An implementation of HTMLRewriter using a WebAssembly version of lol-html. This was primarily written for 🔥 Miniflare, but may be useful for other projects too. Many thanks to @inikulin for their work on lol-html's JavaScript API which this package's Rust code is based on.

import { HTMLRewriter } from "html-rewriter-wasm";

const encoder = new TextEncoder();
const decoder = new TextDecoder();

const rewriter = new HTMLRewriter((outputChunk) => {
  if (outputChunk.length === 0) {
    // Remember to free memory on last chunk
    queueMicrotask(() => rewriter.free());
  } else {
    console.log(decoder.decode(outputChunk)); // <p>new</p>
  }
});

rewriter.on("p", {
  element(element) {
    element.setInnerContent("new");
  },
});

await rewriter.write(encoder.encode("<p>old</p>"));
await rewriter.end();

See test/index.ts for a more traditional HTMLRewriter implementation that doesn't have the caveats listed below, but restricts input and output to strings.

• end may only be called once per HTMLRewriter instance. This means you must create a new HTMLRewriter instance for each transformation:

  // ❌
  const rewriter = new HTMLRewriter(...);
  await rewriter.end();
  await rewriter.end(); // not allowed
  
  // ✅
  const rewriter1 = new HTMLRewriter(...);
  await rewriter1.end();
  const rewriter2 = new HTMLRewriter(...);
  await rewriter2.end();

• When using async handlers, you must always await calls to write and end before calling them again. In other words, you cannot have concurrent write and end calls:

  const rewriter = new HTMLRewriter(...).on("p", {
    async element(element) {
      await fetch(...);
      element.setInnerContent("new");
    }
  });
  
  // ❌
  rewriter.write(encoder.encode("<p>1</p>"));
  rewriter.write(encoder.encode("<p>2</p>")); // not allowed
  
  // ❌
  const promise1 = rewriter.write(encoder.encode("<p>1</p>"));
  const promise2 = rewriter.write(encoder.encode("<p>2</p>"));
  await Promise.all([promise1, promise2]); // not allowed
  
  // ✅
  await rewriter.write(encoder.encode("<p>1</p>"));
  await rewriter.write(encoder.encode("<p>2</p>"));

• If using handler classes, you must bind their methods to the class first:

  class Handler {
    constructor(value) {
      this.value = value;
    }
  
    element(element) {
      element.setInnerContent(this.value);
    }
  }
  const rewriter = new HTMLRewriter(...);
  const handler = new Handler("new");
  
  // ❌
  rewriter.on("p", handler);
  
  // ✅
  rewriter.on("p", {
    element: handler.element.bind(handler)
  })

You can build the package by running npm run build. You must do this prior to running tests with npm test. You must have mrbbot's fork of wasm-pack installed. This upgrades binaryen (wasm-opt) to version_92 which exports asyncify_get_state.

html-rewriter-wasm uses lol-html which is BSD 3-Clause licensed:

Copyright (C) 2019, Cloudflare, Inc.
All rights reserved.

Redistribution and use in source and binary forms, with or without modification,
are permitted provided that the following conditions are met:

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list of conditions and the following disclaimer.

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this list of conditions and the following disclaimer in the documentation and/or
other materials provided with the distribution.

3. Neither the name of the copyright holder nor the names of its contributors
may be used to endorse or promote products derived from this software without
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