}',
+      '{{__proto__}}',
+      '{{lookup this "constructor"}}',
+      '{{lookup this "__defineGetter__"}}',
+      '{{lookup this "__defineSetter__"}}',
+      '{{lookup this "__lookupGetter__"}}',
+      '{{lookup this "__proto__"}}',
+    ];
+
+    templates.forEach((template) => {
+      describe('access should be denied to ' + template, () => {
+        it('by default', () => {
+          expectTemplate(template).withInput({}).toCompileTo('');
+        });
+      });
+    });
+  });
+
+  describe('escapes template variables', () => {
+    it('in default mode', () => {
+      expectTemplate("{{'a\\b'}}").withCompileOptions().withInput({ 'a\\b': 'c' }).toCompileTo('c');
+    });
+
+    it('in strict mode', () => {
+      expectTemplate("{{'a\\b'}}")
+        .withCompileOptions({ strict: true })
+        .withInput({ 'a\\b': 'c' })
+        .toCompileTo('c');
+    });
+  });
+});
diff --git a/dev/lib/handlebars/src/spec/index.strict.test.ts b/dev/lib/handlebars/src/spec/index.strict.test.ts
new file mode 100644
index 00000000..a8f294b9
--- /dev/null
+++ b/dev/lib/handlebars/src/spec/index.strict.test.ts
@@ -0,0 +1,164 @@
+/*
+ * This file is forked from the handlebars project (https://github.com/handlebars-lang/handlebars.js),
+ * and may include modifications made by Elasticsearch B.V.
+ * Elasticsearch B.V. licenses this file to you under the MIT License.
+ * See `packages/kbn-handlebars/LICENSE` for more information.
+ */
+
+import { expectTemplate } from '../__jest__/test_bench';
+
+describe('strict', () => {
+  describe('strict mode', () => {
+    it('should error on missing property lookup', () => {
+      expectTemplate('{{hello}}')
+        .withCompileOptions({ strict: true })
+        .toThrow(/"hello" not defined in/);
+    });
+
+    it('should error on missing child', () => {
+      expectTemplate('{{hello.bar}}')
+        .withCompileOptions({ strict: true })
+        .withInput({ hello: { bar: 'foo' } })
+        .toCompileTo('foo');
+
+      expectTemplate('{{hello.bar}}')
+        .withCompileOptions({ strict: true })
+        .withInput({ hello: {} })
+        .toThrow(/"bar" not defined in/);
+    });
+
+    it('should handle explicit undefined', () => {
+      expectTemplate('{{hello.bar}}')
+        .withCompileOptions({ strict: true })
+        .withInput({ hello: { bar: undefined } })
+        .toCompileTo('');
+    });
+
+    it('should error on missing property lookup in known helpers mode', () => {
+      expectTemplate('{{hello}}')
+        .withCompileOptions({
+          strict: true,
+          knownHelpersOnly: true,
+        })
+        .toThrow(/"hello" not defined in/);
+    });
+
+    it('should error on missing context', () => {
+      expectTemplate('{{hello}}').withCompileOptions({ strict: true }).toThrow(Error);
+    });
+
+    it('should error on missing data lookup', () => {
+      const xt = expectTemplate('{{@hello}}').withCompileOptions({
+        strict: true,
+      });
+
+      xt.toThrow(Error);
+
+      xt.withRuntimeOptions({ data: { hello: 'foo' } }).toCompileTo('foo');
+    });
+
+    it('should not run helperMissing for helper calls', () => {
+      expectTemplate('{{hello foo}}')
+        .withCompileOptions({ strict: true })
+        .withInput({ foo: true })
+        .toThrow(/"hello" not defined in/);
+
+      expectTemplate('{{#hello foo}}{{/hello}}')
+        .withCompileOptions({ strict: true })
+        .withInput({ foo: true })
+        .toThrow(/"hello" not defined in/);
+    });
+
+    it('should throw on ambiguous blocks', () => {
+      expectTemplate('{{#hello}}{{/hello}}')
+        .withCompileOptions({ strict: true })
+        .toThrow(/"hello" not defined in/);
+
+      expectTemplate('{{^hello}}{{/hello}}')
