sax

What is sax?

The sax npm package is a streaming XML parser that is designed for speed and simplicity. It follows the SAX parsing approach, which is an event-based model for parsing XML documents. This allows developers to handle different parts of the document as they are parsed without keeping the entire document in memory.

What are sax's main functionalities?

Parsing XML

This code demonstrates how to parse an XML string. It creates a new SAX parser, sets up an event listener for the 'opentag' event to log the name and attributes of each tag, and then writes an XML string to the parser.

const sax = require('sax'),
  parser = sax.parser(true);
parser.onopentag = function (node) {
  // node has attributes with string values
  console.log(node.name + ' - ' + JSON.stringify(node.attributes));
};
parser.write('<xml><tag attr="value">content</tag></xml>').close();

Stream Parsing

This code demonstrates how to parse XML from a file stream. It creates a SAX stream, sets up an event listener for the 'opentag' event, and then pipes a read stream from a file into the SAX stream.

const sax = require('sax'),
  fs = require('fs'),
  saxStream = sax.createStream(true);
saxStream.on('opentag', function (node) {
  console.log(node.name + ' - ' + JSON.stringify(node.attributes));
});
fs.createReadStream('file.xml').pipe(saxStream);

Error Handling

This code demonstrates how to handle errors during parsing. It sets up an error event listener on the SAX parser to handle any parsing errors.

const sax = require('sax'),
  parser = sax.parser(true);
parser.onerror = function (e) {
  // an error happened.
};
parser.write('<xml>this is some malformed xml</xml>').close();

Other packages similar to sax

xml2js

xml2js is a similar npm package that provides an XML to JavaScript object converter. Unlike sax, which is a streaming parser, xml2js reads the entire XML document and converts it into a JavaScript object. This can be more convenient for small documents but less efficient for large ones.

fast-xml-parser

fast-xml-parser is another alternative that offers both a streaming mode and a non-streaming mode for XML parsing. It claims to be very fast and flexible, providing options to validate, parse, and traverse XML. It is a good alternative to sax when performance is a critical factor and when additional features like validation are needed.

libxmljs

libxmljs is a binding to the libxml C library, providing a more traditional DOM-based approach to parsing XML. It is different from sax in that it builds an in-memory representation of the entire document, which can then be queried and manipulated. This is more powerful but also more resource-intensive than the event-based approach of sax.

README

sax js

A sax-style parser for XML and HTML.

Designed with node in mind, but should work fine in the browser or other CommonJS implementations.

What This Is

• A very simple tool to parse through an XML string.
• A stepping stone to a streaming HTML parser.
• A handy way to deal with RSS and other mostly-ok-but-kinda-broken XML docs.

What This Is (probably) Not

• An HTML Parser - That's a fine goal, but this isn't it. It's just XML.
• A DOM Builder - You can use it to build an object model out of XML, but it doesn't do that out of the box.
• XSLT - No DOM = no querying.
• 100% Compliant with (some other SAX implementation) - Most SAX implementations are in Java and do a lot more than this does.
• An XML Validator - It does a little validation when in strict mode, but not much.
• A Schema-Aware XSD Thing - Schemas are an exercise in fetishistic masochism.
• A DTD-aware Thing - Fetching DTDs is a much bigger job.

Regarding <!DOCTYPEs and <!ENTITYs

The parser will handle the basic XML entities in text nodes and attribute values: & < > ' ". It's possible to define additional entities in XML by putting them in the DTD. This parser doesn't do anything with that. If you want to listen to the ondoctype event, and then fetch the doctypes, and read the entities and add them to parser.ENTITIES, then be my guest.

Unknown entities will fail in strict mode, and in loose mode, will pass through unmolested.

Usage

var sax = require("./lib/sax"),
  strict = true, // set to false for html-mode
  parser = sax.parser(strict);

parser.onerror = function (e) {
  // an error happened.
};
parser.ontext = function (t) {
  // got some text.  t is the string of text.
};
parser.onopentag = function (node) {
  // opened a tag.  node has "name" and "attributes"
};
parser.onattribute = function (attr) {
  // an attribute.  attr has "name" and "value"
};
parser.onend = function () {
  // parser stream is done, and ready to have more stuff written to it.
};

parser.write('<xml>Hello, <who name="world">world</who>!</xml>').close();

// stream usage
// takes the same options as the parser
var saxStream = require("sax").createStream(strict, options)
saxStream.on("error", function (e) {
  // unhandled errors will throw, since this is a proper node
  // event emitter.
  console.error("error!", e)
  // clear the error
  this._parser.error = null
  this._parser.resume()
})
saxStream.on("opentag", function (node) {
  // same object as above
})
// pipe is supported, and it's readable/writable
// same chunks coming in also go out.
fs.createReadStream("file.xml")
  .pipe(saxStream)
  .pipe(fs.createWriteStream("file-copy.xml"))

Arguments

Pass the following arguments to the parser function. All are optional.

strict - Boolean. Whether or not to be a jerk. Default: false.

opt - Object bag of settings regarding string formatting. All default to false.

