sax-wasm

SAX (Simple API for XML) for Web Assembly

When you absolutely, positively have to have the fastest parser in the room, accept no substitutes.

The first streamable, low memory XML, HTML, and JSX parser for WebAssembly. And yes, it will parse JSX!

Sax Wasm is a sax style parser for XML, HTML and JSX written in Rust, compiled for Web Assembly with the sole motivation to bring near native speeds to xml and JSX parsing for node and the web.

This is a port of sax js to Rust for Web Assembly with optimizations for speed and JSX specific syntax. Since there are no built-ins with WebAssembly, file size is larger but execution time is faster, sometimes much, much faster.

Installation

npm i -s sax-wasm

Usage in Node

const fs = require('fs');
const { SaxEventType, SAXParser } = require('sax-wasm');

// Get the path to the Web Assembly binary and load it
const saxPath = require.resolve('sax-wasm/lib/sax-wasm.wasm');
const saxWasmBuffer = fs.readFileSync(saxPath);

// Instantiate 
const parser = new SAXParser(0, SaxEventType.Attribute | SaxEventType.OpenTag);
parser.eventHandler = (event, data) => {
  if (event === SaxEventType.Attribute ) {
    // process attribute
  } else {
    // process open tag
  }
}

// Instantiate and prepare the wasm for parsing
parser.prepareWasm(saxWasmBuffer).then(ready => {
  if (ready) {
    parser.write('<div class="modal"></div>');
  }
})

Usage for the web

import { SaxEventType, SAXParser } from 'sax-wasm';

async function loadAndPrepareWasm() {
  const saxWasmResponse = await fetch('./path/to/wasm/sax-wasm.wasm');
  const saxWasmbuffer = await saxWasmResponse.arrayBuffer();
  const parser = new SAXParser(0, SaxEventType.Attribute | SaxEventType.OpenTag);
  
  // Instantiate and prepare the wasm for parsing
  const ready = await parser.prepareWasm(new Uint8Array(saxWasmbuffer));
  if (ready) {
    return parser;
  }
}

loadAndPrepareWasm().then(parser => {
  parser.eventHandler = (event, data) => {
      if (event === SaxEventType.Attribute ) {
          // process attribute
        } else {
          // process open tag
        }
    }
    parser.write('<div class="modal"></div>')
});

Differences from sax-js

Besides being incredibly fast, there are some notable differences between sax-wasm and sax-js that may affect some users when migrating:

1. JSX is supported including JSX fragments. Things like <foo bar={this.bar()}></bar> will parse as expected.
2. No attempt is made to validate the document. sax-wasm reports what it sees. If you need "strict mode", it could be recreated by applying rules to the events that are reported by the parser.
3. Namespaces are reported in attributes. No special events dedicated to namespaces
4. The parser is ready as soon as the promise is handled.

Streaming

Streaming is supported with sax-wasm by writing utf-8 encoded text to the parser instance. Writes can occur safely anywhere except withing the eventHandler function or within the eventTrap (when extending SAXParser class). Doing so anyway risks overwriting memory still in play.

Events

Events are subscribed to using a bitmask composed from flags representing the event type. Bit positions along a 15 bit integer can be masked on to tell the parser to emit the event of that type. For example, passing in the following bitmask to the parser instructs it to emit events for text, open tags and attributes:

import { SaxEventType } from 'sax-wasm';
parser.events = SaxEventType.Text | SaxEventType.OpenTag | SaxEventType.Attribute;

Complete list of event/argument pairs:

Event	Argument
SaxEventType.Text	text: string
SaxEventType.ProcessingInstruction	procInst: string
SaxEventType.SGMLDeclaration	sgmlDecl: string
SaxEventType.Doctype	doctype: string
SaxEventType.Comment	comment: string
SaxEventType.OpenTagStart	tag: Tag
SaxEventType.Attribute	attribute: Attribute
SaxEventType.OpenTag	tag: Tag
SaxEventType.CloseTag	tag: Tag
SaxEventType.OpenCDATA	start: Position
SaxEventType.CDATA	cdata: string
SaxEventType.CloseCDATA	end: Position
SaxEventType.CloseNamespace	ns: Namespace
SaxEventType.Script	script: string
SaxEventType.OpenNamespace	ns: Namespace

Building from source

Prerequisites

This project requires rust v1.29+ since it contains the wasm32-unknown-unknown target out of the box. This is currently only available in the nightly build.

Install rust:

curl https://sh.rustup.rs -sSf | sh

Install the nightly compiler and switch to it

rustup install nightly
rustup default nightly

Install node with npm then run the following command from the project root.

npm install

The project can now be built using:

npm run build