Rollup

Configuration

Rollup configuration to help you get started building modern web applications. You write modern javascript using the latest browser features, rollup will optimize your code for production ensure it runs on all supported browsers.

The input for rollup is the same index.html you use for development. Any module scripts in your index are run through rollup and your index is updated with the output from rollup.

Setup

New project

npm init @open-wc

Existing project

npm init @open-wc
# Upgrade > Building > Rollup

Manual setup

  1. Install the required dependencies:
npm i -D @open-wc/building-rollup rollup rimraf es-dev-server
  1. Create a rollup.config.js file and pass in your app's index.html:
import createDefaultConfig from '@open-wc/building-rollup/modern-config';

// if you need to support IE11 use "modern-and-legacy-config" instead.
// import createDefaultConfig from '@open-wc/building-rollup/modern-and-legacy-config';

export default createDefaultConfig({ input: './index.html' });
  1. Create an index.html:
<!doctype html>
<html>
  <head></head>
  <body>
    <my-app></my-app>

    <script type="module" src="./src/my-app.js"></script>
  </body>
</html>

We use rollup-plugin-index-html which takes your index.html as input for rollup. It scans for any <script type="module" src="..."> and sends them to rollup for bundling, and outputs your index.html in the output directory.

  1. Add the following commands to your package.json:
{
  "scripts": {
    "build": "rimraf dist && rollup -c rollup.config.js",
    "start:build": "es-dev-server --app-index dist/index.html --open"
  }
}
  • start runs your app for development, reloading on file changes
  • start:build runs your app after it has been built using the build command
  • build builds your app and outputs it in your dist directory

Supporting legacy browsers

modern-config.js works for browsers which support es modules. If you need to support older browsers such as IE11 you need to use our modern-and-legacy-config.js in your rollup.config.js:

import createDefaultConfig from '@open-wc/building-rollup/modern-and-legacy-config';

export default createDefaultConfig({ input: './index.html' });

In addition to outputting a regular build of your app, it outputs a legacy build which is compatible with older browsers down to IE11.

At runtime we determine which version of your app should be loaded, so that legacy browsers don't force you to ship more and slower code to most users on modern browsers.

Config features

modern-config.js:

  • compatible with browsers which support es modules
  • babel transform based on browser support (no es5 for all browsers)
  • load polyfills when needed:
    • dynamic import
    • webcomponents
  • resolve bare imports ( import { html } from 'lit-html' )
  • preserve import.meta.url value from before bundling
  • minify + treeshake js
  • minify html and css in template literals

modern-and-legacy-config.js:

  • Two build outputs:
    • Modern build:

      • es modules
      • compatible with browsers which support es modules
      • babel transform based on browser support (no es5 for all browsers)
    • Legacy build:

      • systemjs modules
      • compatible down to IE11
      • babel transform down to IE11 (es5)
      • core js polyfills (Promise, Symbol, String.prototype.includes etc.)
    • Both:

      • resolve bare imports ( import { html } from 'lit-html' )
      • preserve import.meta.url value from before bundling
      • load polyfills when needed:
        • dynamic import
        • webcomponents
        • fetch
      • minify + treeshake js
      • minify html and css in template literals

Adjusting browser support for the modern build

The legacy build targets IE11, which is the earliest browser supported by the webcomponents polyfill. For the modern build we target the lates 2 versions of the major browsers (chrome, firefox, safari and edge).

You can adjust this by adding a browserslist configuration. For example by adding a .browserslistrc file to your project, or adding an entry to your package.json. See the browserslist documentation for more information.

Warning: you should not add IE11 or other very early browsers as a target in your browserslist, as it would result in a broken modern build because it makes some assumptions around browser support. Use the --legacy flag for legacy builds.

Customizing the babel config

You can define your own babel plugins by adding a .babelrc or babel.config.js to your project. See babeljs config for more information.

For example to add support for class properties:

{
  "plugins": [
    "@babel/plugin-proposal-class-properties"
  ]
}

Extending the rollup config

A rollup config is just a plain object. It's easy to extend it using javascript:

import createDefaultConfig from '@open-wc/building-rollup/modern-config';

const config = createDefaultConfig({ input: './index.html' });

export default {
  ...config,
  output: {
    ...config.output,
    sourcemap: false,
  },
  plugins: [
    ...config.plugins,
    myAwesomePlugin(),
  ],
};

If you use modern-and-legacy-config.js, it is actually an array of configs so that rollup outputs a modern and a legacy build. Simply map over the array to adjust both configs:

import createDefaultConfig from '@open-wc/building-rollup/modern-and-legacy-config';

const configs = createDefaultConfig({ input: './index.html' });

export default configs.map(config => ({
  ...config,
  output: {
    ...config.output,
    sourcemap: false,
  },
  plugins: [
    ...config.plugins,
    myAwesomePlugin(),
  ],
}));

Common extensions

Some extensions or plugins add non-native or experimental features to your code. This can be bad for the maintenance of your code in the long term, we therefore don't recommend it unless you know what you're doing.

