Polyfills import maps and other ES Modules features on top of the baseline native ESM support in browsers.

With import maps now supported by all major browsers, ES Module Shims entirely bypasses processing for the 91% of users with native import maps support.

For the remaining users, the highly performant (see benchmarks) production and CSP-compatible shim kicks in to rewrite module specifiers driven by the Web Assembly ES Module Lexer.

The following modules features are polyfilled:

• Import Maps polyfill.
• JSON and CSS modules with import assertions (when enabled).
• Wasm modules with support for Source Phase Imports (when enabled).

When running in shim mode, module rewriting is applied for all users and custom resolve and fetch hooks can be implemented allowing for custom resolution and streaming in-browser transform workflows.

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.

Built with Chomp

Include ES Module Shims with a async attribute on the script, then include an import map and module scripts normally:

<script async src="https://ga.jspm.io/npm:[email protected]/dist/es-module-shims.js"></script>

<!-- https://generator.jspm.io/#U2NhYGBkDM0rySzJSU1hKEpNTC5xMLTQM9Az0C1K1jMAAKFS5w0gAA -->
<script type="importmap">
{
  "imports": {
    "react": "https://ga.jspm.io/npm:[email protected]/index.js"
  },
  "scopes": {
    "https://ga.jspm.io/npm:[email protected]/": {
      "object-assign": "https://ga.jspm.io/npm:[email protected]/index.js"
    }
  }
}
</script>

<script type="module">
  import react from 'react';
  console.log(react);
</script>

When running the previous example in a browser without import maps support, the browser will output the following console error:

Uncaught TypeError: Failed to resolve module specifier "react". Relative references must start with either "/", "./", or "../".
  at <anonymous>:1:15

This error is important - it means that the native browser loader didn't execute any of the code at all because this error happens at link time, and before execution time. And this is what allows the polyfill to be able to reexecute the modules and their dependencies without risk of duplicate execution.

The ES Module Shims polyfill will analyze the browser to see if it supports import maps. If it does, it doesn't do anything more, otherwise it will analyze all module scripts on the page to see if any of them have bare specifier imports that will fail like this. If one is found, it will then be reexecuted through ES Module Shims using its internal shimming of modules features.

When the polyfill kicks in another console log message is output(which can be disabled or customized via the polyfill hook):

^^ Module error above is polyfilled and can be ignored ^^

Only static link-time errors are polyfilled, not runtime errors.

Module feature errors that are not static errors but rather runtime errors will bypass the polyfill detection.

For example:

<script type="module">
  import './supported-relative-import.js';
  console.log('Static Ok');
  import('unsupported-import-map').then(x => {
    console.log('Dynamic Ok');
  }, err => {
    console.log('Dynamic Fail');
  });
</script>

In the above, the native browser loader without import maps support will execute the above module, but fail the dynamic import.

See the log output in various scenarios:

• Native with Import Maps: Static Ok, Dynamic Ok
• Native without Import Maps: Static Ok, Dynamic Fail
• Native without Import Maps running ES Module Shims: Static Ok, Dynamic Fail

ES Module Shims does not polyfill the dynamic import. The reason for this is that if it did, it would need to reexecute the entire outer module resulting in Static Ok, Dynamic Fail, Static Ok, Dynamic Ok. This double execution would be a change of the normal execution in running code twice that would not be deemed suitable for calling it a polyfill.

This is why it is advisable to always ensure modules use a bare specifier early to avoid non-execution.

If a static failure is not possible and dynamic import must be used, one alternative is to use the importShim ES Module Shims top-level loader:

<script type="module">
  import './supported-relative-import.js';
  console.log('Static Ok');
  importShim('unsupported-import-map').then(x => {
    console.log('Ok');
  }, err => {
    console.log('Fail');
  });
</script>

importShim will automatically pass-through to the native dynamic import or polyfill as necessary, just like it does for script tags.

