golden/modules/test_util.js

/** @module golden/test_util
 * @description
 *
 * <p>
 *  A general set of useful functions for tests and demos,
 *  e.g. reducing boilerplate.
 * </p>
 */

/**
 * Takes a DOM element name (e.g. 'my-component-sk') and returns a factory
 * function that can be used to obtain new instances of that element.
 *
 * The element returned by the factory function is attached to document.body,
 * and an afterEach() hook is set to automatically remove the element from the
 * DOM after each test.
 *
 * The returned factory function optionally takes a callback function that will
 * be called with the newly instantiated element before it is attached to the
 * DOM, giving client code a chance to finish setting up the element before e.g.
 * the element's connecetedCallback() method is invoked.
 *
 * Sample usage:
 *
 *   describe('my-component-sk', () => {
 *     const newInstance = setUpElementUnderTest('my-component-sk');
 *
 *     it('should be correctly instantiated', () => {
 *       const myComponentSk = newInstance((el) => {
 *         // This is called before attaching the element to the DOM.
 *         el.setAttribute('hello', 'world');
 *       });
 *
 *       expect(myComponentSk.parentElement).to.equal(document.body);
 *       expect(myComponentSk.getAttribute('hello')).to.equal('world');
 *     });
 *   });
 *
 * @param elementName {string} Name of the element to test, e.g. 'foo-sk'.
 * @return {Function} A factory function that optionally takes a callback which
 *     is invoked with the newly instantiated element before it is attached to
 *     the DOM.
 */
export function setUpElementUnderTest(elementName) {
  let element;

  afterEach(() => {
    if (element) {
      document.body.removeChild(element);
      element = null;
    }
  });

  return (finishSetupCallbackFn) => {
    element = document.createElement(elementName);
    if (finishSetupCallbackFn !== undefined) {
      finishSetupCallbackFn(element);
    }
    document.body.appendChild(element);
    return element;
  };
}

/**
 * Returns a promise that will resolve when the given DOM event is caught at the
 * document's body element, or reject if the event isn't caught within the given
 * amount of time.
 *
 * Sample usage:
 *
 *   // Code under test.
 *   function doSomethingThatTriggersCustomEvent() {
 *     ...
 *     this.dispatchEvent(
 *         new CustomEvent('my-event', {detail: 'hello world', bubbles: true});
 *   }
 *
 *   // Test.
 *   it('should trigger a custom event', async () => {
 *     const myEvent = eventPromise('my-event');
 *     doSomethingThatTriggersCustomEvent();
 *     const ev = await myEvent;
 *     expect(ev.detail).to.equal('hello world');
 *   });
 *
 * Set timeoutMillis = 0 to skip call to setTimeout(). This is necessary on
 * tests that simulate the passing of time with sinon.useFakeTimers(), which
 * could trigger the timeout before the promise has a chance to catch the event.
 *
 * @param event {string} Name of event to catch.
 * @param timeoutMillis {number} How long to wait for the event before rejecting
 *     the returned promise.
 * @return {Promise} A promise that will resolve to the caught event.
 */
export function eventPromise(event, timeoutMillis = 5000) {
  const eventCaughtCallback = (resolve, _, e) => resolve(e);
  const timeoutCallback = (_, reject) => reject(new Error(`timed out after ${timeoutMillis} ms `
                           + `while waiting to catch event "${event}"`));
  return buildEventPromise(
    event, timeoutMillis, eventCaughtCallback, timeoutCallback,
  );
}

/**
 * Returns a promise that will resolve if the given DOM event is *not* caught at
 * the document's body element after the given amount of time, or reject if the
 * event is caught.
 *
 * Useful for testing code that emits an event based on some condition.
 *
 * Sample usage:
 *
 *   // Code under test.
 *   function maybeTriggerCustomEvent(condition) {
 *     if (condition) {
 *       this.dispatchEvent(
 *         new CustomEvent('my-event', {detail: 'hello world', bubbles: true});
 *     } else {
 *       // Do nothing.
 *     }
 *   }
 *
 *   // Test.
 *   it('should not trigger a custom event', async () => {
 *     const noEvent = noEventPromise('my-event');
 *     maybeTriggerCustomEvent(false);
 *     await noEvent;
 *   });
 *
 * Set timeoutMillis = 0 to skip call to setTimeout(). This is necessary on
 * tests that simulate the passing of time with sinon.useFakeTimers(), which
 * could trigger the timeout before the promise has a chance to catch the event.
 *
 * @param event {string} Name of event to catch.
 * @param timeoutMillis {number} How long to wait for the event before rejecting
 *     the returned promise.
 * @return {Promise} A promise that will resolve to the caught event.
 */
export function noEventPromise(event, timeoutMillis = 200) {
  const eventCaughtCallback = (_, reject) => reject(new Error(`event "${event}" was caught when none was expected`));
  const timeoutCallback = (resolve, _) => resolve();
  return buildEventPromise(
    event, timeoutMillis, eventCaughtCallback, timeoutCallback,
  );
}

/**
 * Helper function to construct promises based on DOM events.
 *
 * @param event {string} Name of event to add a listener for at document.body.
 * @param timeoutMillis {number} Milliseconds to wait before timing out.
 * @param eventCaughtCallback {Function} Called when the event is caught with
 *     parameters (resolve, reject, event), where resolve and reject are the
 *     functions passed to the promise's executor function, and event is the
 *     Event object that was caught.
 * @param timeoutCallback Called after timeoutMillis if no event is caught, with
 *     arguments (resolve, reject) as passed to eventCaughtCallback.
 * @return {Promise} A promise that will resolve or reject based exclusively on
 *     what the callback functions do with the resolve and reject parameters.
 */
function buildEventPromise(
  event, timeoutMillis, eventCaughtCallback, timeoutCallback,
) {
  // The executor function passed as a constructor argument to the Promise
  // object is executed immediately. This guarantees that the event handler
  // is added to document.body before returning.
  return new Promise((resolve, reject) => {
    let timeout;
    const handler = (e) => {
      document.body.removeEventListener(event, handler);
      clearTimeout(timeout);
      eventCaughtCallback(resolve, reject, e);
    };
    // Skip setTimeout() call with timeoutMillis = 0. Useful when faking time in
    // tests with sinon.useFakeTimers(). See
    // https://sinonjs.org/releases/v7.5.0/fake-timers/.
    if (timeoutMillis !== 0) {
      timeout = setTimeout(() => {
        document.body.removeEventListener(event, handler);
        timeoutCallback(resolve, reject);
      }, timeoutMillis);
    }
    document.body.addEventListener(event, handler);
  });
}

/**
 * Asserts that there the given string exactly matches the current query string
 * of the url bar. For example, expectQueryStringToEqual('?foo=bar&foo=orange');
 */
export function expectQueryStringToEqual(expected) {
  expect(window.location.search).to.equal(expected);
}