Testing Helpers

In order to efficiently test Web Components you will need some helpers to register and instantiate them for you.

This is part of the default open-wc testing recommendation

Testing helpers requires as a peer dependency lit-html. You can install it inside your project with npm :

npm i --save lit-html

Test a custom element

import { fixture } from '@open-wc/testing-helpers';

it('can instantiate an element', async () => {
  const el = await fixture('<my-el foo="bar"></my-el>');
  expect(el.getAttribute('foo')).to.equal('bar');
}

Test a custom element with properties

import { html, fixture } from '@open-wc/testing-helpers';

it('can instantiate an element with properties', async () => {
  const el = await fixture(html`<my-el .foo=${'bar'}></my-el>`);
  expect(el.foo).to.equal('bar');
}

Test a custom class

If you're testing a mixin, or have multiple base classes that offer a various set of options you might find yourself in the situation of needing multiple custom element names in your tests. This can be dangerous as custom elements are global, so you don't want to have overlapping names in your tests. Therefore we recommend using a the following function to avoid that.

import { fixture, defineCE } from '@open-wc/testing-helpers';

const tag = defineCE(class extends MyMixin(HTMLElement) {
  constructor() {
    super();
    this.foo = true;
  }
});
const el = await fixture(`<${tag}></${tag}>`);
expect(el.foo).to.be.true;

Test a custom class with properties

For lit-html it's a little tougher as it does not support dynamic tag names by default. This uses a workaround that's not performant for rerenders, which is fine for testing, but do NOT use this in production code.

import { html, fixture, defineCE, unsafeStatic } from '@open-wc/testing-helpers';

const tagName = defineCE(class extends MyMixin(HTMLElement) {
  constructor() {
    super();
    this.foo = true;
  }
});
const tag = unsafeStatic(tagName);
const el = await fixture(html`<${tag} .bar=${'baz'}></${tag}>`);
expect(el.bar).to.equal('baz');

Timings

By default fixture awaits the elements "update complete" Promise.

If none of those specfic Promise hooks are found, it will wait for one frame via await nextFrame().
Note: this does not guarantee that the element is done rendering - it just waits for the next JavaScript tick.

Essentially, fixture creates a synchronous fixture, then waits for the element to finish updating, checking updateComplete first, then falling back to componentReady(), and nextFrame() as a last resort.

This way, you can write your tests more succinctly, without having to explicitly await those hooks yourself.

const el = await fixture(html`<my-el .foo=${'bar'}></my-el>`);
expect(el.foo).to.equal('bar');

// vs

const el = fixtureSync(html`<my-el .foo=${'bar'}></my-el>`);
await elementUpdated(el);
expect(el.foo).to.equal('bar');

nextFrame

Uses requestAnimationFrame to wait for the next frame.

await nextFrame();

aTimeout

Waits for x ms via setTimeout;

await aTimeout(10); // would wait 10ms

Testing Events

If you want to interact with web components you will sometimes need to await a specific event before you can continue testing. Ordinarily, you might pass the done callback to a test, and call it in the body of an event handler. This does not work with async test functions, though, which must return a promise instead of calling the done callback. The oneEvent function helps you handle events in the context of the kinds of async test functions that we recommend. oneEvent resolves with the event specified when it fires on the element specified.

import { oneEvent } from '@open-wc/testing';

class FiresDone extends HTMLElement {
  fireDone() {
    this.done = true;
    this.dispatchEvent(new CustomEvent('done', { detail: this.done }));
  }
}

it('can await an event', async () => {
  const tag = defineCE(FiresDone);

  const el = await fixture(`<${tag}></${tag}>`);

  setTimeout(() => el.fireDone());

  const { detail } = await oneEvent(el, 'done');

  expect(el.done).to.be.true;
  expect(detail).to.be.true;
});

Testing Focus & Blur on IE11

Focus and blur events are synchronous events in all browsers except IE11. If you need to support that browser in your tests, you can await triggerFocusFor and triggerBlurFor helper functions.

import { triggerFocusFor, triggerBlurFor } from '@open-wc/testing';

it('can be focused and blured', async () => {
  const el = await fixture('<input type="text">');

  await triggerFocusFor(el);
  expect(document.activeElement === el).to.be.true;

  await triggerBlurFor(el);
  expect(document.activeElement === el).to.be.false;
});

Fixture Cleanup

By default, if you import anything via import { ... } from '@open-wc/testing-helpers';, it will automatically register a side-effect that cleans up your fixtures. If you want to be in full control you can do so by using

import { fixture, fixtureCleanup } from '@open-wc/testing-helpers/index-no-side-effects.js';

it('can instantiate an element with properties', async () => {
  const el = await fixture(html`<my-el .foo=${'bar'}></my-el>`);
  expect(el.foo).to.equal('bar');
  fixtureCleanup();
}

// Alternatively, you can add the fixtureCleanup in the afterEach function, but note that this is exactly what the automatically registered side-effect does.
afterEach(() => {
  fixtureCleanup();
});
Last Updated: 1/13/2019, 10:46:34 AM