When running in polyfill mode, it can be thought of that are effectively two loaders running on the page - the ES Module Shims polyfill loader, and the native loader.

Note that instances are not shared between these loaders for consistency and performance, since some browsers do not properly share the fetch cache and native loader cache resulting in a double fetch which would be inefficient.

As a result, if you have two module graphs - one native and one polyfilled, they will not share the same dependency instance, for example:

<script type="importmap">
{
  "imports": {
    "dep": "/dep.js"
  }
}
</script>
<script type="module">
import '/dep.js';
</script>
<script type="module">
import 'dep';
</script>

console.log('DEP');

When polyfilling import maps, ES Module Shims will pick up on the second import failure and reexecute /dep.js as a new instance, logging "DEP" twice.

For this reason it is important to always ensure all modules hit the polyfill path, either by having all graphs use import maps at the top-level, or via importShim directly.

If you really need to support instance sharing with the native loader, a useful workaround is to use the skip option to list modules which should always be loaded via the native loader:

<script type="esms-options">
{
  "skip": ["/dep.js"]
}
</script>

The above would then fully cause dependency module instance to be shared between ES Module Shims and the native loader, with the polyfill then logging "DEP" only once.

If the polyfill is analyzing or applying to a module script that doesn't need to or shouldn't be polyfilled, adding the "noshim" attribute to the script tag will ensure that ES Module Shims ignores processing this script entirely:

<script type="module" noshim>
  // ...
</script>

If using more modern features like CSS Modules or JSON Modules, these need to be manually enabled via the polyfillEnable init option to raise the native baseline from just checking import maps to also checking that browsers support these features:

<script>
window.esmsInitOptions = { polyfillEnable: ['css-modules', 'json-modules', 'wasm-modules'] }
</script>

To verify when the polyfill is actively engaging as opposed to relying on the native loader, a polyfill hook is also provided.

Shim mode is an alternative to polyfill mode and doesn't rely on native modules erroring - instead it is triggered by the existence of any <script type="importmap-shim"> or <script type="module-shim">, or when explicitly setting the shimMode init option.

In shim mode, only the above importmap-shim and module-shim tags will be parsed and executed by ES Module Shims.

Shim mode also provides some additional features that aren't yet natively supported such as supporting multiple import maps, external import maps with a "src" attribute, dynamically injecting import maps, and reading current import map state, which can be useful in certain applications.

ES Module Shims is designed for production performance. A comprehensive benchmark suite tracks multiple loading scenarios for the project.

Benchmark summary:

• ES Module Shims Chrome Passthrough (for 72% of users) results in ~5ms extra initialization time over native for ES Module Shims fetching, execution and initialization, and on a slow connection the additional non-blocking bandwidth cost of its 10KB compressed download as expected.
• ES Module Shims Polyfilling (for the remaining 28% of users) is on average 1.4x - 1.5x slower than native module loading, and up to 1.8x slower on slow networks (most likely due to the browser preloader), both for cached and uncached loads, and this result scales linearly up to 10MB and 20k modules loaded executing on the fastest connection in just over 2 seconds in Firefox.
• Very large import maps (100s of entries) cost only a few extra milliseconds upfront for the additional loading cost.

Works in all browsers with baseline ES module support.

Browser Compatibility on baseline ES modules support with ES Module Shims:

Browser compatibility without ES Module Shims:

Stability: WhatWG Standard, implemented in all browsers although only recently in Firefox and Safari

Import maps allow for importing "bare specifiers" in JavaScript modules, which prior to import maps throw in all browsers with native modules support.

Using this polyfill we can write:

<script type="importmap-shim">
{
  "imports": {
    "test": "/test.js"
  },
  "scopes": {
    "/": {
      "test-dep": "/test-dep.js"
    }
  },
  "integrity": {
    "/test.js": "sha386-..."
  }
}
</script>
<script type="module-shim">
  import test from "test";
  console.log(test);
</script>

All modules are still loaded with the native browser module loader, but with their specifiers rewritten then executed as Blob URLs, so there is a relatively minimal overhead to using a polyfill approach like this.

