Shimport

A 2kb shim for import and export. Allows you to use JavaScript modules in all browsers, including dynamic import().

Quick start

Suppose you have a module called js/app.js. We want to:

check to see if modules are fully supported in the current browser, including dynamic imports
if they are, just use the native module loader
if not, use Shimport

We can do this by adding a simple script tag to our index.html file:

<script>
  function shimport(src) {
    try {
      new Function('import("' + src + '")')();
    } catch (e) {
      var s = document.createElement('script');
      s.src = 'https://unpkg.com/shimport';
      s.dataset.main = src;
      document.head.appendChild(s);
    }
  }

  // Load 'js/app.js' via built-in import(), falling back to shimport if needed:
  shimport('./js/app.js');
</script>

Installing locally

In the example above we loaded Shimport from the unpkg CDN. You can also grab the latest copy and include alongside your app's other files.

You can also npm install shimport, in which case it will be available as node_modules/shimport/index.js.

API

Most of the time you won't need to interact directly with Shimport, but it's useful to understand how it works. The script creates a global variable, __shimport__, with the following methods:

load(url: string) => Promise<module> — url must be fully qualified
transform(source: string) => string — converts a JavaScript module to a Shimport module
define(id: string, deps: string[], factory: (...) => void) — used internally to construct modules

Using with Rollup and code-splitting

Since Rollup can already output JavaScript modules, it's easy to use with Shimport. Just use the esm output format:

// rollup.config.js
export default {
  input: 'src/app.js',
  output: {
    dir: 'js',
    format: 'esm'
  },
  experimentalCodeSplitting: true
};

Skipping feature detection

If you want to always use Shimport, regardless of environment, you can create a script that loads Shimport with a data-main attribute:

<script src="path/to/shimport.js" data-main="path/to/my/module.js"></script>

Using with a web worker

In a web worker environment, Shimport can't auto-start based on a script with data-main. Instead, use the API:

importScripts('path/to/shimport.js');

const { href } = new URL('path/to/my/module.js', location.href);
__shimport__.load(href).then(mod => {
	// module is loaded
});

Is it fast?

Blazingly. The code transformation is fast enough that you probably don't need to worry about it, unless you're shipping far too much JavaScript in the first place.

A future version of Shimport may use web workers to do the transformation off the main thread.

Browser support

Shimport only transpiles the import and export statements in your code. If you want to use other features in browsers that do not support them, they will need to be transpiled separately.

Shimport also expects to be able to use Array.from, fetch, Map and URL. On browsers like Internet Explorer, you will need to bring your own polyfills.

Caveats

The JavaScript module specification is complex, and extremely hard to implement completely with the techniques Shimport uses. It is designed to meet the 98% of cases you encounter in the real world, rather than covering the entire spec at the cost of becoming prohibitively slow and complex.

Specifically, it will not correctly handle cyclical dependencies or live bindings.

Because Shimport uses fetch, and evaluates the transformed result, it will not work with some CSP and CORS configurations.

License

MIT