We believe that testing is fundamental to every production-ready product.

We recommend using BDD(Behavior Driven Development) as it seem to be easier when talking to non tech collegues. However note that this can still be a personal preference - we give this recommendation to promote unity within everyone using this recommendation.



npm init @open-wc
# Upgrade > Testing


yarn add @open-wc/testing --dev

Add to your test:

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

This will have the following side effects:

Automating Tests

Ideally, you'll want some way of automatically running all of your tests. To do that, we recommend karma via @open-wc/testing-karma and browserstack via @open-wc/testing-karma-bs. If you use a different test runner or a different browser grid you may skip these steps.


Already part of npm init @open-wc testing

Manual Install

  • Install via yarn add @open-wc/testing-karma --dev
  • Copy karma.conf.js to karma.conf.js
  • Add these scripts to package.json
    "scripts": {
      "test": "karma start --coverage",
      "test:watch": "karma start --auto-watch=true --single-run=false"

If you need to support legacy browsers

  "scripts": {
    "test:legacy": "karma start --legacy --coverage",
    "test:legacy:watch": "karma start --legacy --auto-watch=true --single-run=false",

For more details, please see testing-karma.

Automating Tests via Browserstack

To make sure your project is production ready, we recommend running tests in all the browsers you want to support. If you do not have access to all browsers, we recommend using a service like Browserstack to make sure your project works as intended.

The following step connects the automatic karma runner with browserstack.


Already part of npm init @open-wc testing

Manual Install

  • Install via yarn add @open-wc/testing-karma-bs --dev
  • Copy karma.bs.config.js to karma.bs.config.js
  • Add these scripts to your package.json
    "scripts": {
      "test:bs": "karma start karma.bs.config.js --legacy --coverage"

For more details, please see testing-karma-bs.

Example Tests

A typical Web Component test will look something like this:

/* eslint-disable no-unused-expressions */
import {
} from '@open-wc/testing';

import '../src/get-result.js';

describe('True Checking', () => {
  it('is false by default', async () => {
    const el = await fixture('<get-result></get-result>');

  it('false values will have a light-dom of <p>NOPE</p>', async () => {
    const el = await fixture('<get-result></get-result>');

  it('true values will have a light-dom of <p>YEAH</p>', async () => {
    const foo = 1;
    const el = await fixture(html`<get-result .success=${foo === 1}></get-result>`);

If you run your tests automatically with npm run test, the output should look like this:

> karma start demo/karma.conf.js
Webpack bundling...
Hash: 268cd16a4849f1191bd5
Version: webpack 4.26.1
Time: 1590ms
Built at: 12/18/2018 2:01:09 PM
             Asset       Size           Chunks             Chunk Names
        commons.js   1.93 MiB          commons  [emitted]  commons
get-result.test.js  337 bytes  get-result.test  [emitted]  get-result.test
        runtime.js   14.2 KiB          runtime  [emitted]  runtime
Entrypoint get-result.test = runtime.js commons.js get-result.test.js
18 12 2018 14:01:10.156:INFO [karma-server]: Karma v3.1.3 server started at
18 12 2018 14:01:10.160:INFO [launcher]: Launching browsers ChromeHeadlessNoSandbox with concurrency unlimited
18 12 2018 14:01:10.166:INFO [launcher]: Starting browser ChromeHeadless
18 12 2018 14:01:10.833:INFO [HeadlessChrome 70.0.3538 (Windows 10.0.0)]: Connected on socket 6PnebgDwW91bPrHzAAAA with id 55371387
  True Checking
    ✔ is false by default
    ✔ false values will have a light-dom of <p>NOPE</p>
    ✔ true values will have a light-dom of <p>YEAH</p>
Finished in 0.12 secs / 0.029 secs @ 14:01:11 GMT+0100 (GMT+01:00)
✔ 3 tests completed
=============================== Coverage summary ===============================
Statements   : 100% ( 8/8 )
Branches     : 100% ( 2/2 )
Functions    : 100% ( 3/3 )
Lines        : 100% ( 8/8 )


Make sure you have Chrome (or Chromium) installed. Additionally you may need to set your CHROME_BIN env variable export CHROME_BIN=/usr/bin/chromium-browser.

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/index-no-side-effects.js';

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

// Alternatively, you can add the fixtureCleanup in the afterEach function.
// Note that this is exactly what the automatically registered side-effect does.
afterEach(() => {
Last Updated: 1/19/2019, 12:51:15 PM