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Testing: Events

Testing events can be tricky; the test needs to set up it's own test-specific event listener before the event fires. In most cases, this can be easily accomplished with the oneEvent helper.

it('throws an event with the expected value', async () => {
  const listener = oneEvent(component, 'change');
  const { detail } = await listener;
  expect(detail).to.equal('expected value');

Note that the return type of oneEvent is Promise<Event>. In the above snippet, we first define the promise, then execute the user action (click), then finally await the promise' resolution. We could also accomplish that by cueing the action up in a setTimeout callback.

it('throws an event with the expected value', async () => {
  const clickButton = () => component.shadowRoot.querySelector('button').click();
  const { detail } = await oneEvent(component, 'change');
  expect(detail).to.equal('expected value');

That's how we can easily test events fired as a result of user actions. Now we'll modify the above technique for events which are fired automatically during lifecycle callbacks.

Events Fired from Lifecycle Callbacks

Let's say you have a mixin that dispatches an event on firstUpdated and connectedCallback:

const mixin = superclass =>
  class extends superclass {
    firstUpdated() {
      this.dispatchEvent(new CustomEvent('first-updated'));

    connectedCallback() {
      this.dispatchEvent(new CustomEvent('connected-callback'));

These are the suggested ways of testing these events:


Testing firstUpdated is a little tricky. As soon as you add the component to the DOM, it is executed - with an arbitrary delay depending on the work your component does see lit-element lifecycle.

To test this, you can use fixtureSync and await oneEvent to resolve the event.

it('dispatches a custom event on firstUpdated', async () => {
  const tag = class extends mixin(LitElement) {};
  const el = fixtureSync(`<${tag}></${tag}>`);
  const ev = await oneEvent(el, 'first-updated');


For connectedCallback, since this callback is fired immediately after calling fixture, you can't catch it anymore. What you can do is define a new component and test its callback function in a setTimeout.

it('test', async () => {
  const tag = defineCE(class extends mixin(LitElement) {});
  const foo = document.createElement(`${tag}`);
  setTimeout(() => foo.connectedCallback());
  const ev = await oneEvent(foo, 'connected-callback');