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Generic interruptible "parser" mixin for Transform & Writable streams





Generic interruptible "parser" mixin for Transform & Writable streams

Build Status

This module offers the stream-parser mixin, which provides an easy-to-use API for parsing bytes from Writable and/or Transform stream instances. This module is great for implementing streaming parsers for standardized file formats.

For Writable streams, the parser takes control over the _write callback function. For Transform streams, the parser controls the _transform callback function.


$ npm install stream-parser


Let's create a quick Transform stream subclass that utilizes the parser's _bytes() and _passthrough() functions to parse a theoretical file format that has an 8-byte header we want to parse, and then pass through the rest of the data.

var Parser = require('stream-parser');
var inherits = require('util').inherits;
var Transform = require('stream').Transform;

// create a Transform stream subclass
function MyParser () {;

  // buffer the first 8 bytes written
  this._bytes(8, this.onheader);
inherits(MyParser, Transform);

// mixin stream-parser into MyParser's `prototype`

// invoked when the first 8 bytes have been received
MyParser.prototype.onheader = function (buffer, output) {
  // parse the "buffer" into a useful "header" object
  var header = {};
  header.type = buffer.readUInt32LE(0); = buffer.toString('utf8', 4);
  this.emit('header', header);

  // it's usually a good idea to queue the next "piece" within the callback

// now we can *use* it!
var parser = new MyParser();
parser.on('header', function (header) {
  console.error('got "header"', header);

Here's an example of manually creating a Transform stream and turning it into a "pass through" stream equivalent to the one built into node core:

var Parser = require('stream-parser');
var Transform = require('stream').Transform;

// create a Transform instance and extend it with "stream-parser"
var p = new Transform();

// pass through `Infinity` bytes... forever...

// now `p` is equivalent to a stream.PassThrough instance

See the test directory for some more example code in the test cases.

A list of known concrete implementations is here (send pull requests for more!):



The Parser stream mixin works with either Writable or Transform stream instances/subclasses. Provides a convenient generic "parsing" API:

_bytes(n, cb) - buffers "n" bytes and then calls "cb" with the "chunk"
_skipBytes(n, cb) - skips "n" bytes and then calls "cb" when done

If you extend a Transform stream, then the _passthrough() function is also added:

_passthrough(n, cb) - passes through "n" bytes untouched and then calls "cb"

._bytes(n, cb)

Buffers n bytes and then invokes cb once that amount has been collected.

._skipBytes(n, cb)

Skips over the next n bytes and then invokes cb once that amount has been discarded.

._passthrough(n, cb)

Passes through n bytes to the readable side of this stream untouched, then invokes cb once that amount has been passed through. This function is only defined when stream-parser is extending a Transform stream.