James Garbutt

Software engineer. Front-end @crispthinking.

LitElement with Rollup, Babel & Karma

September 25, 2018

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LitElement is a cool new project from the Polymer team. It serves as a great base to start from when building web components, providing common necessities such as data binding, performant template rendering and life-cycle methods.

As a regular contributor to many Polymer projects, I find myself playing around with Lit quite a lot. Things appear to be fine, everything nicely structured and everything works just as the team has defined…

However, in reality, a lot of people who discover Polymer and Lit will be coming from backgrounds where they have used something completely different or at least something with a very different development workflow.

One of those common workflows is the combination of Rollup, Babel and Karma.

So let’s take a look at how we can apply those while using LitElement…

Project Structure

To begin with, I threw up a pretty basic project structure:

- src/
  - my-element.js
  - index.js
- package.json
- package-lock.json
- .editorconfig

The idea is that we will use Babel to transpile our sources into a lib/ directory and use Rollup to bundle those into a bundle.js.

Our Element

Our obvious dependencies are needed as a start:

$ npm i -S @polymer/lit-element

The element being used for this setup is as follows:

// src/my-element.js
import {LitElement, html, property} from '@polymer/lit-element';

export class MyElement extends LitElement {
  @property({type: String})
  foo = 'test';

  render() {
    return html`<h1>${this.foo}</h1>`;

customElements.define('my-element', MyElement);

// src/index.js
export {MyElement} from './my-element';

As you can see, it is fairly straight forward. More information on the lifecycle methods and property setup can be found here and here.

We’re just defining a very simple element which renders its only property, foo.


First of all, we need to install Babel along with any plugins and presets:

$ npm i -D @babel/core @babel/cli @babel/preset-env
$ npm i -D @babel/plugin-proposal-class-properties @babel/plugin-proposal-decorators

The first three are pretty much to allow us to transpile from ES2015 and beyond to a given level (in our case, to ES modules).

The two plugins are needed to allow us the use of Lit’s decorators as you saw above:

export class MyElement extends LitElement {
  @property({type: String})
  foo = 'test';
  /* ... */

After installing these, a very simple babelrc will do:

  "presets": [
        "modules": false,
        "targets": {
          "esmodules": true
  "plugins": [
      { "legacy": true }
      { "loose": true }

To try explain this a little, we:

  • Enable @babel/preset-env to turn on common Babel features
  • Disable module generation ("modules": false) to prevent Babel outputting CommonJS modules but rather have it output ES Modules
  • Enable the ES modules target so only browsers which support it are targeted
  • Enable legacy decorators (until LitElement supports the newer decorator proposal)
  • Enable loose class properties (until we can stop using legacy decorators)

This combination of things will result in us being able to use the provided decorators and have our element properties defined as class properties.

If we didn’t have any of this, our class would instead use the static properties getter:

export class MyElement extends LitElement {
  static get properties() {
    return {
      foo: { type: String }

Now if we:

$ node_modules/.bin/babel src -d lib
Successfully compiled 2 files with Babel.

We will have a populated lib/ directory containing our transpiled sources.

We should add this as a script in package.json to make things easier:

"scripts": {
  "build": "babel src -d lib",

So we can:

$ npm run build
Successfully compiled 2 files with Babel.

ESLint (optional)

It is fairly easy from this point to add ESLint to the workflow, though not required at all:

$ npm i -D eslint babel-eslint eslint-plugin-lit

babel-eslint is an ESLint parser so we can have ESLint directly understand our pre-transpiled sources. eslint-plugin-lit introduces some handy lint rules specific to Lit.

I used a config like so:

  "extends": "eslint:recommended",
  "parser": "babel-eslint",
  "rules": {
    "lit/no-legacy-template-syntax": "warn"
  "plugins": ["lit"]

You can add whichever rules you want from the Lit plugin, or none at all if you wish.

Then you can simply introduce a new NPM script:

"scripts": {
  "build": "babel src -d lib",

  "lint": "eslint \"src/**/*.js\""

To run:

$ npm run lint



So far, we do have some sources a browser can understand. However, we do not have support for Node (CommonJS) modules or the Node resolution algorithm.

This is where Rollup comes in:

// without Rollup or a loader/polyfill
import {LitElement} from './node_modules/@polymer/lit-element/lit-element.js';

// with Rollup
import {LitElement} from '@polymer/lit-element';
import someModule from 'a-node-module';

If we use Rollup, we can use all our Node modules as normal and simply have them bundled at build time.

Installing is pretty simple:

$ npm i -D rollup rollup-plugin-commonjs rollup-plugin-node-resolve

The two plugins here give Rollup a helping hand with resolving Node dependencies and understanding CommonJS modules (as opposed to the more modern ES modules).

