🍣 A Rollup plugin which enables import assertions for top-level CSS and JSON modules.

This package is available on the npm registry under the name rollup-plugin-import-assert and can be installed with npm, yarn or however else you consume dependencies.

npm:

npm i -D rollup-plugin-import-assert

yarn:

yarn add -D rollup-plugin-import-assert

Once the plugin is installed, you will also need to make sure you have the acorn-import-assertions package installed. You can then add both items to your Rollup configuration as below:

import { importAssertionsPlugin } from 'rollup-plugin-import-assert';
import { importAssertions } from 'acorn-import-assertions';
export default {
  input: 'path/to/file.js'
  output: {
    format: 'esm',
    dir: 'lib' // only necessary to enable dynamic imports
  },
  acornInjectPlugins: [ importAssertions ],
  plugins: [ importAssertionsPlugin() ]
}

These two plugins will enable the import assertion syntax and behavior in your Rollup build. In your input file, you can import files using type assertions as follows:

import styles from './styles.css' assert { type: 'css' };
class MyCustomElement extends HTMLElement {
  connectedCallback() {
    const root = this.attachShadow({ mode: 'open' });
    root.innerHTML = `<h1>Hello world</h1>`;
    root.adoptedStyleSheets = [ styles ];
  }
}
customElements.define('my-custom-element', MyCustomElement);

Assuming valid CSS in styles.css, the contents of the the CSS will be transformed to use CSS module scripts for use with DocumentOrShadowRoot.prototype.adoptedStyleSheets. Currently this API only exists in Chrome, but a polyfill exists to port the behavior back to IE11.

This plugin will ignore dynamic imports with dynamic values, e.g.:

import(`./foo/${bar}.json`, { assert: { type: 'json' } }); // will be ignored
const foo = './foo.json';
import(foo, { assert: { type: 'json' } }); // will be ignored