This document details the current state of ES module support in Service Workers.
ES modules have been a developer favorite for a while now — in addition to a number of other benefits, they offer the promise of a universal module format where shared code can be released once and run in browsers, as well as in alternative runtimes like Node.js. The spec can be found here (refer to type in the section).
The ideal use case for ES modules inside of Service Workers is for loading in a modern library or configuration code that's shared with other runtimes that support ES modules.
A module script can be used by setting type to ‘module’ when registering the Service Worker. The feature can be used in the following way.
// index.html
const registration = await navigator.serviceWorker.register(‘sw.js’, {type: ‘module’});
// sw.js (called as ‘main script’ or ‘top-level script’)
import * from ‘./imported_script.js’; // static import
ES modules can be imported in one of two ways: either statically, using the import ... from ‘...’ syntax, or dynamically, using an import(‘...’) statement. Inside of a Service Worker, only the static import is currently supported. Dynamic import is currently blocked in Service Workers, but it will change in the future. The status of supporting dynamic import inside a Service Worker can be tracked here.
This limitation is analogous to a similar restriction placed on importScripts() usage. Dynamic calls to importScripts() do not work inside of a Service Worker, and all importScripts() calls, which are inherently synchronous, must complete before the Service Worker completes its install phase. This restriction ensures that the browser knows about, and is able to store, all JavaScript code needed for a Service Worker's implementation during installation.
Import maps allow runtime environments to rewrite module specifiers, to, for example, prepend the URL of a preferred CDN from which the ES modules can be loaded.
While Chrome version 89 and above supports import maps, they currently cannot be used with Service Workers.