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ES Module Shims

90% of users are now running browsers with baseline support for ES modules.

But modules features like Import Maps will take a while to be supported in browsers.

It turns out that we can actually polyfill new modules features on top of these baseline implementations in a performant 7KB shim.

This includes support for:

  • Import Maps support.
  • import.meta and import.meta.url.
  • Dynamic import() shimming when necessary in eg older Firefox versions.

In addition a custom fetch hook can be implemented allowing for streaming in-browser transform workflows to support custom module types.

Because we are still using the native module loader the edge cases work out comprehensively, including:

  • Live bindings in ES modules
  • Dynamic import expressions (import('src/' + varname'))
  • Circular references, with the execption that live bindings are disabled for the first unexecuted circular parent.

Due to the use of a tiny Web Assembly JS tokenizer for ES module syntax only, with very simple rewriting rules, transformation is very fast, although in complex cases of hundreds of modules it can be a few hundred milliseconds slower than using SystemJS or native ES modules. See the SystemJS performance comparison for a full performance breakdown in a complex loading scenario.

Browser Support

Works in all browsers with baseline ES module support.

Browser Compatibility with ES Module Shims:

ES Modules Features Chrome (61+) Firefox (60+) Safari (10.1+) Edge (16+)
Executes Modules in Correct Order ✔️ ✔️ ✔️ ✔️1
Dynamic Import ✔️ ✔️ ✔️ ✔️
import.meta.url ✔️ ✔️ ✔️ ✔️
import.meta.resolve ✔️ ✔️ ✔️ ✔️
Module Workers ✔️ ~68+ 2 2 2
Import Maps ✔️ ✔️ ✔️ ✔️
  • 1: The Edge parallel execution ordering bug is corrected by ES Module Shims with an execution chain inlining approach.
  • 2: Module worker support cannot be implemented without dynamic import support in web workers.

Current browser compatibility of modules features without ES module shims:

ES Modules Features Chrome (61+) Firefox (60+) Safari (10.1+) Edge (16+)
Executes Modules in Correct Order ✔️ ✔️ ✔️ 1
Dynamic Import ✔️ 63+ ✔️ 67+ ✔️ 11.1+
import.meta.url ✔️ ~76+ ✔️ ~67+ ✔️ ~12+ 1
Module Workers ✔️ ~68+
Import Maps 2
  • 1: Edge executes parallel dependencies in non-deterministic order. (ChakraCore bug).
  • 2: Enabled under the Experimental Web Platform Features flag in Chrome 76.
  • ~: Indicates the exact first version support has not yet been determined (PR's welcome!).
  • 1: On module redirects, Safari returns the request URL in import.meta.url instead of the response URL as per the spec.

Import Maps

Stability: Draft browser standard, Chrome flagged implementation only

The goal is for this project to eventually become a true polyfill for import maps in older browsers, but this will only happen once the spec is implemented in more than one browser and demonstrated to be stable.

In order to import bare package specifiers like import "lodash" we need import maps, which are still an experimental specification.

Using this polyfill we can write:

<!doctype html>
<!-- either user "defer" or load this polyfill after the scripts below-->
<script defer src="es-module-shims.js"></script>
<script type="importmap-shim">
  "imports": {
    "test": "/test.js"
  "scopes": {
    "/": {
      "test-dep": "/test-dep.js"
<script type="module-shim">
  import test from "test";

All modules are still loaded with the native browser module loader, just as Blob URLs, meaning there is minimal overhead to using a polyfill approach like this.

Dynamic Import

Stability: Stable browser standard

Dynamic import(...) within any modules loaded will be rewritten as importShim(...) automatically providing full support for all es-module-shims features through dynamic import.

To load code dynamically (say from the browser console), importShim can be called similarly:

importShim('/path/to/module.js').then(x => console.log(x));


Stability: Stable browser standard

import.meta.url provides the full URL of the current module within the context of the module execution.


Stability: No current browser standard

import.meta.resolve provides a contextual resolver within modules. It is asynchronous, like the Node.js implementation, to support waiting on any in-flight import map loads when import maps are loaded dynamically.