+        .withCompileOptions({ strict: true })
+        .toThrow(/"hello" not defined in/);
+
+      expectTemplate('{{#hello.bar}}{{/hello.bar}}')
+        .withCompileOptions({ strict: true })
+        .withInput({ hello: {} })
+        .toThrow(/"bar" not defined in/);
+    });
+
+    it('should allow undefined parameters when passed to helpers', () => {
+      expectTemplate('{{#unless foo}}success{{/unless}}')
+        .withCompileOptions({ strict: true })
+        .toCompileTo('success');
+    });
+
+    it('should allow undefined hash when passed to helpers', () => {
+      expectTemplate('{{helper value=@foo}}')
+        .withCompileOptions({
+          strict: true,
+        })
+        .withHelpers({
+          helper(options) {
+            expect('value' in options.hash).toEqual(true);
+            expect(options.hash.value).toBeUndefined();
+            return 'success';
+          },
+        })
+        .toCompileTo('success');
+    });
+
+    it('should show error location on missing property lookup', () => {
+      expectTemplate('\n\n\n   {{hello}}')
+        .withCompileOptions({ strict: true })
+        .toThrow('"hello" not defined in [object Object] - 4:5');
+    });
+
+    it('should error contains correct location properties on missing property lookup', () => {
+      try {
+        expectTemplate('\n\n\n   {{hello}}')
+          .withCompileOptions({ strict: true })
+          .toCompileTo('throw before asserting this');
+      } catch (error) {
+        expect(error.lineNumber).toEqual(4);
+        expect(error.endLineNumber).toEqual(4);
+        expect(error.column).toEqual(5);
+        expect(error.endColumn).toEqual(10);
+      }
+    });
+  });
+
+  describe('assume objects', () => {
+    it('should ignore missing property', () => {
+      expectTemplate('{{hello}}').withCompileOptions({ assumeObjects: true }).toCompileTo('');
+    });
+
+    it('should ignore missing child', () => {
+      expectTemplate('{{hello.bar}}')
+        .withCompileOptions({ assumeObjects: true })
+        .withInput({ hello: {} })
+        .toCompileTo('');
+    });
+
+    it('should error on missing object', () => {
+      expectTemplate('{{hello.bar}}').withCompileOptions({ assumeObjects: true }).toThrow(Error);
+    });
+
+    it('should error on missing context', () => {
+      expectTemplate('{{hello}}')
+        .withCompileOptions({ assumeObjects: true })
+        .withInput(undefined)
+        .toThrow(Error);
+    });
+
+    it('should error on missing data lookup', () => {
+      expectTemplate('{{@hello.bar}}')
+        .withCompileOptions({ assumeObjects: true })
+        .withInput(undefined)
+        .toThrow(Error);
+    });
+
+    it('should execute blockHelperMissing', () => {
+      expectTemplate('{{^hello}}foo{{/hello}}')
+        .withCompileOptions({ assumeObjects: true })
+        .toCompileTo('foo');
+    });
+  });
+});
diff --git a/dev/lib/handlebars/src/spec/index.subexpressions.test.ts b/dev/lib/handlebars/src/spec/index.subexpressions.test.ts
new file mode 100644
index 00000000..4dee24b7
--- /dev/null
+++ b/dev/lib/handlebars/src/spec/index.subexpressions.test.ts
@@ -0,0 +1,214 @@
+/*
+ * This file is forked from the handlebars project (https://github.com/handlebars-lang/handlebars.js),
+ * and may include modifications made by Elasticsearch B.V.
+ * Elasticsearch B.V. licenses this file to you under the MIT License.
+ * See `packages/kbn-handlebars/LICENSE` for more information.
+ */
+
+import Handlebars, { type HelperOptions } from '../..';
+import { expectTemplate } from '../__jest__/test_bench';
+
+describe('subexpressions', () => {
+  it('arg-less helper', () => {
+    expectTemplate('{{foo (bar)}}!')
