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Packet simplifies the construction an maintenance of libraries that convert binary to JavaScript and back. The name Packet may make you think that it is designed solely for binary network protocols, but it is also great for reading and writing binary file formats.

Incremental — Node packet creates incremental parsers and serializers that are almost as fast as the parser you'd write by hand, but a lot easier to define and maintain.

Declarative — Packet defines a binary structure using a pattern language inspired by Perl's pack. The binary patterns are used to define both parsers and serializers. If you have a protocol specification, or even just a C header file with structures that define your binary data, you can probably translate that directly into Packet patterns.

For parsers, you associate the patterns to callbacks invoked with captured values when the pattern is extracted from the stream. For serializers you simply give the values to write along with the pattern to follow when writing them.

Expressive — The pattern language can express


Parsing not searching — Packet is not a pattern matching library. It does not search binary streams for patterns. Packet is used for parsing well-defined streams of binary data.

8-bit boundaries — I'm unable to think of an example in contemporary computing that doesn't align to an 8-bit boundary, but the world is big and I am small, so I plan on being surprised. I can also imagine that someone might want to unleash Packet on legacy data someday, from way back when a byte was whatever a computer manufacturer said it was.

Therefore, It's worth noting that Packet parses 8-bit bytes and expects bytes to align to an 8-bit boundary. Packet can parse 7-bit ASCII formats like tar archives, because they are 8-bit aligned with the top bit ignored. Packet can also parse and serialize bit packed integers, so it does support awkward integer sizes, but within an 8-bit aligned integer.


Install Packet using NPM.

npm install packet

The source is available on GitHub.

Parsers and Serializers

Packet defines a binary format using a binary pattern language inspired by Perl's pack function. The pattern language is used in a Parser to define the parameters passed to callback when enough bytes are read from the input stream to satisfy the pattern. The pattern language is used in a Serializer to define how JavaScript primitives passed to the serialize method are written to stream.


Patterns are a series of element declarations joined by commas.

parser.extract("length: b16, address: b32, name: b8z", function (object) {
  console.log(object.length, object.address,;
parser.parse([ 0x01, 0xFF, 0x01, 0x00, 0x00, 0x00, 0x01, 0x02, 0x00 ]);

You can also name the elements in a pattern. If named, parsers will be create an object to pass to callbacks, serializers will serialize an object's properties.

parser.extract("b16 => length, b32 => address, b8z => name", function (record) {
parser.parse([ 0x01, 0xFF, 0x01, 0x00, 0x00, 0x00, 0x01, 0x02, 0x00 ]);

Unnamed elements are good for short, simple patterns. For longer patterns it is easier to have a parser build an object for you.

The following example shows a complicated pattern, the invariable portion of an IP header, the first 20 bytes before options, if any.

// Define an IP header pattern using a joined array to explode the pattern.
parser.packet('ip', 'b8{version: b4, headerLength: b4},         \
                     typeOfService: b8,                         \
                     length: b16,                               \
                     identification: b16,                       \
                     b16{flags: b3, fragmentOffset: b13},       \
                     timeToLive: b8,                            \
                     protocol: b8,                              \
                     checksum: b16,                             \
                     sourceAddress: b32,                        \
                     destinationAddress: b32                    \

// The pattern is then used to defined parser and serializer actions.
parser.extract("ip", function (header) {

Both parsers and serializers can define patterns using the packet method. The packet method allows you to pre-compile patterns.

// Create a parser and define a header.
var parser = require('packet').createParser();
parser.packet("header", "b8 => type, b16 => length, b32 => extra");
parser.packet("data", "b16 => sequence, b16 => crc");

// Now you can define your serializer using your parser as a prototype. The
// serializer will inherit the parser's packet definitions.
var serializer = parser.createSerializer();

Parsers and serializers maintain internal state so that they can be used incrementally. If you're going to parse streams incrementally, you're going to need a parser for each stream your parsing. Same goes for serializers and serializing.

