An implementation of HTMLRewriter using a WebAssembly version of lol-html. This was primarily written for ๐ฅ 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.
- ๐ Supports all handler types, properties and methods
- โฐ Supports synchronous and asynchronous handlers
- ๐ Supports class handlers with correctly bound methods
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("<p>old</p>"));
await rewriter.end();
console.log(output); // <p>new</p>
} finally {
rewriter.free(); // Remember to free memory
}
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.
To enable the
html_rewriter_treats_esi_include_as_void_tag
compatibility flag, set enableEsiTags
when constructing the HTMLRewriter
:
const rewriter = new HTMLRewriter((outputChunk) => { ... }, {
enableEsiTags: true,
});
-
Once
write
orend
has been called, you cannot add any more handlers. You must register all handlers before you start transforming:const rewriter = new HTMLRewriter(...); // โ rewriter.on("h1", { ... }); await rewriter.write(encoder.encode("<h1>1</h1")); rewriter.on("p", { ... }); // not allowed await rewriter.write(encoder.encode("<p>2</p>")); // โ rewriter.on("h1", { ... }); rewriter.on("p", { ... }); await rewriter.write(encoder.encode("<h1>1</h1")); await rewriter.write(encoder.encode("<p>2</p>"));
-
end
may only be called once perHTMLRewriter
instance. This means you must create a newHTMLRewriter
instance for each transformation:// โ const rewriter = new HTMLRewriter(...); await rewriter.end(); await rewriter.end(); // not allowed // โ const rewriter1 = new HTMLRewriter(...); await rewriter1.end(); const rewriter2 = new HTMLRewriter(...); await rewriter2.end();
-
When using
async
handlers, you must alwaysawait
calls towrite
andend
before calling them again. In other words, you cannot have concurrentwrite
andend
calls:const rewriter = new HTMLRewriter(...).on("p", { async element(element) { await fetch(...); element.setInnerContent("new"); } }); // โ rewriter.write(encoder.encode("<p>1</p>")); rewriter.write(encoder.encode("<p>2</p>")); // not allowed // โ 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 // โ await rewriter.write(encoder.encode("<p>1</p>")); await rewriter.write(encoder.encode("<p>2</p>"));
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
feature of Binaryen to implement
this. See
this article for
more details.
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
:
$ cargo install --git https://github.com/mrbbot/wasm-pack
$ npm run build
$ npm test
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.
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.
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