+      .withHelpers({
+        foo(val) {
+          return val + val;
+        },
+        bar() {
+          return 'LOL';
+        },
+      })
+      .toCompileTo('LOLLOL!');
+  });
+
+  it('helper w args', () => {
+    expectTemplate('{{blog (equal a b)}}')
+      .withInput({ bar: 'LOL' })
+      .withHelpers({
+        blog(val) {
+          return 'val is ' + val;
+        },
+        equal(x, y) {
+          return x === y;
+        },
+      })
+      .toCompileTo('val is true');
+  });
+
+  it('mixed paths and helpers', () => {
+    expectTemplate('{{blog baz.bat (equal a b) baz.bar}}')
+      .withInput({ bar: 'LOL', baz: { bat: 'foo!', bar: 'bar!' } })
+      .withHelpers({
+        blog(val, that, theOther) {
+          return 'val is ' + val + ', ' + that + ' and ' + theOther;
+        },
+        equal(x, y) {
+          return x === y;
+        },
+      })
+      .toCompileTo('val is foo!, true and bar!');
+  });
+
+  it('supports much nesting', () => {
+    expectTemplate('{{blog (equal (equal true true) true)}}')
+      .withInput({ bar: 'LOL' })
+      .withHelpers({
+        blog(val) {
+          return 'val is ' + val;
+        },
+        equal(x, y) {
+          return x === y;
+        },
+      })
+      .toCompileTo('val is true');
+  });
+
+  it('GH-800 : Complex subexpressions', () => {
+    const context = { a: 'a', b: 'b', c: { c: 'c' }, d: 'd', e: { e: 'e' } };
+    const helpers = {
+      dash(a: any, b: any) {
+        return a + '-' + b;
+      },
+      concat(a: any, b: any) {
+        return a + b;
+      },
+    };
+
+    expectTemplate("{{dash 'abc' (concat a b)}}")
+      .withInput(context)
+      .withHelpers(helpers)
+      .toCompileTo('abc-ab');
+
+    expectTemplate('{{dash d (concat a b)}}')
+      .withInput(context)
+      .withHelpers(helpers)
+      .toCompileTo('d-ab');
+
+    expectTemplate('{{dash c.c (concat a b)}}')
+      .withInput(context)
+      .withHelpers(helpers)
+      .toCompileTo('c-ab');
+
+    expectTemplate('{{dash (concat a b) c.c}}')
+      .withInput(context)
+      .withHelpers(helpers)
+      .toCompileTo('ab-c');
+
+    expectTemplate('{{dash (concat a e.e) c.c}}')
+      .withInput(context)
+      .withHelpers(helpers)
+      .toCompileTo('ae-c');
+  });
+
+  it('provides each nested helper invocation its own options hash', () => {
+    let lastOptions: HelperOptions;
+    const helpers = {
+      equal(x: any, y: any, options: HelperOptions) {
+        if (!options || options === lastOptions) {
+          throw new Error('options hash was reused');
+        }
+        lastOptions = options;
+        return x === y;
+      },
+    };
+    expectTemplate('{{equal (equal true true) true}}').withHelpers(helpers).toCompileTo('true');
+  });
+
+  it('with hashes', () => {
+    expectTemplate("{{blog (equal (equal true true) true fun='yes')}}")
+      .withInput({ bar: 'LOL' })
+      .withHelpers({
+        blog(val) {
+          return 'val is ' + val;
+        },
+        equal(x, y) {
+          return x === y;
+        },
+      })
+      .toCompileTo('val is true');
+  });
+
+  it('as hashes', () => {
+    expectTemplate("{{blog fun=(equal (blog fun=1) 'val is 1')}}")
+      .withHelpers({
+        blog(options) {
+          return 'val is ' + options.hash.fun;
+        },
+        equal(x, y) {
+          return x === y;
+        },
+      })
+      .toCompileTo('val is true');
+  });
+
+  it('multiple subexpressions in a hash', () => {
+    expectTemplate('{{input aria-label=(t "Name") placeholder=(t "Example User")}}')
+      .withHelpers({
+        input(options) {
+          const hash = options.hash;
+          const ariaLabel = Handlebars.Utils.escapeExpression(hash['aria-label']);
+          const placeholder = Handlebars.Utils.escapeExpression(hash.placeholder);
+          return new Handlebars.SafeString(
+            '<input aria-label="' + ariaLabel + '" placeholder="' + placeholder + '" />'
+          );
+        },
+        t(defaultString) {
+          return new Handlebars.SafeString(defaultString);
+        },
+      })
+      .toCompileTo('<input aria-label="Name" placeholder="Example User" />');
+  });
+
+  it('multiple subexpressions in a hash with context', () => {
+    expectTemplate('{{input aria-label=(t item.field) placeholder=(t item.placeholder)}}')
+      .withInput({
+        item: {
+          field: 'Name',
+          placeholder: 'Example User',
+        },
+      })
+      .withHelpers({
+        input(options) {
+          const hash = options.hash;
+          const ariaLabel = Handlebars.Utils.escapeExpression(hash['aria-label']);
+          const placeholder = Handlebars.Utils.escapeExpression(hash.placeholder);
+          return new Handlebars.SafeString(
+            '<input aria-label="' + ariaLabel + '" placeholder="' + placeholder + '" />'
+          );
+        },
+        t(defaultString) {
+          return new Handlebars.SafeString(defaultString);
+        },
+      })
+      .toCompileTo('<input aria-label="Name" placeholder="Example User" />');
+  });
+
+  it('subexpression functions on the context', () => {
+    expectTemplate('{{foo (bar)}}!')
