Testing experiments

Testing experiments with RSpec

In the course of working with experiments, you’ll probably want to utilize the RSpec tooling that’s built in. This happens automatically for files in spec/experiments, but for other files and specs you want to include it in, you can specify the :experiment type:

it "tests experiments nicely", :experiment do
end

Stub helpers

You can stub experiments using stub_experiments. Pass it a hash using experiment names as the keys, and the variants you want each to resolve to, as the values:

# Ensures the experiments named `:example` & `:example2` are both "enabled" and
# that each will resolve to the given variant (`:my_variant` and `:control`
# respectively).
stub_experiments(example: :my_variant, example2: :control)

experiment(:example) do |e|
  e.enabled? # => true
  e.assigned.name # => 'my_variant'
end

experiment(:example2) do |e|
  e.enabled? # => true
  e.assigned.name # => 'control'
end

Exclusion, segmentation, and behavior matchers

You can also test things like the registered behaviors, the exclusions, and segmentations using the matchers.

class ExampleExperiment < ApplicationExperiment
  control { }
  candidate { '_candidate_' }
    
  exclude { context.actor.first_name == 'Richard' }
  segment(variant: :candidate) { context.actor.username == 'jejacks0n' }
end

excluded = double(username: 'rdiggitty', first_name: 'Richard')
segmented = double(username: 'jejacks0n', first_name: 'Jeremy')

# register_behavior matcher
expect(experiment(:example)).to register_behavior(:control)
expect(experiment(:example)).to register_behavior(:candidate).with('_candidate_')

# exclude matcher
expect(experiment(:example)).to exclude(actor: excluded)
expect(experiment(:example)).not_to exclude(actor: segmented)

# segment matcher
expect(experiment(:example)).to segment(actor: segmented).into(:candidate)
expect(experiment(:example)).not_to segment(actor: excluded)

Tracking matcher

Tracking events is a major aspect of experimentation. We try to provide a flexible way to ensure your tracking calls are covered.

You can do this on the instance level or at an “any instance” level:

subject = experiment(:example)

expect(subject).to track(:my_event)

subject.track(:my_event)

You can use the on_next_instance chain method to specify that it will happen on the next instance of the experiment. This helps you if you’re calling experiment(:example).track downstream:

expect(experiment(:example)).to track(:my_event).on_next_instance

experiment(:example).track(:my_event)

A full example of the methods you can chain onto the track matcher:

expect(experiment(:example)).to track(:my_event, value: 1, property: '_property_')
  .on_next_instance
  .with_context(foo: :bar)
  .for(:variant_name)

experiment(:example, :variant_name, foo: :bar).track(:my_event, value: 1, property: '_property_')

Test with Jest

Stub Helpers

You can stub experiments using the stubExperiments helper defined in spec/frontend/__helpers__/experimentation_helper.js.

import { stubExperiments } from 'helpers/experimentation_helper';
import { getExperimentData } from '~/experimentation/utils';

describe('when my_experiment is enabled', () => {
  beforeEach(() => {
    stubExperiments({ my_experiment: 'candidate' });
  });

  it('sets the correct data', () => {
    expect(getExperimentData('my_experiment')).toEqual({ experiment: 'my_experiment', variant: 'candidate' });
  });
});
note
This method of stubbing in Jest specs will not automatically un-stub itself at the end of the test. We merge our stubbed experiment in with all the other global data in window.gl. If you need to remove the stubbed experiments after your test or ensure a clean global object before your test, you’ll need to manage the global object directly yourself:
describe('tests that care about global state', () => {
  const originalObjects = [];

  beforeEach(() => {
    // For backwards compatibility for now, we're using both window.gon & window.gl
    originalObjects.push(window.gon, window.gl);
  });

  afterEach(() => {
    [window.gon, window.gl] = originalObjects;
  });

  it('stubs experiment in fresh global state', () => {
    stubExperiment({ my_experiment: 'candidate' });
    // ...
  });
})