Binary-parser is a parser builder for JavaScript that enables you to write efficient binary parsers in a simple and declarative manner.

It supports all common data types required to analyze a structured binary data. Binary-parser dynamically generates and compiles the parser code on-the-fly, which runs as fast as a hand-written parser (which takes much more time and effort to write). Supported data types are:

• Integers (8, 16, 32 and 64 bit signed and unsigned integers)
• Floating point numbers (32 and 64 bit floating point values)
• Bit fields (bit fields with length from 1 to 32 bits)
• Strings (fixed-length, variable-length and zero terminated strings with various encodings)
• Arrays (fixed-length and variable-length arrays of builtin or user-defined element types)
• Choices (supports integer keys)
• Pointers
• User defined types (arbitrary combination of builtin types)

Binary-parser was inspired by BinData and binary.

1. Create an empty Parser object with new Parser() or Parser.start().
2. Chain methods to build your desired parser. (See API for detailed document of each method)
3. Call Parser.prototype.parse with a Buffer/Uint8Array object passed as an argument.
4. The parsed result will be returned as an object.

// Module import
var Parser = require("binary-parser").Parser;

// Build an IP packet header Parser
var ipHeader = new Parser()
  .endianess("big")
  .bit4("version")
  .bit4("headerLength")
  .uint8("tos")
  .uint16("packetLength")
  .uint16("id")
  .bit3("offset")
  .bit13("fragOffset")
  .uint8("ttl")
  .uint8("protocol")
  .uint16("checksum")
  .array("src", {
    type: "uint8",
    length: 4
  })
  .array("dst", {
    type: "uint8",
    length: 4
  });

// Prepare buffer to parse.
var buf = Buffer.from("450002c5939900002c06ef98adc24f6c850186d1", "hex");

// Parse buffer and show result
console.log(ipHeader.parse(buf));

Create an empty parser object that parses nothing.

Parse a Buffer/Uint8Array object buffer with this parser and return the resulting object. When parse(buffer) is called for the first time, the associated parser code is compiled on-the-fly and internally cached.

Set the constructor function that should be called to create the object returned from the parse method.

Parse bytes as an integer and store it in a variable named name. name should consist only of alphanumeric characters and start with an alphabet. Number of bits can be chosen from 8, 16, 32 and 64. Byte-ordering can be either l for little endian or b for big endian. With no prefix, it parses as a signed number, with u prefixed as an unsigned number. The runtime type returned by the 8, 16, 32 bit methods is number while the type returned by the 64 bit is bigint.

Note: [u]int64{be,le} methods only work if your runtime is node v12.0.0 or greater. Lower version will throw a runtime error.

var parser = new Parser()
  // Signed 32-bit integer (little endian)
  .int32le("a")
  // Unsigned 8-bit integer
  .uint8("b")
  // Signed 16-bit integer (big endian)
  .int16be("c");
  // signed 64-bit integer (big endian)
  .int64be("d")

Parse bytes as a bit field and store it in variable name. There are 32 methods from bit1 to bit32 each corresponding to 1-bit-length to 32-bits-length bit field.

Parse bytes as a floating-point value and stores it to a variable named name.

var parser = new Parser()
  // 32-bit floating value (big endian)
  .floatbe("a")
  // 64-bit floating value (little endian)
  .doublele("b");

Parse bytes as a string. name should consist only of alpha numeric characters and start with an alphabet. options is an object which can have the following keys:

• encoding - (Optional, defaults to utf8) Specify which encoding to use. Supported encodings include "hex" and all encodings supported by TextDecoder.
• length - (Optional) Length of the string. Can be a number, string or a function. Use number for statically sized arrays, string to reference another variable and function to do some calculation.
• zeroTerminated - (Optional, defaults to false) If true, then this parser reads until it reaches zero.
• greedy - (Optional, defaults to false) If true, then this parser reads until it reaches the end of the buffer. Will consume zero-bytes.
• stripNull - (Optional, must be used with length) If true, then strip null characters from end of the string

Parse bytes as a buffer. Its type will be the same as the input to parse(buffer). name should consist only of alpha numeric characters and start with an alphabet. options is an object which can have the following keys:

• clone - (Optional, defaults to false) By default, buffer(name [,options]) returns a new buffer which references the same memory as the parser input, but offset and cropped by a certain range. If this option is true, input buffer will be cloned and a new buffer referencing a new memory region is returned.
• length - (either length or readUntil is required) Length of the buffer. Can be a number, string or a function. Use number for statically sized buffers, string to reference another variable and function to do some calculation.
• readUntil - (either length or readUntil is required) If "eof", then this parser will read till it reaches the end of the Buffer/Uint8Array object. If it is a function, this parser will read the buffer until the function returns true.