+      .withInput({
+        bar() {
+          return 'LOL';
+        },
+      })
+      .withHelpers({
+        foo(val) {
+          return val + val;
+        },
+      })
+      .toCompileTo('LOLLOL!');
+  });
+
+  it("subexpressions can't just be property lookups", () => {
+    expectTemplate('{{foo (bar)}}!')
+      .withInput({
+        bar: 'LOL',
+      })
+      .withHelpers({
+        foo(val) {
+          return val + val;
+        },
+      })
+      .toThrow();
+  });
+});
diff --git a/dev/lib/handlebars/src/spec/index.utils.test.ts b/dev/lib/handlebars/src/spec/index.utils.test.ts
new file mode 100644
index 00000000..6350bc7c
--- /dev/null
+++ b/dev/lib/handlebars/src/spec/index.utils.test.ts
@@ -0,0 +1,24 @@
+/*
+ * This file is forked from the handlebars project (https://github.com/handlebars-lang/handlebars.js),
+ * and may include modifications made by Elasticsearch B.V.
+ * Elasticsearch B.V. licenses this file to you under the MIT License.
+ * See `packages/kbn-handlebars/LICENSE` for more information.
+ */
+
+import Handlebars from '../..';
+import { expectTemplate } from '../__jest__/test_bench';
+
+describe('utils', function () {
+  describe('#SafeString', function () {
+    it('constructing a safestring from a string and checking its type', function () {
+      const safe = new Handlebars.SafeString('testing 1, 2, 3');
+      expect(safe).toBeInstanceOf(Handlebars.SafeString);
+      expect(safe.toString()).toEqual('testing 1, 2, 3');
+    });
+
+    it('it should not escape SafeString properties', function () {
+      const name = new Handlebars.SafeString('<em>Sean O&#x27;Malley</em>');
+      expectTemplate('{{name}}').withInput({ name }).toCompileTo('<em>Sean O&#x27;Malley</em>');
+    });
+  });
+});
diff --git a/dev/lib/handlebars/src/spec/index.whitespace_control.test.ts b/dev/lib/handlebars/src/spec/index.whitespace_control.test.ts
new file mode 100644
index 00000000..1f7cf019
--- /dev/null
+++ b/dev/lib/handlebars/src/spec/index.whitespace_control.test.ts
@@ -0,0 +1,88 @@
+/*
+ * This file is forked from the handlebars project (https://github.com/handlebars-lang/handlebars.js),
+ * and may include modifications made by Elasticsearch B.V.
+ * Elasticsearch B.V. licenses this file to you under the MIT License.
+ * See `packages/kbn-handlebars/LICENSE` for more information.
