TagSoup

TagSoup is the fastest pure JS SAX/DOM XML/HTML parser.

• It is the fastest;
• Tiny and tree-shakable, just 6.5 kB gzipped;
• Streaming support with SAX and DOM parsers for XML and HTML;
• Extremely low memory consumption;
• Forgives malformed tag nesting and missing end tags;
• Parses HTML attributes in the same way your browser does, see tests for more details;
• Recognizes CDATA, processing instructions, and DOCTYPE;

npm install --save-prod tag-soup

Usage

⚠️ API documentation is available here.

SAX

import {createSaxParser} from 'tag-soup';

// Or use
// import {createXmlSaxParser, createHtmlSaxParser} from 'tag-soup';

const saxParser = createSaxParser({

  startTag(token) {
    console.log(token); // → {tokenType: 1, name: 'foo', …} 
  },

  endTag(token) {
    console.log(token); // → {tokenType: 101, data: 'okay', …} 
  },
});

saxParser.parse('<foo>okay');

SAX parser invokes callbacks during parsing.

Callbacks receive tokens which represent structures read from the input. Tokens are pooled objects so when handler callback finishes they are returned to the pool and reused. Object pooling drastically reduces memory consumption and allows passing a lot of data to the callback.

If you need to retain token after callback finishes use token.clone() which returns the deep copy of the token.

startTag and endTag callbacks are always invoked in the correct order even if tags in the input were incorrectly nested or missed. For self-closing tags only startTag callback in invoked.

Defaults

All SAX parser factories accept two arguments the handler with callbacks and options. The most generic parser factory createSaxParser doesn't have any defaults.

For createXmlSaxParser defaults are xmlParserOptions:

• CDATA sections, processing instructions and self-closing tags are recognized;
• XML entities are decoded in text and attribute values;
• Tag and attribute names are preserved as is;

For createHtmlSaxParser defaults are htmlParserOptions:

• CDATA sections and processing instructions are treated as comments;
• Self-closing tags are treated as a start tags;
• Tags like p, li, td and others follow implicit end rules, so <p>foo<p>bar is parsed as <p>foo</p><p>bar</p>;
• Tag and attribute names are converted to lower case;
• Legacy HTML entities are decoded in text and attribute values.

You can alter how the parser works through options which give you fine-grained control over parsing dialect.

By default, TagSoup uses speedy-entites to decode XML and HTML entities. Parser created by createHtmlSaxParser decodes only legacy HTML entities. This is done to reduce the bundle size.

To decode all HTML entities use this snippet below. It would add 10 kB gzipped to the bundle size.

import {decodeHtml} from 'speedy-entities/lib/full';

const htmlParser = createHtmlSaxParser({
  decodeText: decodeHtml,
  decodeAttribute: decodeHtml,
});

With speedy-entites you can create a custom decoder that would recognize custom entities.

The list of legacy HTML entities

aacute, Aacute, acirc, Acirc, acute, aelig, AElig, agrave, Agrave, amp, AMP, aring, Aring, atilde, Atilde, auml, Auml, brvbar, ccedil, Ccedil, cedil, cent, copy, COPY, curren, deg, divide, eacute, Eacute, ecirc, Ecirc, egrave, Egrave, eth, ETH, euml, Euml, frac12, frac14, frac34, gt, GT, iacute, Iacute, icirc, Icirc, iexcl, igrave, Igrave, iquest, iuml, Iuml, laquo, lt, LT, macr, micro, middot, nbsp, not, ntilde, Ntilde, oacute, Oacute, ocirc, Ocirc, ograve, Ograve, ordf, ordm, oslash, Oslash, otilde, Otilde, ouml, Ouml, para, plusmn, pound, quot, QUOT, raquo, reg, REG, sect, shy, sup1, sup2, sup3, szlig, thorn, THORN, times, uacute, Uacute, ucirc, Ucirc, ugrave, Ugrave, uml, uuml, Uuml, yacute, Yacute, yen and yuml

Streaming

SAX parsers support streaming. You can use saxParser.write(chunk) to parse input data chunk by chunk.

const saxParser = createSaxParser({/*callbacks*/});

saxParser.write('<foo>ok');
// Triggers startTag callabck for "foo" tag.

saxParser.write('ay');
// Doesn't trigger any callbacks.

saxParser.write('</foo>');
// Triggers text callback for "okay" and endTag callback for "foo" tag.