When parsing/serializing incrementally create new parsers and serializers using the prototype pattern above.

// Create a serializer and define a header.
var serializer = require("packet").createSerializer();
serializer.packet("header", "b8 => type, b16 => length, b32 => extra");
serializer.packet("data", "b16 => sequence, b16 => crc");

// Now you can define parsers for a using the serializer as a prototype.
net.createServer(function (socket) {
  var parser = serializer.createParser();
  socket.on("data", function (buffer) {
  parser.extract("header", function (header) {


Parsers parse a buffer extracting a specified pattern. The pattern is specified using the extract method. The extract method accept either a pattern or the name a pre-compiled pattern.

After the pattern has been specified, the parser will extract the data from one or more buffers according to the specified pattern.

When the pattern specified by extract has been read from the series of buffers, it will invoke a callback with the extracted values. The extract callback has the option of calling the extract method specifying a subsequent pattern for the parser to extract. When the callback returns, the parser will immediately continue to parse the newly specified pattern from the series of buffers.

The parser callback receives the values either as positioned function arguments or as an object. How the callback is invoked is based on the pattern and the arity of the callback function.

To receive an object in the callback, we defined named elements. When the pattern has at least one named element, and the callback has only a single argument, an object is passed to the callback containing the values using the element names as keys.

Unnamed elements are excluded, but there's no good reason not name them. Use a skip pattern to skip over unwanted bytes instead.

You can still get positioned arguments using a named pattern. Just provide a callback with more than one argument and it will be invoked with the extract values as parameters.

A callback for a pattern without any named elements is always invoked with values as parameters regardless of arity.


First you call serialize with a pattern and arguments to serialize, then you call write with a series of buffers.

Binary Pattern Fields

The binary pattern language is used to specify the fields binary structures in data streams, using a comma separated field pattern.

Big-Endian Byte Ordering

To define a big-endian byte ordering for a field, prefix the bit size with b.

mnemonic: The letter `b` stands for big-endian.
    pattern: world: b16
      word: 256
    description: Big-endian 32 bit number.
    pattern: byte: b8
      byte: 1
    description: Endianess of a single byte is irrelevant.
    pattern: word: l16, byte: b8
      word: 1
      byte: 1
    parsed: 0x01, 0x00, 0x01

Mnemonic: The letter b stands for big-endian.

"b16"             // Big-endian 32 bit number.
"b8"              // Endianess of a single byte is irrelevant.
"l16, b8"         // Big-endian 16 bit integer followed by a byte.

Little-Endian Byte Ordering

To define a little-endian byte ordering for a field, prefix the bit size with l.

Mnemonic: The letter l stands for little-endian.

"l32"             // Little-endian 32 bit integer.
"l8"              // Endianess of a single byte is irrelevant.
"l16, b8"         // Little endian 16 bit integer followed by a byte.

Skipping Bytes

You can skip over bytes your pattern with x.

Mnemonic: The letter x means to cross-out, which is kind of like skipping.

"b8, x16, l16"    // A byte, two skipped bytes, and a little-endian 16 bit
                  // integer.

Signed Versus Unsigned Integers

All numbers are assumed to be unsigned, unless prefixed by a negative symbol.

Mnemonic: The - symbol indicates the possibility of negative numbers.

"-b32"            // Big-endian 32 bit signed integer.
"-l32"            // Little-endian 32 bit signed integer.
"b32"             // Big-endian 32 bit unsigned integer.

IEEE 754 Floating Point Numbers

The number type for JavaScript is the 64 bit IEEE 754 floating point. Packet can read write 64 bit and 32 bit IEEE 754 floating point numbers.

To indicated that the type is a floating point number, use the f type suffix. This is indicated with a f suffix.

Mnemonic: The letter f stands for floating-point.

"b64f"            // Big-endian 64 bit IEEE 754 double floating point number.
"l32f"            // Little-endian 32 bit IEEE 754 single floating point number.

The floating-point numbers can be stored in little-endian or big-endian byte order.