Multiple import maps are not currently supported in any native implementation, Chromium support is currently being tracked in https://bugs.chromium.org/p/chromium/issues/detail?id=927119.

In polyfill mode, multiple import maps are therefore not supported.

In shim mode, support for multiple importmap-shim scripts follows the import map extensions proposal.

External import maps (using a "src" attribute) are not currently supported in any native implementation.

In polyfill mode, external import maps are therefore not supported.

In shim mode, external import maps are fully supported.

Support for dynamically injecting import maps with JavaScript via eg:

document.body.appendChild(Object.assign(document.createElement('script'), {
  type: 'importmap',
  innerHTML: JSON.stringify({ imports: { x: './y.js' } }),
}));

is supported in Chromium, provided it is injected before any module loads and there is no other import map yet loaded (multiple import maps are not supported).

Both modes in ES Module Shims support dynamic injection using DOM Mutation Observers.

While in polyfill mode the same restrictions apply that multiple import maps, import maps with a src attribute, and import maps loaded after the first module load are not supported, in shim mode all of these behaviours are fully enabled for "importmap-shim".

To make it easy to keep track of import map state, es-module-shims provides a importShim.getImportMap utility function, available only in shim mode.

const importMap = importShim.getImportMap();

// importMap will be an object in the same shape as the json in a importmap script

To make it easy to set the import map state, es-module-shims provides a importShim.addImportMap utility function, available only in shim mode.

// importMap will be an object in the same shape as the json in a importmap script
const importMap = { imports: {/*...*/}, scopes: {/*...*/} };

importShim.addImportMap(importMap);

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: WhatWG Standard, Single Browser Implementer

Preloading of modules can be achieved by including a <link rel="modulepreload" href="/module.js" /> tag in the HTML or injecting it dynamically.

This tag also supports the "integrity", "crossorigin" and "referrerpolicy" attributes as supported on module scripts.

This tag just initiates a fetch request in the browser and thus works equally as a preload polyfill in both shimmed and unshimmed modes, with integrity validation support.

In browsers that don't support modulepreload, polyfilled preloading behaviour is provided using an early fetch() call with the same request options as the module script, resulting in network-level cache sharing.

Unlike the browser specification, the modulepreload polyfill does not request dependency modules by default, in order to avoid unnecessary code analysis in the polyfill scenarios, it is always recommended to preload deep imports so that this feature shouldn't be necessary.

When in shim mode, <link rel="modulepreload-shim" href="/module.js" /> must be used to properly cache the preloaded modules.

By default ES Module Shims provides full support for CSP by using the asm.js ES Module Lexer build. This is absolutely identical in performance to the Wasm version in Firefox and Chrome (in Safari the asm.js version is actually faster than Wasm).

The CSP nonce to use for module scripts will be picked up from the first script on the page or via the nonce init option.

A full example of such a CSP workflow is provided below:

<meta http-equiv="Content-Security-Policy" content="script-src 'self' 'nonce-n0nce'" />
<script async src="es-module-shims.js"></script>
<script type="importmap" nonce="n0nce">
{
  "imports": {
    "pkg": "/pkg.js"
  }
}
</script>
<script type="module" nonce="n0nce">
import pkg from 'pkg';
</script>

To use the Web Assembly / non-CSP build of ES Module Shims, this is available as a self-contained single file at es-module-shims/wasm or es-module-shims/dist/es-module-shims.wasm.js in the package folder.

The "integrity" field for import maps is supported when possible, throwing an error in es-module-shims when the integrity does not match the expected value:

<script type="importmap">
{
  "imports": {
    "pkg": "/pkg.js"
  },
  "integrity": {
    "/pkg.js": "sha384-..."
  }
}
</script>
<script>
import "/pkg.js";
</script>

Note integrity can only be validated when in shim mode or when the polyfill is definitely engaging.