DOM

import {createDomParser} from 'tag-soup';

// Or use
// import {createXmlDomParser, createHtmlDomParser} from 'tag-soup';

// Minimal DOM handler example
const domParser = createDomParser<any>({

  element(token) {
    return {tagName: token.name, children: []};
  },

  appendChild(parentNode, node) {
    parentNode.children.push(node);
  },
});

const domNode = domParser.parse('<foo>okay');

console.log(domNode[0].children[0].data); // → 'okay'

DOM parser assembles a node three using a handler that describes how nodes are created and appended.

The generic parser factory createDomParser requires a handler to be provided.

Both createXmlDomParser and createHtmlDomParser use domHandler if no other handler was provided and use default options (xmlParserOptions and htmlParserOptions respectively) which can be overridden.

Streaming

DOM parsers support streaming. You can use domParser.write(chunk) to parse input data chunk by chunk.

const domParser = createXmlDomParser();

domParser.write('<foo>ok');
// → [{nodeType: 1, tagName: 'foo', children: [], …}]

domParser.write('ay');
// → [{nodeType: 1, tagName: 'foo', children: [], …}]

domParser.write('</foo>');
// → [{nodeType: 1, tagName: 'foo', children: [{nodeType: 3, data: 'okay', …}], …}]

Performance

To run a performance test use npm ci && npm run build && npm run perf.

Large input

Performance was measured when parsing the 3.81 MB HTML file.

Results are in operations per second. The higher number is better.

SAX benchmark

	Ops/sec
createSaxParser ¹	36.3 ± 0.8%
createXmlSaxParser ¹	30.7 ± 0.5%
createHtmlSaxParser ¹	23.7 ± 0.5%
createSaxParser	29.2 ± 0.5%
createXmlSaxParser	26.1 ± 0.5%
createHtmlSaxParser	19.9 ± 0.5%
@fb55/htmlparser2	14.3 ± 0.5%
@isaacs/sax-js	1.7 ± 4.6%

¹ Parsers were provided a handler with a single text callback. This configuration can be useful if you want to strip tags from the input.

DOM benchmark

	Ops/sec
createDomParser	12.1 ± 4.3%
createXmlDomParser	11.1 ± 4.3%
createHtmlDomParser	8.9 ± 2.4%
@fb55/htmlparser2	6.0 ± 1.5%
@inikulin/parse5	2.0 ± 1.8%

Small input

The performance was measured when parsing 258 files with 95 kB in size on average from htmlparser-benchmark.

Results are in operations per second. The higher number is better.

SAX benchmark

	Ops/sec
createSaxParser	1 755 ± 0.1%
createXmlSaxParser	1 450 ± 0.1%
createHtmlSaxParser	1 162 ± 0.1%
@fb55/htmlparser2	546 ± 5.7%

DOM benchmark

	Ops/sec
createDomParser	839 ± 3.2%
createXmlDomParser	718 ± 3.1%
createHtmlDomParser	612 ± 3.2%
@fb55/htmlparser2	437 ± 3.1%
@inikulin/parse5	37 ± 3.1%

Limitations

TagSoup doesn't resolve some weird element structures that malformed HTML may cause.

For example, assume the following markup:

<p><strong>okay
<p>nope

With DOMParser this markup would be transformed to:

<p><strong>okay</strong></p>
<p><strong>nope</strong></p>

TagSoup doesn't insert the second strong tag:

<p><strong>okay</strong></p>
<p>nope</p> <!-- Note the absent "strong" tag  -->