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 documentation of each method)
3. Call Parser.prototype.parse with a Buffer/Uint8Array object passed as its only argument.
4. The parsed result will be returned as an object.
   • If parsing failed, an exception will be thrown.

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

// Alternative way to import the module
// import { Parser } from "binary-parser";

// Build an IP packet header Parser
const ipHeader = new Parser()
  .endianness("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.
const buf = Buffer.from("450002c5939900002c06ef98adc24f6c850186d1", "hex");

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

You can install binary-parser via npm:

npm install binary-parser

The npm package provides entry points for both CommonJS and ES modules.

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 le for little endian or be for big endian. With no prefix, it parses as a signed number, with u prefix 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 versions will throw a runtime error.

const 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.

const 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 a 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.

const 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.

const parser1 = ...;
const parser2 = ...;
const parser3 = ...;

const 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 a 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.

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

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

Set an alias to this parser, so that it can be referred to by name in methods like .array, .nest and .choice, without the requirement to have an instance of this parser.

Especially, the parser may reference itself:

const stop = new Parser();

const 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

const 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 parsers:

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

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

const 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"
  }
});

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

parser.parse(buffer);

Read data, then wrap it by transforming it by a function for further parsing. It works similarly to a buffer where it reads a block of data. But instead of returning the buffer it will pass the buffer on to a parser for further processing.

The result will be stored in the key name. If name is an empty string or null, or if it is omitted, the parsed result is directly embedded into the current object.

• wrapper - (Required) A function taking a buffer and returning a buffer ((x: Buffer | Uint8Array ) => Buffer | Uint8Array) transforming the buffer into a buffer expected by type.
• type - (Required) A Parser object to parse the buffer returned by wrapper.
• length - (either length or readUntil is required) Length of the buffer. Can be a number, string or a function. Use a number for statically sized buffers, a string to reference another variable and a 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.

const zlib = require("zlib");
// A parser to run on the data returned by the wrapper
const textParser = Parser.start()
  .string("text", {
    zeroTerminated: true,
  });

const mainParser = Parser.start()
  // Read length of the data to wrap
  .uint32le("length")
  // Read wrapped data
  .wrapped("wrappedData", {
    // Indicate how much data to read, like buffer()
    length: "length",
    // Define function to pre-process the data buffer
    wrapper: function (buffer) {
      // E.g. decompress data and return it for further parsing
      return zlib.inflateRawSync(buffer);
    },
    // The parser to run on the decompressed data
    type: textParser,
  });

mainParser.parse(buffer);

Returns how many bytes this parser consumes. If the size of the parser cannot be statically determined, a NaN is returned.

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.

  const 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 (the parsed result) and if it returns false, an exception is thrown.

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

You can use some special fields while parsing to traverse your structure. These context variables will be removed after the parsing process. Note that this feature is turned off by default for performance reasons, and you need to call .useContextVars() to enable it.

• $parent - This field references the parent structure. This variable will be null while parsing the root structure.

  var parser = new Parser()
    .useContextVars()
    .nest("header", {
      type: new Parser().uint32("length"),
    })
    .array("data", {
      type: "int32",
      length: function() {
        return this.$parent.header.length;
      }
    });

• $root - This field references the root structure.

  const parser = new Parser()
    .useContextVars()
    .nest("header", {
      type: new Parser().uint32("length"),
    })
    .nest("data", {
      type: new Parser()
        .uint32("value")
        .array("data", {
          type: "int32",
          length: function() {
            return this.$root.header.length;
          }
        }),
    });

• $index - This field references the actual index in array parsing. This variable will be available only when using the length mode for arrays.

  const parser = new Parser()
    .useContextVars()
    .nest("header", {
      type: new Parser().uint32("length"),
    })
    .nest("data", {
      type: new Parser()
        .uint32("value")
        .array("data", {
          type: new Parser().nest({
            type: new Parser().uint8("_tmp"),
            formatter: function(item) {
              return this.$index % 2 === 0 ? item._tmp : String.fromCharCode(item._tmp);
            }
          }),
          length: "$root.header.length"
        }),
    });

See example/ for real-world examples.

A benchmark script to compare the parsing performance with binparse, structron and destruct.js is available under benchmark/.

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

To contribute code, first clone this repo, then install the dependencies:

git clone https://github.com/keichi/binary-parser.git
cd binary-parser
npm install

If you added a feature or fixed a bug, update the test suite under test/ and then run it like this:

npm run test

Make sure all the tests pass before submitting a pull request.