Parse bytes as an array. options is an object which can have the following keys:

• type - (Required) Type of the array element. Can be a string or an user defined Parser object. If it's a string, you have to choose from [u]int{8, 16, 32}{le, be}.
• length - (either length, lengthInBytes, or readUntil is required) Length of the array. Can be a number, string or a function. Use number for statically sized arrays.
• lengthInBytes - (either length, lengthInBytes, or readUntil is required) Length of the array expressed in bytes. Can be a number, string or a function. Use number for statically sized arrays.
• readUntil - (either length, lengthInBytes, or readUntil is required) If "eof", then this parser reads until the end of the Buffer/Uint8Array object. If function it reads until the function returns true.

var parser = new Parser()
  // Statically sized array
  .array("data", {
    type: "int32",
    length: 8
  })

  // Dynamically sized array (references another variable)
  .uint8("dataLength")
  .array("data2", {
    type: "int32",
    length: "dataLength"
  })

  // Dynamically sized array (with some calculation)
  .array("data3", {
    type: "int32",
    length: function() {
      return this.dataLength - 1;
    } // other fields are available through `this`
  })

  // Statically sized array
  .array("data4", {
    type: "int32",
    lengthInBytes: 16
  })

  // Dynamically sized array (references another variable)
  .uint8("dataLengthInBytes")
  .array("data5", {
    type: "int32",
    lengthInBytes: "dataLengthInBytes"
  })

  // Dynamically sized array (with some calculation)
  .array("data6", {
    type: "int32",
    lengthInBytes: function() {
      return this.dataLengthInBytes - 4;
    } // other fields are available through `this`
  })

  // Dynamically sized array (with stop-check on parsed item)
  .array("data7", {
    type: "int32",
    readUntil: function(item, buffer) {
      return item === 42;
    } // stop when specific item is parsed. buffer can be used to perform a read-ahead.
  })

  // Use user defined parser object
  .array("data8", {
    type: userDefinedParser,
    length: "dataLength"
  });

Choose one parser from multiple parsers according to a field value and store its parsed result to key name. If name is null or omitted, the result of the chosen parser is directly embedded into the current object. options is an object which can have the following keys:

• tag - (Required) The value used to determine which parser to use from the choices Can be a string pointing to another field or a function.
• choices - (Required) An object which key is an integer and value is the parser which is executed when tag equals the key value.
• defaultChoice - (Optional) In case if the tag value doesn't match any of choices, this parser is used.

var parser1 = ...;
var parser2 = ...;
var parser3 = ...;

var parser = new Parser().uint8("tagValue").choice("data", {
  tag: "tagValue",
  choices: {
    1: parser1, // if tagValue == 1, execute parser1
    4: parser2, // if tagValue == 4, execute parser2
    5: parser3 // if tagValue == 5, execute parser3
  }
});

Combining choice with array is an idiom to parse TLV-based binary formats.

Execute an inner parser and store its result to key name. If name is null or omitted, the result of the inner parser is directly embedded into the current object. options is an object which can have the following keys:

• type - (Required) A Parser object.

Jump to offset, execute parser for type and rewind to previous offset. Useful for parsing binary formats such as ELF where the offset of a field is pointed by another field.

• type - (Required) Can be a string [u]int{8, 16, 32, 64}{le, be} or an user defined Parser object.
• offset - (Required) Indicates absolute offset from the beginning of the input buffer. Can be a number, string or a function.

Save the current buffer offset as key name. This function is only useful when called after another function which would advance the internal buffer offset.

var parser = new Parser()
  // this call advances the buffer offset by
  // a variable (i.e. unknown to us) number of bytes
  .string('name', {
    zeroTerminated: true
  })
  // this variable points to an absolute position
  // in the buffer
  .uint32('seekOffset')
  // now, save the "current" offset in the stream
  // as the variable "currentOffset"
  .saveOffset('currentOffset')
  // finally, use the saved offset to figure out
  // how many bytes we need to skip
  .seek(function() {
    return this.seekOffset - this.currentOffset;
  })
  ... // the parser would continue here

Move the buffer offset for relOffset bytes from the current position. Use a negative relOffset value to rewind the offset. This method was previously named skip(length).