We can then throw up a config file as simple as:

import resolve from 'rollup-plugin-node-resolve';
import common from 'rollup-plugin-commonjs';

export default {
  input: 'lib/index.js',
  output: {
    file: 'bundle.js',
    format: 'esm'
  plugins: [

This is as simple as it looks, it will use the two plugins mentioned earlier and bundle our lib/index.js along with its dependencies into a bundle.js.

We can now introduce yet another NPM script:

"scripts": {
  "build": "babel src -d lib",
  "lint": "eslint \"src/**/*.js\"",

  "bundle": "npm run build && rollup -c"

You can see how we chained the Babel build so when we call:

$ npm run bundle

We will actually transpile the sources, then bundle the output.

Now if we were to load our bundle in a browser:

<script type="module" src="bundle.js"></script>

We would make <my-element> available just as expected.


The last piece of the puzzle, which nobody should ever forget about, is testing!

You may be wondering why we don’t use something like Jest here. The answer to that is very simple: Jest does not (yet?) support browser testing; instead it uses JSDOM which does not yet support the web components APIs.

So, for now (or if you prefer Karma), we can use Karma instead:

$ npm i -D karma karma-chrome-launcher karma-mocha chai

This is a pretty common bunch of dependencies: Karma with Mocha support and Chai assertions.

We can also install Puppeteer to automatically download an appropriate Chrome binary for us:

$ npm i -D puppeteer

Our karma.conf.js is simple too:

// Trick to use the auto-downloaded puppeteer chrome binary
process.env.CHROME_BIN = require('puppeteer').executablePath();

module.exports = function(config) {
    basePath: '',
    frameworks: ['mocha'],
    files: [
    reporters: ['progress'],
    autoWatch: true,
    browsers: ['ChromeHeadless'],
    singleRun: false

We are going to create a second bundle specifically for tests (bundle.test.js) and want to use headless Chrome.

To create this second bundle, firstly we need another rollup plugin:

$ npm i -D rollup-plugin-multi-entry

The reason for this is that Rollup doesn’t provide multiple entry point support by default, rather it expects a single entry point and a single output (src/index.js -> lib/index.js).

With this plugin, we can now create a rollup.test.config.js:

import resolve from 'rollup-plugin-node-resolve';
import common from 'rollup-plugin-commonjs';
import multiEntry from 'rollup-plugin-multi-entry';

export default {
  input: 'lib/test/**/*.js',
  output: {
    file: 'bundle.test.js',
    format: 'esm'
  plugins: [
      namedExports: {
        'chai': ['expect']

This is all fairly simple… we now use a glob input (lib/test/**/*.js) and have added multiEntry() as a plugin.

We have also specified namedExports because the Rollup plugin has trouble automatically detecting what chai exports, unfortunately.

Another two NPM scripts are then added:

"scripts": {
  "build": "babel src -d lib",
  "lint": "eslint \"src/**/*.js\"",
  "bundle": "npm run build && rollup -c",

  "bundle:test": "npm run build && rollup -c rollup.test.config.js",
  "test": "npm run bundle:test && karma start --singleRun"

The first (bundle:test) simply runs our existing Babel build and calls Rollup with our rollup.test.config.js.

The second (test) will run bundle:test and start the karma launcher to execute our test suite.

Example Test

An example test would be src/test/my-element.js:

import {expect} from 'chai';
// Very important to ensure any elements we test are loaded
import '../my-element';

describe('my-element', () => {
  let node;

  beforeEach(async () => {
    // Use createElement to test it is registered correctly
    node = document.createElement('my-element');

    // Connect to DOM in case there's any `connectedCallback` logic

    // Wait for initial render
    await node.updateComplete;

  afterEach(() => {
    // Remove from DOM, cleanup

  it('should render', () => {
    const span = node.shadowRoot.querySelector('h1');

As you can see, this too is pretty straight forward. We didn’t need any helpers, any fancy testing frameworks or custom loaders. We simply bundle our sources as normal and execute the full test suite in Chrome.


This is a very brief overview of how to set up this common work flow, so I expect there’s plenty missing. I do hope it has helped somewhat, though.

While most of the Polymer team and its community are happy to push the idea of everything being native and working as-is in the browser, it is not always reality.

Maybe you’re required to use this workflow to remain consistent with your other projects, or maybe you just like being able to use Node modules. Either way, this should at least give you an idea of how to stick to that process while also adopting something as new as Lit.

Now whether you use bundlers or not, you can try Lit out. Have a play around!