Stability: WhatWG Standard, Single Browser Implementer

In shim mode, JSON modules are always supported. In polyfill mode, JSON modules require the polyfillEnable: ['json-modules'] init option.

JSON Modules are currently supported in Chrome when using them via an import assertion:

<script type="module">
import json from 'https://site.com/data.json' with { type: 'json' };
</script>

In addition JSON modules need to be served with a valid JSON content type.

Stability: WhatWG Standard, Single Browser Implementer

In shim mode, CSS modules are always supported. In polyfill mode, CSS modules require the polyfillEnable: ['css-modules'] init option.

CSS Modules are currently supported in Chrome when using them via an import assertion:

<script type="module">
import sheet from 'https://site.com/sheet.css' with { type: 'css' };
</script>

In addition CSS modules need to be served with a valid CSS content type.

Stability: WebAssembly Standard, Unimplemented

Implements the WebAssembly ESM Integration spec, including support for source phase imports.

In shim mode, Wasm modules are always supported. In polyfill mode, Wasm modules require the polyfillEnable: ['wasm-modules'] init option.

WebAssembly module exports are made available as module exports and WebAssembly module imports will be resolved using the browser module loader.

When using the source phase import form, this must be enabled separately via the polyfillEnable: ['wasm-modules', 'source-phase'] init option to support source imports to WebAssembly modules.

When enabling 'source-phase', WebAssembly.Module is also polyfilled to extend from AbstractModuleSource per the source phase proposal.

WebAssembly modules require native top-level await support to be polyfilled, see the compatibility table above.

<script type="module">
import { fn } from './app.wasm';
</script>

And for the source phase:

<script type="module">
import source mod from './app.wasm';
const instance = await WebAssembly.instantiate(mod, { /* ...imports */ });
</script>

If using CSP, make sure to add 'unsafe-wasm-eval' to script-src which is needed when the shim or polyfill engages, note this policy is much much safer than eval due to the Wasm secure sandbox. See https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Content-Security-Policy/script-src#unsafe_webassembly_execution.

ES Module Shims can be used in module workers in browsers that provide dynamic import in worker environments, which at the moment are Chrome(80+), Edge(80+), Firefox(~113+) and Safari(15+).

By default, when there is no DOM present, ES Module Shims will switch into shim mode. An example of ES Module Shims usage through shim mode in web workers is provided below:

/**
 * 
 * @param {string} aURL a string representing the URL of the module script the worker will execute.
 * @returns {string} The string representing the URL of the script the worker will execute.
 */
function getWorkerScriptURL(aURL) {
  // baseURL, esModuleShimsURL are considered to be known in advance
  // esModuleShimsURL - must point to the non-CSP build of ES Module Shims, 
  // namely the `es-module-shim.wasm.js` output: es-module-shims/dist/es-module-shims.wasm.js

  return URL.createObjectURL(new Blob(
    [
      `importScripts('${new URL(esModuleShimsURL, baseURL).href}');
      importShim.addImportMap(${JSON.stringify(importShim.getImportMap())});
      importShim('${new URL(aURL, baseURL).href}').catch(e => setTimeout(() => { throw e; }))`
    ],
    { type: 'application/javascript' }))
}

const worker = new Worker(getWorkerScriptURL('myEsModule.js'));

Web workers must use the non-CSP build of ES Module Shims via es-module-shim.wasm.js from the dist/ folder, since the CSP build currently assumes a DOM.

