SAX (Simple API for XML) for WebAssembly

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

The first streamable, low memory XML, HTML, JSX and Angular Template parser for WebAssembly.

Sax Wasm is a sax style parser for XML, HTML, JSX and Angular Templates written in Rust, compiled for WebAssembly with the sole motivation to bring the fastest possible speeds to XML and JSX parsing for node and the web. Inspired by sax js and rebuilt with Rust for WebAssembly, 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.

Entity Start and End Positions

The sax-wasm parser provides precise location information for each entity encountered during the parsing process. Here's how you can access and utilize this information:

Entity Types

• Elements: Both opening and closing tags.
• Attributes: Within elements.
• Text: Text content, CDATA, Entities, etc.
• ProcInst: Processing instructions.

Position Data

For each entity, sax-wasm returns a Position object:

• Start and End Positions: Objects with:
  • line: The line number where the entity begins.
  • character: The column or "character" number where the entity begins.

The position data 100% works with xml.substring(start, end) or xml.slice(start, end) and takes into account 2-4 byte graphemes such as emojis, cyrillic or utf-16 encoded documents.

Example Output

When parsing <div class="myDiv">This is my div</div>, you might receive output like this:

{
  openStart: {
    line: 0,
    character: 0,
  },
  openEnd: {
    line: 0,
    character: 19,
  },
  closeStart: {
    line: 0,
    character: 33,
  },
  closeEnd: {
    line: 0,
    character: 39,
  },
  name: "div",
  attributes: [
    {
      name: {
        start: {
          line: 0,
          character: 5,
        },
        end: {
          line: 0,
          character: 10,
        },
        value: "class",
      },
      value: {
        start: {
          line: 0,
          character: 12,
        },
        end: {
          line: 0,
          character: 17,
        },
        value: "myDiv",
      },
      type: 0,
    },
  ],
  textNodes: [
    {
      start: {
        line: 0,
        character: 19,
      },
      end: {
        line: 0,
        character: 33,
      },
      value: "This is my div",
    },
  ],
  selfClosing: false,
}

Benchmarks (Node v20.18.1 / 2.7 GHz Quad-Core Intel Core i7)

All parsers are tested using a large XML document (3 MB) containing a variety of elements and is streamed from memory to remove variations in disk access latency and focus on benchmarking just the parser alone. Other libraries test benchmarks using a very small XML fragment such as <foo bar="baz">quux</foo> which does not hit all code branches responsible for processing the document and heavily skews the results in their favor.

Parser with Advanced Features	time/ms (lower is better)	JS	Runs in browser
sax-wasm	18.54	☑	☑
saxes	41.01	☑	☑
ltx(using Saxes as the parser)	44.56	☑	☑
sax-js	116.98	☑	☑*
node-xml	124.49	☑	☐
node-expat	149.61	☑	☐
*built for node but should run in the browser

Installation

npm i -s sax-wasm

Usage in Node

import fs from 'fs';
import path from 'path';
import { fileURLToPath } from 'url';
import { SaxEventType, SAXParser } from 'sax-wasm';

const wasmUrl = new URL(import.meta.resolve('sax-wasm/lib/sax-wasm.wasm'));
const saxWasm = await readFile(wasmUrl);
const parser = new SAXParser(SaxEventType.Cdata | SaxEventType.OpenTag);

if (await parser.prepareWasm(saxWasm)) {
  const xmlPath = import.meta.resolve('../src/xml.xml');
  const readable = createReadStream(new URL(xmlPath));
  const webReadable = Readable.toWeb(readable);
  for await (const [event, detail] of parser.parse(webReadable.getReader())) {
    if (event === SaxEventType.Cdata) {
      // process Cdata
    } else {
      // process open tag
    }
  }
}

Usage for the web

1. Instantiate and prepare the wasm for parsing
2. Pipe the document stream to sax-wasm using ReadableStream.getReader()

NOTE This uses WebAssembly.instantiateStreaming under the hood to load the wasm.

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

// Fetch the WebAssembly binary
const wasmUrl = new URL(import.meta.resolve('sax-wasm/lib/sax-wasm.wasm'));
const response = fetch(wasmUrl);

// Instantiate
const parser = new SAXParser(SaxEventType.Attribute | SaxEventType.OpenTag);

// Instantiate and prepare the wasm for parsing
const ready = await parser.prepareWasm(response);
if (ready) {
  // Fetch the XML document
  const xmlResponse = await fetch('path/to/document.xml');
  const reader = xmlResponse.body.getReader();

  for await (const [event, detail] of parser.parse(reader)) {
    if (event === SaxEventType.Attribute) {
      // process attribute
    } else {
      // process open tag
    }
  }
}

The 'Lifetime' of events

Tag, Attribute, ProcInst and Text objects received from the parsing operation have a 'lifetime' that is limited to the eventHandler() or the function loop body for the *parse() generator. Data sent across the FFI (Foreign Function Interface) boundary is read directly from WASM memory which is partly why sax-wasm is so fast. This comes with the tradeoff that this memory is temporary because it is overwritten on the next write operation. If you need to persist the event data for long term use, call toJSON() on each object as needed. This comes at a slight performance cost and should not be necessary for the vast majority of use cases.

