Over 75% of users are now running browsers with baseline support for ES modules.
But a lot of the useful features of modules come from new specifications which either aren't implemented yet, or are only available in some browsers.
It turns out that we can actually polyfill most of the newer modules specifications on top of these baseline implementations in a performant 3.5KB shim.
This includes support for:
- Dynamic
import()shimming when necessary in eg older Firefox versions. import.metaandimport.meta.url.- Import Maps support.
- Importing Web Assembly (note, there is an open issue on how to handle the 4KB imposed limit)
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 dedicated JS tokenizer for ES module syntax only, with very simple rewriting rules, transformation is instant.
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>
<script type="module-shim">
import test from "test";
console.log(test);
</script>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(...) 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));To load Web Assembly, just import it:
import { fn } from './test.wasm';Web Assembly imports are in turn supported.
Import map support is provided both for mapping into Web Assembly URLs, as well as mapping import specifiers to JS or WebAssembly from within WASM.
- Tokenizing handles the full language grammar including nested template strings, comments, regexes and division operator ambiguity based on backtracking.
- Rewriting is based on fetching the sources, turning them into BlobURLs and executing up the graph.
- 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.
- The approach will only work in browsers supporting ES modules.
- CSP is not supported as we're using fetch and modular evaluation.
- 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.
- In order for Web Assembly to execute in the module graph as a blob: URL we need to use
new WebAssembly.Instancefor synchronous execution, but this has a 4KB size limit in Chrome and Firefox which will throw for larger binaries. There is no known workaround currently. Tracking in guybedford#1. - Exports are snapshotted on execution. Unexecuted circular dependencies will be snapshotted as empty imports. This matches the current integration plans for Web Assembly.
Huge thanks to Rich Harris for inspiring this approach with Shimport.
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