Arrays of Raw Bytes

A value will be converted to a big-endian array of bytes if followed by an a suffix.

Mnemonic: The letter a stands for array.

"l128a"           // Unsigned little-endian 128 bit integer as big-endian array
                  // of bytes.

Note that big-endian means that the most significant byte is at index 0 of the array.

This can be surprising if you're expecting the significance of the bytes will increase with the index of the array, but then that's what little-endian is all about. (Big-endian orders like Arabic numerals, while little-endian orders like offsets into memory.)

If you'd prefer a little-endian array, simply call reverse on the array passed to you.

Arrays of Common Types

It is often the case that a binary format contains an array of values. The most common case are arrays of bytes representing ASCII or UTF-8 strings.

Arrays are specified with an subscript and a count.

Mnemonic: The square brackets are used as array subscripts in JavaScript, and used to declare array length in other C dialect languages.

"b32[4]"          // An array of four big-endian 32 bit numbers.
"b8[16]"          // An array of 16 bytes.

The array notation produces an array of the type before the subscript.

Zero Terminated Arrays

Zero terminated series are specified with a z qualifier.

You can specify both termination and width together. Why would you need this? This occurs in underlying C structures when there is a fixed width character array in a structure, but the structure still contains a zero terminated string.

Upcoming: Chose your own terminator.

Mnemonic: The letter z stands for zero.

"l16z"            // Little-endian 16 bit numbers terminated by a zero value.
"b8z"             // Byte string terminated by zero.
"b8[12]z"         // Byte string terminated by zero in a field 12 bytes long.

Array Padding

You can specify a padding value immediately after the array brackets using curly braces. This should be the numeric value, or character code for padding. If you want to zero pad, use 0. If you want to pad with ASCII spaces use 32.

Mnemonic: Curly braces are used to define array literals in C.

"b16[12]{0}"      // Array of 12 big-endian 16 bit integers, zero padded.
"b8[12]{32}z"     // Byte string terminated by zero in a field 12 bytes long
                  // ascii space padded.

Length Encoded Arrays

Length encoded arrays are specified by joining a count type and a value type with a forward slash character /.

Mnemonic: Inspired by Perl's pack, which uses the slash to separate count and type.

"b8/b8"           // Length encoded byte array with a byte length.
"l16/b8"          // Length encoded byte array with 16 bit little-endian length.

Bit Packed Integers

Integers are often divided into smaller integers through a process called bit packing. Bit packing is specified by following an integer specification with a curly brace enclosed series series of integers patterns whose total size in bits equals the size of the packed integer.

Packed integers are always big-endian and can be either singed or unsigned.

Mnemonic Curly braces are used to define structures in C and bit packing is kind of like a structure.

"b16{b3,x6,-b7}"  // A 16 bit big-endian integer divided into a 3-bit integer,
                  // 6 skipped bits, and a signed 7-bit integer.

You can also name the packed fields.

"b16{b3 => type, x6, -b7 => count}"

Alternate Patterns

A common pattern in binary formats is using the value of a byte, or the high order bits of a byte to specify the type of data to follow. Length Encoded Arrays are one example of this practice, where a an integer count indicates the length of a following string or array.

With an alternate pattern, Packet will extract an integer from the byte stream, then choose a pattern based on the value of that integer. The pattern is applied at the index where the integer used to choose the pattern was extracted. That is, the bytes used to choose the pattern are included when the pattern is applied. It is a peek and switch.

Alternate patterns are grouped by parenthesis ( and ) and delimited by pipes |. Each alternative maps a match to a pattern separated by a colon :.

Mnemonic — Parenthesis and pipes are used to indicate alternation in regular expressions, while colons are used to delineate switch options in C.

// MySQL length coded binary; if the byte is less than 252, use the byte value,
// otherwise the byte value indicates the length of the following word.
"b8(0-251: b8 | 252: x8, b16 | 253: x8, b24 | 254: x8, b64)"

Conditions are either a value to match exactly or a range of values. Packet tests each condition is tested in order. Packet uses the alternation of the first condition to match the extracted integer is used. An alternate without a condition will always match. This is used to specify a default pattern.

