Add experiment vue component

This adds a Vue component that can be used for experiments that are
conducted with `gitlab-experiment`.

app/assets/javascripts/experimentation/components/experiment.vue

+<script>
+import { getExperimentVariant } from '../utils';
+
+export default {
+  props: {
+    name: {
+      type: String,
+      required: true,
+    },
+  },
+  render() {
+    return this.$slots?.[getExperimentVariant(this.name)];
+  },
+};
+</script>

app/assets/javascripts/experimentation/constants.js

 export const TRACKING_CONTEXT_SCHEMA = 'iglu:com.gitlab/gitlab_experiment/jsonschema/1-0-0';
+export const DEFAULT_VARIANT = 'control';

app/assets/javascripts/experimentation/utils.js

 // This file only applies to use of experiments through https://gitlab.com/gitlab-org/gitlab-experiment
 import { get } from 'lodash';
+import { DEFAULT_VARIANT } from './constants';
 
 export function getExperimentData(experimentName) {
   return get(window, ['gon', 'experiment', experimentName]);
@@ -8,3 +9,7 @@ export function getExperimentData(experimentName) {
 export function isExperimentVariant(experimentName, variantName) {
   return getExperimentData(experimentName)?.variant === variantName;
 }
+
+export function getExperimentVariant(experimentName) {
+  return getExperimentData(experimentName)?.variant || DEFAULT_VARIANT;
+}

doc/development/experiment_guide/gitlab_experiment.md

@@ -501,6 +501,64 @@ Any experiment that's been run in the request lifecycle surfaces in `window.gon.
 and matches [this schema](https://gitlab.com/gitlab-org/iglu/-/blob/master/public/schemas/com.gitlab/gitlab_experiment/jsonschema/1-0-0)
 so you can use it when resolving some concepts around experimentation in the client layer.
 
+### Using experiments in Vue
+
+With the `experiment` component, you can define slots that match the name of the variants pushed to `window.gon.experiment`. For example an experiment with the default variants `control` and `candidate` could be implemented the following:
+
+```ruby
+def show
+  experiment(:button_color) do |e|
+    e.use { } # control
+    e.try { } # candidate
+  end.run
+end
+```
+
+```vue
+<script>
+import Experiment from '~/experimentation/components/experiment.vue';
+
+export default {
+  components: { Experiment }
+}
+</script>
+
+<template>
+  <experiment name="button_name">
+    <template #control>
+      <button>Click me</button>
+    </template>
+
+    <template #candidate>
+      <button>You will not believe what happens when you click this button</button>
+    </template>
+  </experiment>
+</template>
+```
+
+When using a multivariate experiment, the names of the variant names can be used, e.g. with the `pill_color` experiment from before, the Vue component would look like this:
+
+```vue
+<template>
+  <experiment name="pill_color">
+    <template #control>
+      <button class="bg-default">Click default button</button>
+    </template>
+
+    <template #red>
+      <button class="bg-red">Click red button</button>
+    </template>
+
+    <template #blue>
+      <button class="bg-blue">Click blue button</button>
+    </template>
+  </experiment>
+</template>
+```
+
+NOTE:
+When there is no experiment defined in the frontend via `experiment(:experiment_name)`, then `control` will be rendered if it exists.
+
 ## Notes on feature flags
 
 NOTE:

spec/frontend/experimentation/components/experiment_spec.js

+import { shallowMount } from '@vue/test-utils';
+import { extendedWrapper } from 'helpers/vue_test_utils_helper';
+import ExperimentComponent from '~/experimentation/components/experiment.vue';
+
+const defaultProps = { name: 'experiment_name' };
+const defaultSlots = {
+  candidate: `<p>Candidate</p>`,
+  control: `<p>Control</p>`,
+};
+
+describe('ExperimentComponent', () => {
+  const oldGon = window.gon;
+  let wrapper;
+
+  const createComponent = (propsData = defaultProps, slots = defaultSlots) => {
+    wrapper = extendedWrapper(shallowMount(ExperimentComponent, { propsData, slots }));
+  };
+
+  const mockVariant = (expectedVariant) => {
+    window.gon = { experiment: { experiment_name: { variant: expectedVariant } } };
+  };
+
+  afterEach(() => {
+    wrapper.destroy();
+    wrapper = null;
+    window.gon = oldGon;
+  });
+
+  describe('when variant and experiment is set', () => {
+    it('renders control when it is the active variant', () => {
+      mockVariant('control');
+
+      createComponent();
+
+      expect(wrapper.text()).toBe('Control');
+    });
+
+    it('renders candidate when it is the active variant', () => {
+      mockVariant('candidate');
+
+      createComponent();
+
+      expect(wrapper.text()).toBe('Candidate');
+    });
+  });
+
+  describe('when variant or experiment is not set', () => {
+    it('renders the control slot when no variant is defined', () => {
+      mockVariant(undefined);
+
+      createComponent();
+
+      expect(wrapper.text()).toBe('Control');
+    });
+
+    it('renders nothing when behavior is not set for variant', () => {
+      mockVariant('non-existing-variant');
+
+      createComponent(defaultProps, { control: `<p>First</p>`, other: `<p>Other</p>` });
+
+      expect(wrapper.text()).toBe('');
+    });
+
+    it('renders nothing when there are no slots', () => {
+      mockVariant('control');
+
+      createComponent(defaultProps, {});
+
+      expect(wrapper.text()).toBe('');
+    });
+  });
+});

spec/frontend/experimentation/utils_spec.js

+import { DEFAULT_VARIANT } from '~/experimentation/constants';
 import * as experimentUtils from '~/experimentation/utils';
 
 const TEST_KEY = 'abc';
@@ -35,4 +36,17 @@ describe('experiment Utilities', () => {
       expect(experimentUtils.isExperimentVariant(...input)).toEqual(output);
     });
   });
+
+  describe('getExperimentVariant', () => {
+    it.each`
+      gon                                                         | input         | output
+      ${{ experiment: { [TEST_KEY]: { variant: 'control' } } }}   | ${[TEST_KEY]} | ${'control'}
+      ${{ experiment: { [TEST_KEY]: { variant: 'candidate' } } }} | ${[TEST_KEY]} | ${'candidate'}
+      ${{}}                                                       | ${[TEST_KEY]} | ${DEFAULT_VARIANT}
+    `('with input=$input and gon=$gon, returns $output', ({ gon, input, output }) => {
+      window.gon = gon;
+
+      expect(experimentUtils.getExperimentVariant(...input)).toEqual(output);
+    });
+  });
 });