Provide a esmsInitOptions on the global scope before es-module-shims is loaded to configure various aspects of the module loading process:

• enforceIntegrity
• fetch
• mapOverrides
• modulepreload
• noLoadEventRetriggers
• globalLoadEventRetrigger
• nonce
• onerror
• onpolyfill
• polyfillEnable
• resolve
• revokeBlobURLs
• shimMode
• skip

<script>
window.esmsInitOptions = {
  // Enable Shim Mode
  shimMode: true, // default false
  // Enable newer modules features
  polyfillEnable: ['css-modules', 'json-modules'], // default empty
  // Custom CSP nonce
  nonce: 'n0nce', // default is automatic detection
  // Don't retrigger load events on module scripts (DOMContentLoaded, domready)
  noLoadEventRetriggers: true, // default false
  // Retrigger window 'load' event (will be combined into load event above on next major)
  globalLoadEventRetrigger: true, // default false
  // Skip source analysis of certain URLs for full native passthrough
  skip: /^https:\/\/cdn\.com/, // defaults to null
  // Clean up blob URLs after execution
  revokeBlobURLs: true, // default false
  // Secure mode to not support loading modules without integrity
  // (integrity is always verified even when disabled though)
  enforceIntegrity: true, // default false
  // Permit overrides to import maps
  mapOverrides: true, // default false

  // -- Hooks --
  // Module load error
  onerror: (e) => { /*...*/ }, // default noop
  // Called when polyfill mode first engages
  onpolyfill: () => {}, // default logs to the console
  // Hook all module resolutions
  resolve: (id, parentUrl, resolve) => resolve(id, parentUrl), // default is spec resolution
  // Hook source fetch function
  fetch: (url, options) => fetch(url, options), // default is native
  // Hook import.meta construction
  meta: (meta, url) => void // default is noop
  // Hook top-level imports
  onimport: (url, options, parentUrl) => void // default is noop
}
</script>
<script async src="es-module-shims.js"></script>

<script type="esms-options"> can also be used:

<script type="esms-options">
{
  "shimMode": true,
  "polyfillEnable": ["css-modules", "json-modules"],
  "nonce": "n0nce",
  "onpolyfill": "polyfill"
}
</script>

This can be convenient when using a CSP policy. Function strings correspond to global function names.

Shim Mode can be overridden using the shimMode option:

<script type="esms-options">
{
  "shimMode": true
}
</script>

For example, if lazy loading <script type="module-shim"> scripts alongside static native module scripts, shim mode would not be enabled at initialization time.

DOM load events are fired for all "module-shim" scripts both for success and failure just like for native module scripts.

The polyfillEnable option allows enabling polyfill features which are newer and would otherwise result in unnecessary polyfilling in modern browsers that haven't yet updated.

This options supports "css-modules", "json-modules", "wasm-modules", "source-phase".

<script type="esms-options">
{
  "polyfillEnable": ["css-modules", "json-modules"]
}
</script>

The above is necessary to enable CSS modules and JSON modules.

The reason the polyfillEnable option is needed is because ES Module Shims implements a performance optimization where if a browser supports modern modules features to an expected baseline of import maps support, it will skip all polyfill source analysis resulting in full native passthrough performance.

If the application code then tries to use modern features like CSS modules beyond this baseline it won't support those features. As a result all modules features which are considered newer or beyond the recommended baseline require explicit enabling. This common baseline itself will change to track the common future modules baseline supported by this project for each release cycle.

This option can also be set to true to entirely disable the native passthrough system and ensure all sources are fetched and analyzed through ES Module Shims. This will still avoid duplicate execution since module graphs are still only reexecuted when they use unsupported native features, but there is a small extra cost in doing the analysis.

While integrity is always verified and validated when available, when enabled, enforceIntegrity will ensure that all modules must have integrity defined when loaded through ES Module Shims either on a <link rel="modulepreload" integrity="...">, a <link rel="modulepreload-shim" integrity="..."> preload tag in shim mode, or the "integrity" field in the import map. Modules without integrity will throw at fetch time.

For example in the following, only the listed app.js, dep.js and another.js modules will be able to execute with the provided integrity:

<script type="importmap">
{
  "integrity": {
    "/another.js": "sha384-..."
  }
}
</script>
<script type="esms-options">{ "enforceIntegrity": true }</script>
<link rel="modulepreload-shim" href="/app.js" integrity="sha384-..." />\
<link rel="modulepreload-shim" href="/dep.js" integrity="sha384-..." />
<script type="module-shim">
  import '/app.js';
  import '/another.js';
</script>

Strong execution guarantees are only possible in shim mode since in polyfill mode it is not possible to stop the native loader from executing code without an integrity.