Settings supported:

• trim - Boolean. Whether or not to trim text and comment nodes.
• normalize - Boolean. If true, then turn any whitespace into a single space.
• lowercase - Boolean. If true, then lowercase tag names and attribute names in loose mode, rather than uppercasing them.
• xmlns - Boolean. If true, then namespaces are supported.
• position - Boolean. If false, then don't track line/col/position.
• strictEntities - Boolean. If true, only parse predefined XML entities (&, ', >, <, and ")

Methods

write - Write bytes onto the stream. You don't have to do this all at once. You can keep writing as much as you want.

close - Close the stream. Once closed, no more data may be written until it is done processing the buffer, which is signaled by the end event.

resume - To gracefully handle errors, assign a listener to the error event. Then, when the error is taken care of, you can call resume to continue parsing. Otherwise, the parser will not continue while in an error state.

Members

At all times, the parser object will have the following members:

line, column, position - Indications of the position in the XML document where the parser currently is looking.

startTagPosition - Indicates the position where the current tag starts.

closed - Boolean indicating whether or not the parser can be written to. If it's true, then wait for the ready event to write again.

strict - Boolean indicating whether or not the parser is a jerk.

opt - Any options passed into the constructor.

tag - The current tag being dealt with.

And a bunch of other stuff that you probably shouldn't touch.

Events

All events emit with a single argument. To listen to an event, assign a function to on<eventname>. Functions get executed in the this-context of the parser object. The list of supported events are also in the exported EVENTS array.

When using the stream interface, assign handlers using the EventEmitter on function in the normal fashion.

error - Indication that something bad happened. The error will be hanging out on parser.error, and must be deleted before parsing can continue. By listening to this event, you can keep an eye on that kind of stuff. Note: this happens much more in strict mode. Argument: instance of Error.

text - Text node. Argument: string of text.

doctype - The <!DOCTYPE declaration. Argument: doctype string.

processinginstruction - Stuff like <?xml foo="blerg" ?>. Argument: object with name and body members. Attributes are not parsed, as processing instructions have implementation dependent semantics.

sgmldeclaration - Random SGML declarations. Stuff like <!ENTITY p> would trigger this kind of event. This is a weird thing to support, so it might go away at some point. SAX isn't intended to be used to parse SGML, after all.

opentagstart - Emitted immediately when the tag name is available, but before any attributes are encountered. Argument: object with a name field and an empty attributes set. Note that this is the same object that will later be emitted in the opentag event.

opentag - An opening tag. Argument: object with name and attributes. In non-strict mode, tag names are uppercased, unless the lowercase option is set. If the xmlns option is set, then it will contain namespace binding information on the ns member, and will have a local, prefix, and uri member.

closetag - A closing tag. In loose mode, tags are auto-closed if their parent closes. In strict mode, well-formedness is enforced. Note that self-closing tags will have closeTag emitted immediately after openTag. Argument: tag name.

attribute - An attribute node. Argument: object with name and value. In non-strict mode, attribute names are uppercased, unless the lowercase option is set. If the xmlns option is set, it will also contains namespace information.

comment - A comment node. Argument: the string of the comment.

opencdata - The opening tag of a <![CDATA[ block.

cdata - The text of a <![CDATA[ block. Since <![CDATA[ blocks can get quite large, this event may fire multiple times for a single block, if it is broken up into multiple write()s. Argument: the string of random character data.

closecdata - The closing tag (]]>) of a <![CDATA[ block.

opennamespace - If the xmlns option is set, then this event will signal the start of a new namespace binding.

closenamespace - If the xmlns option is set, then this event will signal the end of a namespace binding.

end - Indication that the closed stream has ended.

ready - Indication that the stream has reset, and is ready to be written to.

noscript - In non-strict mode, <script> tags trigger a "script" event, and their contents are not checked for special xml characters. If you pass noscript: true, then this behavior is suppressed.

Reporting Problems

It's best to write a failing test if you find an issue. I will always accept pull requests with failing tests if they demonstrate intended behavior, but it is very hard to figure out what issue you're describing without a test. Writing a test is also the best way for you yourself to figure out if you really understand the issue you think you have with sax-js.