// Simpiler, but will also match 255 which is invalid, which is fine if you test
// the value in your callback.
"b8(252: x8, b16 | 253: x8, b24 | 254: x8, b64 | b8)"

The values can be expressed in binary, decimal or hexadecimal.

Bitwise Alternate Patterns

You can also indicate a branch based on set bits by prefixing the value with an ampersand. Packet will use the value as a bit mask. If the result of a logical and with the bit mask equals the bit mask, then Packet use use that alternative.

Mnemonic The & performs a logical and in C and is used to check to see if bits in a bit mask are set.

"b8(&0x80: b16{x1,b15} | b8)"   // A 15-bit word if the first bit is set,
                                // otherwise a byte.

Bitwise conditions cannot be used in to choose a pattern for serialization. Upon serialization, the field value is a JavaScript number, not an stream of bytes. The bit flag simply does not exist.

Instead, we need to perform a range check to determine which pattern. To delimit alternate tests for reading and writing, we use a slash in the condition.

// A 15-bit word if the first bit is set, otherwise a byte.
"b8(&0x80/0x80-0xffff: b16{x1{1},b15} | b8)"

Multi-Field Alternate Patterns

Alternate patterns can define either a single field or multiple field. Alternate patterns can contain bit-packed patterns, but they cannot contain still more alternate patterns.

// Two alternate patterns with a different number of fields.
"b8(&0x80/1: b16{b1, b15}, b16|b32{b1, b31})"

In the above example, the serialization test would be applied to the field value in the position of the b1 field for either alternate.

Named Alternate Patterns

Names can be applied either to the entire alternation if the alternation produces a single field, or else to individual members of the alternate patterns.

// A single field mapped to a name.
"b8(&0x80/0x80-0xffff: b16{x1{1},b15} | b8) => number"

When serializing named multi-field patterns, for each alternate, Packet will use the first value of the property in the alternate for the serialization condition. Packet reads the property from the object we're serializing. If the value is not null, it is tested against the serialization condition. If it is null, the test is skipped. We use the first alternate whose object property is not null and whose serialization condition matches the object property.

// Multi-field alternates mapped to names.
"b8(&0x80/1: b16{b1 => control, b15 => type}, x16|b32{b1 => control, b31 => sequence})"


Often there are transformations that you need to perform on an field to get it to its final state. You may need to convert a byte array to string of a particular character encoding, for example. This is done with a transformation functions which are specified with a transformation pipeline.

If the transformation is a fixed transformation, you can perform the transformation by defining a pipeline. A pipeline defines one or more transformations that are invoked on the value after parsing and before serialization. The transformations can accept scalar JavaScript parameters.

function str(encoding, name, field, parsing, value) {
    if (parsing) {
        var buffer = new Buffer(array.length)
        for (var i = 0; i < value.length; i++) {
            buffer[i] = value[i];
        var length = value.length
        if (field.terminator) {
            length += field.terminator.length;
        return buffer.toString(encoding, 0, length);
    } else {
        if (field.terminator) {
            value += field.terminator;
        return new Buffer(value, encoding);

Now you can use the transform in your pattern.

"b8z|str('ascii')"      // An ascii string terminated by zero.
"b8z|str('ascii'), b16" // An ascii string terminated by zero followed by a
                        // big-endian 16 bit integer.

The str transform is defined by default. The transform names are purposely terse to fit with the brevity of the pattern language.

Error Messages

Error messages for pattern parsing.

Change Log

Changes for each release.

Version 0.0.6

Wed Jul 3 21:52:20 UTC 2013

Version 0.0.5

Fri Mar 1 03:05:15 UTC 2013

Version 0.0.4

Mon Nov 6 04:50:16 UTC 2012.

Version 0.0.3

Fri Aug 17 00:40:37 UTC 2012.