Future versions of this option may provide support for origin-specific allow lists.

The nonce option allows setting a CSP nonce to be used with all script injections for full CSP compatibility supported by the CSP build of ES Module Shims.

Alternatively, add a blob: URL policy with the CSP build to get CSP compatibility.

<script type="esms-options">
{
  "nonce": "n0nce"
}
</script>

Because of the extra processing done by ES Module Shims it is possible for static module scripts to execute after the DOMContentLoaded or readystatechange events they expect, which can cause missed attachment.

In addition, script elements will also have their load events refired when polyfilled.

In order to ensure libraries that rely on these event still behave correctly, ES Module Shims will always double trigger these events that would normally have executed before the document ready state transition to completion, once all the static module scripts in the page have been completely executed through ES module shims.

In such a case, this double event firing can be disabled with the noLoadEventRetriggers option:

<script type="esms-options">
{
  // do not re-trigger DOM events (onreadystatechange, DOMContentLoaded)
  "noLoadEventRetriggers": true
}
</script>
<script async src="es-module-shims.js"></script>

In ES Module Shims 1.x, load event retriggers only apply to DOMContentLoaded and readystatechange and not to the window load event. To enable the window / worker 'load' event, set globalLoadEventRetrigger: true.

In the next major version, this will be the default for load events, at which point only noLoadEventRetriggers will remain.

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 being polyfilled to always load and be referenced through the native loader only. This enables instance sharing with the native loader and also 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.

The skip option supports a string regular expression or array of exact module URLs to check:

<script type="esms-options">
{
  "skip": "^https?:\/\/(cdn\.skypack\.dev|jspm\.dev)\/"
}
</script>
<script async src="es-module-shims.js"></script>

When passing an array, relative URLs or paths ending in / can be provided:

<script type="esms-options">
{
  "skip": ["./app.js", "https://jspm.dev/"]
}
</script>
<script async src="es-module-shims.js"></script>

When polyfilling the missing features es-module-shims would create in-memory blobs using URL.createObjectURL() for each processed module. In most cases, memory footprint of these blobs is negligible so there is no need to call URL.revokeObjectURL() for them, and we don't do that by default.

That said, in some scenarios, e.g. when evaluating some continuously changing modules without a page reload, like in a web-based code editor, you might want to reduce the growth of memory usage by revoking those blob URLs after they were already imported.

You can do that by enabling the revokeBlobURLs init option:

<script type="esms-options">
{
  "revokeBlobURLs": true
}
</script>
<script type="module" src="es-module-shims.js"></script>

NOTE: revoking object URLs is not entirely free, while we are trying to be smart about it and make sure it doesn't cause janks, we recommend enabling this option only if you have done the measurements and identified that you really need it.

When dynamically injecting import maps, an error will be thrown in both polyfill and shim modes if the new import map would override existing entries with a different value.

It is possible to disable this behavior in shim mode by setting the mapOverrides option:

<script type="esms-options">
{
  "shimMode": true,
  "mapOverrides": true
}
</script>
<script type="importmap-shim">
{
  "imports": {
    "x": "/x.js"
  }
}
</script>
<script>
// No error will be thrown here
document.body.appendChild(Object.assign(document.createElement('script'), {
  type: 'importmap',
  innerHTML: JSON.stringify({ imports: { x: './y.js' } }),
}));
</script>

This can be useful for HMR workflows.

The polyfill hook is called when running in polyfill mode and the polyfill is kicking in instead of passing through to the native loader.

This can be a useful way to verify that the native passthrough is working correctly in latest browsers for performance, while also allowing eg the ability to analyze or get metrics reports of how many users are getting the polyfill actively applying to their browser application loads.

