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https://github.com/cloudflare/html-rewriter-wasm

WebAssembly version of HTMLRewriter
https://github.com/cloudflare/html-rewriter-wasm

Last synced: 6 days ago
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WebAssembly version of HTMLRewriter

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README 

        
# `html-rewriter-wasm`



An implementation of

[HTMLRewriter](https://developers.cloudflare.com/workers/runtime-apis/html-rewriter)

using a WebAssembly version of

[lol-html](https://github.com/cloudflare/lol-html/). This was primarily written

for [🔥 Miniflare](https://github.com/mrbbot/miniflare), but may be useful for

other projects too. Many thanks to [@inikulin](https://github.com/inikulin) for

their work on

[lol-html's JavaScript API](https://github.com/cloudflare/lol-html/tree/master/js-api)

which this package's Rust code is based on.



## Features



- 🔋 Supports all handler types, properties and methods

- ⏰ Supports synchronous and asynchronous handlers

- 📌 Supports class handlers with correctly bound methods



## Usage



```js

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



const encoder = new TextEncoder();

const decoder = new TextDecoder();



let output = "";

const rewriter = new HTMLRewriter((outputChunk) => {

  output += decoder.decode(outputChunk);

});



rewriter.on("p", {

  element(element) {

    element.setInnerContent("new");

  },

});



try {

  await rewriter.write(encoder.encode("
old
"));

  await rewriter.end();

  console.log(output); // 
new


} finally {

  rewriter.free(); // Remember to free memory

}

```



See [test/index.ts](./test/index.ts) for a more traditional `HTMLRewriter`

implementation that doesn't have the caveats listed below, but restricts input

and output to strings.



To enable the

[`html_rewriter_treats_esi_include_as_void_tag`](https://developers.cloudflare.com/workers/platform/compatibility-dates#htmlrewriter-handling-of-esiinclude)

compatibility flag, set `enableEsiTags` when constructing the `HTMLRewriter`:



```js

const rewriter = new HTMLRewriter((outputChunk) => { ... }, {

  enableEsiTags: true,

});

```



## Caveats



- Once `write` or `end` has been called, you cannot add any more handlers. You

  must register all handlers before you start transforming:



  ```js

  const rewriter = new HTMLRewriter(...);



  // ❌

  rewriter.on("h1", { ... });

  await rewriter.write(encoder.encode("
1
2"));



  // ✅

  rewriter.on("h1", { ... });

  rewriter.on("p", { ... });

  await rewriter.write(encoder.encode("
1
2"));

  ```



- `end` may only be called once per `HTMLRewriter` instance. This means you must

  create a new `HTMLRewriter` instance for each transformation:



  ```js

  // ❌

  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:



  ```js

  const rewriter = new HTMLRewriter(...).on("p", {

    async element(element) {

      await fetch(...);

      element.setInnerContent("new");

    }

  });



  // ❌

  rewriter.write(encoder.encode("
1
"));

  rewriter.write(encoder.encode("
2
")); // not allowed



  // ❌

  const promise1 = rewriter.write(encoder.encode("
1
"));

  const promise2 = rewriter.write(encoder.encode("
2
"));

  await Promise.all([promise1, promise2]); // not allowed



  // ✅

  await rewriter.write(encoder.encode("
1
"));

  await rewriter.write(encoder.encode("
2
"));

  ```



## Internals



`lol-html` doesn't natively support asynchronous handlers. Instead, whenever a

handler returns a `Promise`, we have to unwind the WebAssembly stack into

temporary storage, wait for the promise to resolve, then rewind the stack and

continue parsing. This temporary storage is per `HTMLRewriter` instance, hence

we cannot have concurrent `write` and `end` calls. We use the

[Asyncify](https://github.com/WebAssembly/binaryen/blob/main/src/passes/Asyncify.cpp)

feature of [Binaryen](https://github.com/WebAssembly/binaryen) to implement

this. See

[this article](https://kripken.github.io/blog/wasm/2019/07/16/asyncify.html) for

more details.



## Building



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`:



```shell

$ cargo install --git https://github.com/mrbbot/wasm-pack

$ npm run build

$ npm test

```



## License



`html-rewriter-wasm` uses [lol-html](https://github.com/cloudflare/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:



1. Redistributions of source code must retain the above copyright notice, this

list of conditions and the following disclaimer.



2. Redistributions in binary form must reproduce the above copyright notice,

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

specific prior written permission.



THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND

ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED

WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE

DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE FOR

ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES

(INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;

LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON

ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT

(INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS

SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.

```