See how Stencil fits into the entire Ionic Ecosystem ->
Stencil is part of the Ionic Ecosystem ->

Custom Elements Bundle

The dist-custom-elements-bundle output target is used to generate custom elements as a single bundle. Even though the output ends up as a "single" bundle, it's generated to ensure components are tree-shakable. For example, if a component library has 100 components, but an external project only imported one component from the bundle, then only the code used by that one component would be pulled into the project. This is due to Stencil's use of ES Modules and the compiler generating friendly code for bundlers to parse and understand.

outputTargets: [
  {
    type: 'dist-custom-elements-bundle',
  },
];

Defining Exported Custom Elements

By default, the custom elements bundle will be written to dist/custom-elements/index.js. This directory can be configured using the output target's dir config. The generated index.js file contains exports to each component class and will have their styles included within the bundle. However, this build does not automatically define the custom elements or apply any polyfills.

Below is an example of defining a custom element within the bundle:

import { HelloWorld } from 'my-library/dist/custom-elements';

customElements.define('hello-world', HelloWorld);

If a component is dependent on other components, you'll need to register each one manually. This can be tedious so, for convenience, the bundle also exports a defineCustomElements() method.

When defineCustomElements() is called, it will define every component in the bundle. However, it does not run automatically and it will not be called if it is not imported and executed. It can also result in a larger bundle size if there are unused components being imported.

import { defineCustomElements } from 'my-library/dist/custom-elements';

defineCustomElements();

The generated bundle will export each component class and will already have the styles bundled. However, it does not define the custom elements or apply any polyfills.

Making Assets Available

For performance reasons, the generated bundle does not include local assets built within the JavaScript output, but instead it's recommended to keep static assets as external files. By keeping them external this ensures they can be requested on-demand, rather than either welding their content into the JS file, or adding many URLs for the bundler to add to the output. One method to ensure local assets are available to external builds and http servers is to set the asset path using setAssetPath().

The setAssetPath() function is used to manually set the base path where static assets can be found. For the lazy-loaded output target the asset path is automatically set and assets copied to the correct build directory. However, for custom elements builds, the setAssetPath(path) should be used to customize the asset path depending on where they are found on the http server.

If the component's script is a type="module", it's recommended to use import.meta.url, such as setAssetPath(import.meta.url). Other options include setAssetPath(document.currentScript.src), or using a bundler's replace plugin to dynamically set the path at build time, such as setAssetPath(process.env.ASSET_PATH).

import { defineCustomElements, setAssetPath } from 'my-library/dist/custom-elements';

setAssetPath(document.currentScript.src);
defineCustomElements();

Make sure to copy the assets over to a public directory in your app. This configuration depends on how your script is bundled, or lack of bunding, and where your assets can be loaded from. How the files are copied to the production build directory depends on the bundler or tooling. The configs below provide examples of how to do this automatically with popular bundlers.

Distributing Custom Elements

Your component library can be easily distributed on NPM, similar to how @ionic/core does it. From there, consumers of your library can decide how to import your library into their project. For the dist-custom-elements-bundle, the default import location would be my-library/dist/custom-elements-bundle, but this can get further configured within the package.json file.

To make the custom elements bundle the entry module for a package, set the module property in package.json to:

Also be sure to set @stencil/core as a dependency of the package.

{
  "module": "dist/custom-elements-bundle/index.js",
  "dependencies": {
    "@stencil/core": "latest"
  },
  ...
}

Note: If you are distributing both the dist and dist-custom-elements-bundle, then it's up to you to choose which one of them should be available in the module entry.

Now you can publish your library to Node Package Manager (NPM). For more information about setting up the package.json file, and publishing, see: Publishing Component Library To NPM.

Example Bundler Configs

Instructions for consuming the custom elements bundle vary depending on the bundler you're using. These examples will help your users consume your components with webpack and Rollup.

The following examples assume your component library is published to NPM as my-library. You should change this to the name you actually publish your library with.

Users will need to install your library before importing them.

npm install my-library

webpack.config.js

A webpack config will look something like the one below. Note how assets are copied from the library's node_module folder to dist/assets via the CopyPlugin utility. This is important if your library includes includes local assets.

const path = require('path');
const CopyPlugin = require('copy-webpack-plugin');

module.exports = {
  entry: './src/index.js',
  output: {
    filename: 'main.js',
    path: path.resolve(__dirname, 'dist'),
  },
  module: {
    rules: [
      {
        test: /\.css$/i,
        use: ['style-loader', 'css-loader'],
      },
    ],
  },
  plugins: [
    new CopyPlugin({
      patterns: [
        {
          from: path.resolve(__dirname, 'node_modules/my-library/dist/my-library/assets'),
          to: path.resolve(__dirname, 'dist/assets'),
        },
      ],
    }),
  ],
};

rollup.config.js

A Rollup config will look something like the one below. Note how assets are copied from the library's node_module folder to dist/assets via the rollup-copy-plugin utility. This is important if your library includes local assets.

import path from 'path';
import commonjs from '@rollup/plugin-commonjs';
import copy from 'rollup-plugin-copy';
import postcss from 'rollup-plugin-postcss';
import resolve from '@rollup/plugin-node-resolve';

export default {
  input: 'src/index.js',
  output: [{ dir: path.resolve('dist/'), format: 'es' }],
  plugins: [
    resolve(),
    commonjs(),
    postcss({
      extensions: ['.css'],
    }),
    copy({
      targets: [
        {
          src: path.resolve(__dirname, 'node_modules/my-library/dist/my-library/assets'),
          dest: path.resolve(__dirname, 'dist'),
        },
      ],
    }),
  ],
};

How is this different from the "dist" output target?

The dist-custom-elements-bundle builds each component as a stand-alone class that extends HTMLElement. The output is a standardized custom element with the styles already attached and without any of Stencil's lazy-loading. This may be preferred for projects that are already handling bundling, lazy-loading and defining the custom elements themselves.

The dist output target, on the other hand, is more for projects that want to allow components to lazy-load themselves, without having to setup bundling configurations to do so.

Luckily, both builds can be generated at the same time, using the same source code, and shipped in the same distribution. It would be up to the consumer of your component library to decide which build to use.

Browser Support

If the library is to be used on IE11 we recommend using the dist output target instead since it will only load the required polyfills on-demand. The dist-custom-elements-bundle is only recommended for modern browsers that already support Custom Elements, Shadow DOM, and CSS Variables (basically not IE11 or Edge 12-18). If this build is going to be used within legacy browsers then the project consuming these components will have to provide its own polyfills, and correctly downlevel the output to ES5.

Good news is that these are already widely supported for modern web develoment:

BackNext
Contributors