Customizing index.html output

If you need to customize the output of your index.html you can pass extra options to rollup-plugin-index-html:

import createDefaultConfig from '@open-wc/building-rollup/modern-config';

export default createDefaultConfig({
  input: './index.html',
  indexHTMLPlugin: {
    polyfills: {
      fetch: false,
      intersectionObserver: true,
    },
  }
});

See the plugin docs for all options.

non index.html entrypoint

By default we look for an index.html as entrypoint. If want to use regular entrypoints you will need to provide your index.html for output manually:

import createDefaultConfig from '@open-wc/building-rollup/modern-config';

export default createDefaultConfig({
  input: './my-app.js',
  indexHTMLPlugin: {
    // inline
    indexHTML: `
      <html>
        <head></head>
        <body></body>
      </html>
    `,

    // from file
    indexHTML: fs.readFileSync('/path/to/index.html', 'utf-8'),
  }
});

Resolve commonjs modules

CommonJS is the module format for NodeJS, and not suitable for the browser. Rollup only handles es modules by default, but sometimes it's necessary to be able to import a dependency. To do this, you can add rollup-plugin-commonjs:

import createDefaultConfig from '@open-wc/building-rollup/modern-and-legacy-config';
import commonjs from 'rollup-plugin-commonjs';

const configs = createDefaultConfig({ input: './index.html' });

// map if you use an array of configs, otherwise just extend the config
export default configs.map(config => ({
  ...config,
  plugins: [
    ...config.plugins,
    commonjs(),
  ],
}));

Copy assets

Web apps often include assets such as CSS files and images. These are not part of your regular dependency graph (see above), so they need to be copied into the build directory. Other files you might need to copy this way are e.g. fonts, JSON files, sound or video files, and HTML files (other than the index.html referenced in the input option) etc.

Rollup-plugin-cpy is a plugin that can be used, but there are other options, too.

import cpy from 'rollup-plugin-cpy';
import createDefaultConfig from '@open-wc/building-rollup/modern-and-legacy-config';

const config = createDefaultConfig({
  input: './index.html',
});

// if you use an array of configs, you don't need the copy task to be executed for both builds.
// we can add the plugin only to the first rollup config:
export default [
  // add plugin to the first config
  {
    ...config[0],
    plugins: [
      ...config[0].plugins,
      cpy({
        // copy over all images files
        files: ['**/*.png'],
        dest: 'dist',
        options: {
          // parents makes sure to preserve the original folder structure
          parents: true
        }
      }),
    ],
  },

  // leave the second config untouched
  config[1],
];

Support typescript

To import a typescript file, use the .ts extension in your index.html:

<html>
<head></head>
<body>
  <my-app></my-app>
  <script type="module" src="./src/my-app.ts"></script>
</body>
</html>

Make sure you set your tsconfig.json target and module fields to ESNext. This way tsc, the typescript compiler, won't do any compilation so that a plugin can take care of it.

Within rollup there are two options to add typescript support.

1. Babel

We recommend using the babel typescript plugin. Add it to your babel config file (.babelrc or babel.config.js):

{
  "presets": [
    "@babel/preset-typescript"
  ],
}

You also need to specify .ts in the extensions option, for babel and node to properly recognize ts files:

const configs = createDefaultConfig({
  input: './index.html',
  extensions: ['.js', '.mjs', '.ts'],
});

(keep .js in there, since node will want to resolve javascript files in node_modules)

This is the fastest method, as it strips away types during babel transformation of your code. It will not perform any type checking though. We recommend setting up the type checking as part of your linting setup, so that you don't need to run the typechecker during development for faster builds.

2. Plugin

It is also possible to add the rollup typescript plugin, which does typechecking and compiling for you:

import typescript from 'rollup-plugin-typescript2';
import createDefaultConfig from '@open-wc/building-rollup/modern-and-legacy-config';

const configs = createDefaultConfig({
  input: './index.html',
});

export default configs.map(config => ({
  ...config,
  plugins: [
    ...config.plugins,
    typescript(),
  ],
}));

Disable typescript compilation

We already mentioned this above, but this is really important: Make sure to prevent any compilation done by the typescript compiler (tsc). If you use one of the options above, you put babel or rollup in charge of the compilation of typescript. In no case do you want multiple compilers to interfere with each other.

You can do this by setting the following options in tsconfig.json:

{
  "compilerOptions": {
    "target": "ESNext",
    "module": "ESNext",
  }
}

Import CSS files in lit-html

To separate your lit-html styles in css files, you can use rollup-plugin-lit-css:

import createDefaultConfig from '@open-wc/building-rollup/modern-and-legacy-config';
import litcss from 'rollup-plugin-lit-css';

const configs = createDefaultConfig({ input: './index.html' });

// map if you use an array of configs, otherwise just extend the config
export default configs.map(config => ({
  ...config,
  plugins: [
    ...config.plugins,
    litcss({ include, exclude, uglify })
  ],
}));
Last Updated: 3/2/2019, 10:06:16 PM