The second argument to import.meta.resolve permits a custom parent URL scope for the resolution, which defaults to import.meta.url.

// resolve a relative path to a module
var resolvedUrl = await import.meta.resolve('./relative.js');
// resolve a dependency from a module
var resolvedUrl = await import.meta.resolve('dep');
// resolve a path
var resolvedUrlPath = await import.meta.resolve('dep/');
// resolve with a custom parent scope
var resolvedUrl = await import.meta.resolve('dep', '');

This implementation is as provided experimentally in Node.js -

Skip Processing

Stability: Non-spec feature

When loading modules that you know will only use baseline modules features, it is possible to set a rule to explicitly opt-out modules from rewriting. This improves performance because those modules then do not need to be processed or transformed at all, so that only local application code is handled and not library code.

This can be configured by setting the importShim.skip URL regular expression:

importShim.skip = /^https:\/\/cdn\.com/;

By default, this expression supports, and


Stability: Pre-Draft Standard

Like in SystemJS, a "depcache" property is supported in import maps to enable waterfall flattening.

Dynamic Import Map Updates

Import maps are frozen as soon as the first module load is loaded.

To support dynamic injection of new import maps into the page, call importShim.load() to pick up any new <script type="importmap-shim"> tags.

This can be linked up to mutation observers if desired, with something like:

new MutationObserver(mutations => {
  for (const mutation of mutations) {
    if (mutation.type !== 'childList') continue;
    for (const node of mutation.addedNodes) {
      if (node.tagName === 'SCRIPT' && node.type === 'importmap-shim' && !node.ep) {
}).observe(document, { childList: true, subtree: true });

then allowing dynamic injection of <script type="importmap-shim"> to immediately update the internal import maps.

This follows the dynamic import map specification approach outlined in import map extensions.

Fetch Hook

Stability: Non-spec feature

This is provided as a convenience feature since the pipeline handles the same data URL rewriting and circular handling of the module graph that applies when trying to implement any module transform system.

The ES Module Shims fetch hook can be used to implement transform plugins.

For example:

importShim.fetch = async function (url) {
  const response = await fetch(url);
  if (response.url.endsWith('.ts')) {
    const source = await response.body();
    const transformed = tsCompile(source);
    return new Response(new Blob([transformed], { type: 'application/javascript' }));
  return response;

Because the dependency analysis applies by ES Module Shims takes care of ensuring all dependencies run through the same fetch hook, the above is all that is needed to implement custom plugins.

Streaming support is also provided, for example here is a hook with streaming support for JSON:

importShim.fetch = async function (url) {
  const response = await fetch(url);
  if (!response.ok)
    throw new Error(`${response.status} ${response.statusText} ${response.url}`);
  const contentType = response.headers.get('content-type');
  if (!/^application\/json($|;)/.test(contentType))
    return response;
  const reader = response.body.getReader();
  return new Response(new ReadableStream({
    async start (controller) {
      let done, value;
      controller.enqueue(new Uint8Array([...'export default '].map(c => c.charCodeAt(0))));
      while (({ done, value } = await && !done) {
  }), {
    status: 200,
    headers: {
      "Content-Type": "application/javascript"


Since the Fetch Hook is very new, there are no plugin examples of it yet, but it should be easy to support various workflows such as TypeScript and new JS features this way.

If you work on something here (or even just wrap the examples above into a separate project) please do share to link to from here!

Implementation Details

Import Rewriting

  • Sources are fetched, import specifiers are rewritten to reference exact URLs, and then executed as BlobURLs through the whole module graph.
  • CSP is not supported as we're using fetch and modular evaluation.
  • The tokenizer handles the full language grammar including nested template strings, comments, regexes and division operator ambiguity based on backtracking.
  • When executing a circular reference A -> B -> A, a shell module technique is used to "shim" the circular reference into an acyclic graph. As a result, live bindings for the circular parent A are not supported, and instead the bindings are captured immediately after the execution of A.

Import Maps

  • The import maps specification is under active development and will change, all of the current specification features are implemented, but the edge cases are not currently fully handled. These will be refined as the specification and reference implementation continue to develop.


Huge thanks to Rich Harris for inspiring this approach with Shimport.



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