Transitively includes commonJS modules for use on the web

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1710.1.07 years ago7 years agoMinified + gzip package size for cjs-loader in KB


cjs-loader transitively includes commonJS modules for use on the web, and is useful for developers wanting to migrate to ES6 modules who still have legacy dependencies. Go here for a demo.
We do this using just your browser, without a compile step, by implementing require() and other methods just while loading the module code. For users of cjs-loader, we provide load(moduleName) that returns a Promise of the exports.
šŸ”„šŸ‘Øā€šŸ’»šŸ”„ This is an interesting, successful but terrible idea and should not be used by anyone. It reqiures support for ES6 Modules šŸ› ļø and has only really been tested on Chrome 61+.
First, install any public modules you want to use with NPM or Yarn. Then, use the loader:
<script type="module">
import {load, setup} from './path/to/cjs-loader.js';

load('npm-package-name').then((out) => {
  // do whatever as if you require()'d the package

For example, to use Handlebars (as per the demo):
load('handlebars').then((handlebars) => {
  const Handlebars = handlebars.create();

  const source = '<p>Hello {{name}}, you have {{name.length}} letters</p>';
  const template = Handlebars.compile(source);
  console.info(template({name: 'Sam'}));

Handlebars internally fetches about ~35 modules (via require()), which we wrap.
  1. Stub out module.exports, require() etc.
  2. Load the target module as an ES6 module^1
If a further dependency is unavailable in require(), throw a known exception^2 Catch in a global handler, and load the dependency via step 2 Retry running the target module when done
^1 more or less
^2 require() is synchronous, so we can't block to load more code

Specific Hacks

We abuse
the fact that Chrome reruns but does not need to reload script files with the same path, but a different hash. This allows for "efficient" retries. e.g.:
import * as foo1 from './foo.js#1';
import * as foo2 from './foo.js#2';
foo1 !== foo2  // not the same, but only caused one network request

TODO: document more hacks
Things that we can't fix:
  • Don't use this in production.
It's horrible.
  • Modules can't really determine their path, so if one of your dependenies is from a 302 etc, all bets are off

  • Runtime require() (i.e., not run on initial import) calls will fail if the code isn't available


Things that we can fix:
  • Built-in Node packages don't work (fs, path etc)

  • We should coalesce multiple failures to require() (just return null until an actual error occurs) and request further code in parallel

  • This code is forked from cjs-faker, and still supports AMD, but calling an unplanned require() from within define() doesn't work yet