Testing Web Components

Things to consider when we want to test our components with: Styles, Templates, Component classes.

Webpack and Jest

Jest is used by Facebook to test all JavaScript code including React applications. One of Jest’s philosophies is to provide an integrated “zero-configuration” experience. We observed that when engineers are provided with ready-to-use tools, they end up writing more tests, which in turn results in more stable and healthy code bases.

Full working example is available on GitHub as web-components-webpack-es6-boilerplate

Jest runs tests in NodeJS enviroment with jsdom. The whole process is easy. Let’s consider following webpack setup, assuming our project structure looks like a following example:


A simple directory structure designed to separate the server render logic from the rest. Webpack config.js file would contain following modules:

  resolve: {
    modules: ["node_modules"],
    alias: {
      client: path.join(__dirname, "../src/client"),
      server: path.join(__dirname, "../src/server")
    extensions: [".js", ".json", ".scss"]

We can set up Jest to reflect our Webpack config.

module.exports = {
  setupTestFrameworkScriptFile: "<rootDir>/bin/jest.js",
  mapCoverage: true,
  moduleFileExtensions: ["js", "scss", "html"],
  moduleDirectories: ["node_modules"],
  moduleNameMapper: {
    "src/(.*)$": "<rootDir>/src/$1"
  transform: {
    "^.+\\.(js|html|scss)$": "<rootDir>/bin/preprocessor.js"
  testMatch: ["<rootDir>/test/**/?(*.)(spec|test).js"],
  testPathIgnorePatterns: ["<rootDir>/(node_modules|bin|build)"]

Where should we save this config ?

We can do it in the package.json file under jest key or create as in this example jest.config.js file in the project root.

What we want to achieve is to make sure that our html files are going to be imported correctly. That means by escaping them with custom preprocessor, as using only babel-jest would throw error when trying to parse non js files.

The other important thing here is setupTestFrameworkScriptFile script which actually includes custom elements polyfills to jsdom. Here is how our preprocessor.js looks like:

const babelJest = require("babel-jest");

const STYLE_URLS_REGEX = /styles:\s*\[\s*((?:'|").*\s*(?:'|")).*\s*.*\]/g;
const ESCAPE_TEMPLATE_REGEX = /(\${|\`)/g;

module.exports.process = (src, path, config) => {
  if (path.endsWith(".html")) {
    src = src.replace(ESCAPE_TEMPLATE_REGEX, "\\$1");
    src = "module.exports=`" + src + "`;";
  src = src.replace(STYLE_URLS_REGEX, "styles: []");

  return babelJest.process(src, path, config);

What this script does, is simple: remove style files content as we do not need/want to test it, and escape templates, when we import them for example with require('template.html') syntax. Then it passes down content to babel transformer.

Last important thing to do is to include web components polyfills. As by default jsdom does not support them yet. To do it we can simply add setupTestFrameworkScriptFile in our example it is jest.js with the following content:


This way we can access web components API in jsdom.

After setting up everything we should have structure like this:


Where we keep our tests in the test directory and can run it with command: yarn run jest --no-cache --config $(node jest.config.js).