+ */
+
+import { expectTemplate } from '../__jest__/test_bench';
+
+describe('whitespace control', () => {
+  it('should strip whitespace around mustache calls', () => {
+    const hash = { foo: 'bar<' };
+    expectTemplate(' {{~foo~}} ').withInput(hash).toCompileTo('bar&lt;');
+    expectTemplate(' {{~foo}} ').withInput(hash).toCompileTo('bar&lt; ');
+    expectTemplate(' {{foo~}} ').withInput(hash).toCompileTo(' bar&lt;');
+    expectTemplate(' {{~&foo~}} ').withInput(hash).toCompileTo('bar<');
+    expectTemplate(' {{~{foo}~}} ').withInput(hash).toCompileTo('bar<');
+    expectTemplate('1\n{{foo~}} \n\n 23\n{{bar}}4').toCompileTo('1\n23\n4');
+  });
+
+  describe('blocks', () => {
+    it('should strip whitespace around simple block calls', () => {
+      const hash = { foo: 'bar<' };
+
+      expectTemplate(' {{~#if foo~}} bar {{~/if~}} ').withInput(hash).toCompileTo('bar');
+      expectTemplate(' {{#if foo~}} bar {{/if~}} ').withInput(hash).toCompileTo(' bar ');
+      expectTemplate(' {{~#if foo}} bar {{~/if}} ').withInput(hash).toCompileTo(' bar ');
+      expectTemplate(' {{#if foo}} bar {{/if}} ').withInput(hash).toCompileTo('  bar  ');
+
+      expectTemplate(' \n\n{{~#if foo~}} \n\nbar \n\n{{~/if~}}\n\n ')
+        .withInput(hash)
+        .toCompileTo('bar');
+
+      expectTemplate(' a\n\n{{~#if foo~}} \n\nbar \n\n{{~/if~}}\n\na ')
+        .withInput(hash)
+        .toCompileTo(' abara ');
+    });
+
+    it('should strip whitespace around inverse block calls', () => {
+      expectTemplate(' {{~^if foo~}} bar {{~/if~}} ').toCompileTo('bar');
+      expectTemplate(' {{^if foo~}} bar {{/if~}} ').toCompileTo(' bar ');
+      expectTemplate(' {{~^if foo}} bar {{~/if}} ').toCompileTo(' bar ');
+      expectTemplate(' {{^if foo}} bar {{/if}} ').toCompileTo('  bar  ');
+      expectTemplate(' \n\n{{~^if foo~}} \n\nbar \n\n{{~/if~}}\n\n ').toCompileTo('bar');
+    });
+
+    it('should strip whitespace around complex block calls', () => {
+      const hash = { foo: 'bar<' };
+
+      expectTemplate('{{#if foo~}} bar {{~^~}} baz {{~/if}}').withInput(hash).toCompileTo('bar');
+      expectTemplate('{{#if foo~}} bar {{^~}} baz {{/if}}').withInput(hash).toCompileTo('bar ');
+      expectTemplate('{{#if foo}} bar {{~^~}} baz {{~/if}}').withInput(hash).toCompileTo(' bar');
+      expectTemplate('{{#if foo}} bar {{^~}} baz {{/if}}').withInput(hash).toCompileTo(' bar ');
+      expectTemplate('{{#if foo~}} bar {{~else~}} baz {{~/if}}').withInput(hash).toCompileTo('bar');
+
+      expectTemplate('\n\n{{~#if foo~}} \n\nbar \n\n{{~^~}} \n\nbaz \n\n{{~/if~}}\n\n')
+        .withInput(hash)
+        .toCompileTo('bar');
+
+      expectTemplate('\n\n{{~#if foo~}} \n\n{{{foo}}} \n\n{{~^~}} \n\nbaz \n\n{{~/if~}}\n\n')
+        .withInput(hash)
+        .toCompileTo('bar<');
+
+      expectTemplate('{{#if foo~}} bar {{~^~}} baz {{~/if}}').toCompileTo('baz');
+      expectTemplate('{{#if foo}} bar {{~^~}} baz {{/if}}').toCompileTo('baz ');
+      expectTemplate('{{#if foo~}} bar {{~^}} baz {{~/if}}').toCompileTo(' baz');
+      expectTemplate('{{#if foo~}} bar {{~^}} baz {{/if}}').toCompileTo(' baz ');
+      expectTemplate('{{#if foo~}} bar {{~else~}} baz {{~/if}}').toCompileTo('baz');
+      expectTemplate('\n\n{{~#if foo~}} \n\nbar \n\n{{~^~}} \n\nbaz \n\n{{~/if~}}\n\n').toCompileTo(
+        'baz'
+      );
+    });
+  });
+
+  it('should strip whitespace around partials', () => {
+    expectTemplate('foo {{~> dude~}} ').withPartials({ dude: 'bar' }).toCompileTo('foobar');
+    expectTemplate('foo {{> dude~}} ').withPartials({ dude: 'bar' }).toCompileTo('foo bar');
+    expectTemplate('foo {{> dude}} ').withPartials({ dude: 'bar' }).toCompileTo('foo bar ');
+    expectTemplate('foo\n {{~> dude}} ').withPartials({ dude: 'bar' }).toCompileTo('foobar');
+    expectTemplate('foo\n {{> dude}} ').withPartials({ dude: 'bar' }).toCompileTo('foo\n bar');
+  });
+
+  it('should only strip whitespace once', () => {
+    expectTemplate(' {{~foo~}} {{foo}} {{foo}} ')
+      .withInput({ foo: 'bar' })
+      .toCompileTo('barbar bar ');
+  });
+});