<script>
window.polyfilling = () => console.log('The polyfill is actively applying');
</script>
<script type="esms-options">
{
  "onpolyfill": "polyfilling"
}
</script>

The default hook will log a message to the console with console.info noting that polyfill mode is enabled and that the native error can be ignored.

Overriding this hook with an empty function will disable the default polyfill log output.

In the above, running in latest Chromium browsers, nothing will be logged, while running in an older browser that does not support newer features like import maps the console log will be output.

You can provide a function to handle errors during the module loading process by providing an onerror option:

<script>
  window.esmsInitOptions = {
    onerror: error => console.log(error) // defaults to `((e) => { throw e; })`
  }
</script>
<script async src="es-module-shims.js"></script>

The import hook is supported for both shim and polyfill modes and provides an async hook which can ensure any necessary work is done before a top-level module import or dynamic import() starts further processing.

<script>
  window.esmsInitOptions = {
    onimport: function (url, options, parentUrl) {
      console.log(`Top-level import for ${url}`);
    }
  }
</script>

The resolve hook is supported for both shim and polyfill modes and allows full customization of the resolver, while still having access to the original resolve function.

Note that in polyfill mode the resolve hook may not be called for all modules when native passthrough is occurring and that it still will not affect the native passthrough executions.

If the resolve hook should apply for all modules in the entire module graph, make sure to set polyfillEnable: true to disable the baseline support analysis opt-out.

<script>
  window.esmsInitOptions = {
    shimMode: true,
    resolve: function (id, parentUrl, defaultResolve) {
      if (id === 'custom' && parentUrl.startsWith('https://custom.com/'))
        return 'https://custom.com/custom.js';

      // Default resolve will handle the typical URL and import map resolution
      return defaultResolve(id, parentUrl);
    }
  }
</script>

Support for an asynchronous resolve hook has been deprecated as of 1.5.0 and will be removed in the next major.

Instead async work should be done with the import hook.

The meta hook allows customizing the import.meta object in each module scope.

The function takes as arguments the import.meta object itself (with import.meta.url an import.meta.resolve already present), and the URL of the module as its second argument.

Example:

<script>
  window.esmsInitOptions = {
    shimMode: true,
    meta: function (metaObj, url) {
      metaObj.custom = `custom value for ${url}`;
    }
  }
</script>

Where within the module the following would be supported:

import assert from 'assert';
assert.ok(import.meta.custom.startsWith('custom value'));

The fetch hook is supported for shim mode only.

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

For example TypeScript support:

<script>
  window.esmsInitOptions = {
    shimMode: true,
    fetch: async function (url, options) {
      const res = await fetch(url, options);
      if (!res.ok)
        return res;
      if (res.url.endsWith('.ts')) {
        const source = await res.body();
        const transformed = tsCompile(source);
        return new Response(new Blob([transformed], { type: 'application/javascript' }));
      }
      return res;
    } // defaults to `((url, options) => fetch(url, options))`
  }
</script>
<script async src="es-module-shims.js"></script>

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:

window.esmsInitOptions = {
  shimMode: true,
  fetch: async function (url, options) {
    const res = await fetch(url, options);
    if (!res.ok || !/^application\/json($|;)/.test(res.headers.get('content-type')))
      return res;
    const reader = res.body.getReader();
    const headers = new Headers(res.headers);
    headers.set('Content-Type', 'application/javascript');
    return new Response(new ReadableStream({
      async start (controller) {
        let done, value;
        controller.enqueue(new TextEncoder().encode('export default '));
        while (({ done, value } = await reader.read()) && !done) {
          controller.enqueue(value);
        }
        controller.close();
      }
    }), { headers });
  }
}

• Sources are fetched, import specifiers are rewritten to reference exact URLs, and then executed as BlobURLs through the whole module graph.
• The lexer 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 acyclify into the graph A -> B -> A Shell, with A -> A Shell. The shell module exports an update function which is called by the original once after the last import statement, and again after the last statement of the source file.

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

MIT