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Custom Elements Everywhere 🍻

What is this?

Karma tests for each of the major frameworks to see how they handle working with Custom Elements.

Build Status Renovate enabled


To install all dependencies and build the site:

# Install all the things!
npm ci

# Test all the things!
npm run build

# Preview the site.
npm start

Current List of Libraries/Frameworks

How do I add a library/framework to the project?

Step 1. Copy an existing example

Tests for each library/framework live in the libraries/ directory. The easiest way to start is by copying the test directory from a project that is similar to your own. For example, if the library you use is similar to React/Preact, you might start by copying and renaming the libraries/preact directory.

Your library structure should look like this:

├── .babelrc
├── karma.conf.js
├── meta
│   ├── issues.json
│   └──
├── package-lock.json
├── package.json
├── src
│   ├── advanced-tests.js
|   ├── basic-tests.js
│   └── components.js
└── tests.webpack.js


Your package.json should contain build and test npm scripts. The test script is in charge of actually running karma. The test script should set a variable, LIBRARY_NAME, that corresponds to your library's npm package name. This is used during the build process to grab the library's semver and publish it on the site.


"scripts": {
  "test": "cross-env LIBRARY_NAME=@angular/core karma start",
  "build": "npm run test"


Your Karma configuration. Ideally you shouldn't need to change much in here. The config file uses karma-webpack, so there is a webpack property where you can essentially write your webpack.config.js. You'll need to change this property to tell it how to bundle your library.


This directory contains issues.json where you list any open GitHub issues related to custom element support in your library. There is also a where you write a short description of how the library interacts with custom elements and any known quirks or gotchas.


This directory contains components.js where you create library/framework components which try to communicate with custom elements. You then test these components in basic-tests.js and advanced-tests.js. You'll want to use all of the assertions in the test files but update the actual test implementations to use your library's components.

Note that all frameworks use the custom elements in the /libraries/__shared__/webcomponents/ directory for tests.


This file is consumed by the test runner and tells it to import any files ending in -test. You probably won't need to change this file.

Step 2. Add npm scripts

In the root of the project you'll need to add a couple of npm scripts to make sure your library builds with the rest of the site. You should be able to copy an example from one of the other libraries.

  • In the root of the project, Add an install-* script to package.json and run it during the install script.
  • In the libraries/[your library]/ director, update the build script in package.json to include your library's name.

There's a test runner called index.js that lives in the root of the project. It will detect when you've added a new folder to libraries/ and attempt to run that folder's build command.

Step 3. Run update-goldens

In the root of the project, run npm run update-goldens. This will generate an expectedResults.json file for your library's tests.

This way any change that results in a different summary score for any library shows up in code review, and any change that would accidentally change the summary score will make the tests fail.

What kind of behavior do the tests assume?

  • The library/framework should be able to display elements that use shadow DOM, insert children in them, and handle hiding and showing them.
  • The library/framework should be able to pass primitive data (strings, numbers, booleans) to a custom element as either attributes or properties.
  • The library/framework should be able to pass rich data (objects, arrays) to a custom element using properties.
  • The library/framework should be able to listen to DOM events from a custom element. These DOM events could use any casing style.

How does the site get deployed/maintained?

Any PR landed to the main branch will trigger an automatic publish to GitHub pages.


Copyright 2017 Google, Inc.

Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except in compliance with the License. You may obtain a copy of the License at

Unless required by applicable law or agreed to in writing, software distributed under the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the License for the specific language governing permissions and limitations under the License.


Custom Element + Framework Interoperability Tests.







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