Define what endianess to use in this parser. endianess can be either "little" or "big". The default endianess of Parser is set to big-endian.

var parser = new Parser()
  .endianess("little")
  // You can specify endianess explicitly
  .uint16be("a")
  .uint32le("a")
  // Or you can omit endianess (in this case, little-endian is used)
  .uint16("b")
  .int32("c");

Set an alias to this parser, so there will be an opportunity to refer to it by name in methods like .array, .nest and .choice, instead of requirement to have an instance of it.

Especially, the parser may reference itself:

var stop = new Parser();

var parser = new Parser()
  .namely("self") // use 'self' to refer to the parser itself
  .uint8("type")
  .choice("data", {
    tag: "type",
    choices: {
      0: stop,
      1: "self",
      2: Parser.start()
        .nest("left", { type: "self" })
        .nest("right", { type: "self" }),
      3: Parser.start()
        .nest("one", { type: "self" })
        .nest("two", { type: "self" })
        .nest("three", { type: "self" })
    }
  });

//        2
//       / \
//      3   1
//    / | \  \
//   1  0  2  0
//  /     / \
// 0     1   0
//      /
//     0

var buffer = Buffer.from([
  2,
  /* left -> */ 3,
    /* one   -> */ 1, /* -> */ 0,
    /* two   -> */ 0,
    /* three -> */ 2,
      /* left  -> */ 1, /* -> */ 0,
      /* right -> */ 0,
  /* right -> */ 1, /* -> */ 0
]);

parser.parse(buffer);

For most of the cases there is almost no difference to the instance-way of referencing, but this method provides the way to parse recursive trees, where each node could reference the node of the same type from the inside.

Also, when you reference a parser using its instance twice, the generated code will contain two similar parts of the code included, while with the named approach, it will include a function with a name, and will just call this function for every case of usage.

Note: This style could lead to circular references and infinite recursion, to avoid this, ensure that every possible path has its end. Also, this recursion is not tail-optimized, so could lead to memory leaks when it goes too deep.

An example of referencing other patches:

// the line below registers the name 'self', so we will be able to use it in
// `twoCells` as a reference
var parser = Parser.start().namely("self");

var stop = Parser.start().namely("stop");

var twoCells = Parser.start()
  .namely("twoCells")
  .nest("left", { type: "self" })
  .nest("right", { type: "stop" });

parser.uint8("type").choice("data", {
  tag: "type",
  choices: {
    0: "stop",
    1: "self",
    2: "twoCells"
  }
});

var buffer = Buffer.from([2, /* left */ 1, 1, 0, /* right */ 0]);

parser.parse(buffer);

Compile this parser on-the-fly and cache its result. Usually, there is no need to call this method directly, since it's called when parse(buffer) is executed for the first time.

Dynamically generates the code for this parser and returns it as a string. Useful for debugging the generated code.

These options can be used in all parsers.

• formatter - Function that transforms the parsed value into a more desired form.

  var parser = new Parser().array("ipv4", {
    type: uint8,
    length: "4",
    formatter: function(arr) {
      return arr.join(".");
    }
  });

• assert - Do assertion on the parsed result (useful for checking magic numbers and so on). If assert is a string or number, the actual parsed result will be compared with it with === (strict equality check), and an exception is thrown if they mismatch. On the other hand, if assert is a function, that function is executed with one argument (parsed result) and if it returns false, an exception is thrown.

  // simple maginc number validation
  var ClassFile = Parser.start()
    .endianess("big")
    .uint32("magic", { assert: 0xcafebabe });
  
  // Doing more complex assertion with a predicate function
  var parser = new Parser()
    .int16le("a")
    .int16le("b")
    .int16le("c", {
      assert: function(x) {
        return this.a + this.b === x;
      }
    });

See example/ for real-world examples.

Please report issues to the issue tracker if you have any difficulties using this module, found a bug, or request a new feature. Pull requests are welcomed.