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.

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. Inspired by sax js and rebuilt with Rust for Web Assembly sax-wasm brings optimizations for speed and support for JSX syntax.

Suitable for LSP implementations, sax-wasm provides line numbers and character positions within the document for elements, attributes and text node which provides the raw building blocks for linting, transpilation and lexing.

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(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>');
    parser.end();
  }
});

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(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(processDocument);

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

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> and <><foo/><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 or document validation, it may 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 within 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 12 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	Mask	Argument
SaxEventType.Text	0b1	text: Text
SaxEventType.ProcessingInstruction	0b10	procInst: string
SaxEventType.SGMLDeclaration	0b100	sgmlDecl: string
SaxEventType.Doctype	0b1000	doctype: string
SaxEventType.Comment	0b10000	comment: string
SaxEventType.OpenTagStart	0b100000	tag: Tag
SaxEventType.Attribute	0b1000000	attribute: Attribute
SaxEventType.OpenTag	0b10000000	tag: Tag
SaxEventType.CloseTag	0b100000000	tag: Tag
SaxEventType.OpenCDATA	0b1000000000	start: Position
SaxEventType.CDATA	0b10000000000	cdata: string
SaxEventType.CloseCDATA	0b100000000000	end: Position

SAXParser.js

Methods

• prepareWasm(wasm: Uint8Array): Promise<boolean> - Instantiates the wasm binary with reasonable defaults and stores the instance as a member of the class. Always resolves to true or throws if something went wrong.

• write(buffer: string): void; - writes the supplied string to the wasm stream and kicks off processing. The character and line counters are not reset.

• end(): void; - Ends processing for the stream. The character and line counters are reset to zero and the parser is readied for the next document.

Properties

• events - A bitmask containing the events to subscribe to. See the examples for creating the bitmask

• eventHanlder - A function reference used for event handling. The supplied function must have a signature that accepts 2 arguments: 1. The event which is one of the SaxEventTypes and the body (listed in the table above)

sax-wasm.wasm

Methods

• parser(events: u32) - Prepares the parser struct internally and supplies it with the specified events bitmask. Changing the events bitmask can be done at anytime during processing using this method.

• write(ptr: *mut u8, length: usize) - Supplies the parser with the location and length of the newly written bytes in the stream and kicks off processing. The parser assumes that the bytes are valid utf-8. Writing non utf-8 bytes will may cause unpredictable behavior.

• end() - resets the character and line counts but does not halt processing of the current buffer.

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 the wasm32-unknown-unknown target.

rustup target add wasm32-unknown-unknown --toolchain nightly

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

npm install

Follow the instructions to install wasm-gc based on your platform. The project can now be built using:

npm run build

The artifacts from the build will be located in the /libs directory.