Differences from other parsers

Besides being incredibly fast, there are some notable differences between other SAX style parsers:

1. This repo is maintained
2. UTF-16 encoded documents are supported. 1-4 byte graphemes are fully supported even if streaming causes a break between surrogates.
3. JSX is supported including JSX fragments. Things like <foo bar={this.bar()}></bar> and <><foo/><bar/></> will parse as expected.
4. Angular 2+ templates are supported. Things like <button type="submit" [disabled]=disabled *ngIf=boolean (click)="clickHandler(event)"> will parse as expected.
5. HTML is supported provided it is not a "quirks mode" document that ran in IE9.
6. 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.
7. Namespaces are reported in attributes. No special events dedicated to namespaces.
8. Streaming utf-8 code points in a Uint8Array is required.
9. Whitespace between XML elements is not reported. If you need this, a simple subtraction of the line and character between the end of one tag and the start of the next will reveal where this whitespace exists.

Streaming

Streaming is supported with sax-wasm by writing utf-8 code points (Uint8Array) 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 passed to handler
SaxEventType.Text	0b000000000001	text: Text
SaxEventType.ProcessingInstruction	0b000000000010	procInst: Text
SaxEventType.SGMLDeclaration	0b000000000100	sgmlDecl: Text
SaxEventType.Doctype	0b000000001000	doctype: Text
SaxEventType.Comment	0b000000010000	comment: Text
SaxEventType.OpenTagStart	0b000000100000	tag: Tag
SaxEventType.Attribute	0b000001000000	attribute: Attribute
SaxEventType.OpenTag	0b000010000000	tag: Tag
SaxEventType.CloseTag	0b000100000000	tag: Tag
SaxEventType.CDATA	0b001000000000	start: Position

Speeding things up on large documents

The speed of the sax-wasm parser is incredibly fast and can parse very large documents in a blink of an eye. Although it's performance out of the box is ridiculous, the JavaScript thread must be involved with transforming raw bytes to human readable data, there are times where slowdowns can occur if you're not careful. These are some of the items to consider when top speed and performance is an absolute must:

1. Stream your document from it's source as a Uint8Array - This is covered in the examples above. Things slow down significantly when the document is loaded in JavaScript as a string, then encoded to bytes using Buffer.from(document) or new TextEncoder.encode(document) before being passed to the parser. Encoding on the JavaScript thread is adds a non-trivial amount of overhead so its best to keep the data as raw bytes. Streaming often means the parser will already be done once the document finishes downloading!
2. Keep the events bitmask to a bare minimum whenever possible - the more events that are required, the more work the JavaScript thread must do once sax-wasm.wasm reports back.
3. Limit property reads on the reported data to only what's necessary - this includes things like stringifying the data to json using JSON.stringify(). The first read of a property on a data object reported by the eventHandler will retrieve the value from raw bytes and convert it to a string, number or Position on the JavaScript thread. This conversion time becomes noticeable on very large documents with many elements and attributes. NOTE: After the initial read, the value is cached and accessing it becomes faster.

SAXParser.js

Constructor

SaxParser([events: number, [options: SaxParserOptions]])

Constructs new SaxParser instance with the specified events bitmask and options

Parameters

• events - A number representing a bitmask of events that should be reported by the parser.

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(chunk: Uint8Array, offset: number = 0): void; - writes the supplied bytes to the wasm memory buffer and kicks off processing. An optional offset can be provided if the read should occur at an index other than 0. NOTE: The line and character counters are not reset.

• end(): void; - Ends processing for the stream. The line and character 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

• eventHandler - 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

The methods listed here can be used to create your own implementation of the SaxWasm class when extending it or composing it will not meet the needs of the program.

• 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 grapheme clusters. Writing non utf-8 bytes may cause unpredictable results but probably will not break.

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

Building from source

Prerequisites

This project requires rust v1.30+ since it contains the wasm32-unknown-unknown target out of the box.

Install rust:

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

Install the stable compiler and switch to it.

rustup install stable
rustup default stable

Install the wasm32-unknown-unknown target.

rustup target add wasm32-unknown-unknown --toolchain stable

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

npm install

Install the wasm-bindgen-cli tool

cargo install wasm-bindgen-cli

The project can now be built using:

npm run build

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