Writing a parser for a structured binary format such as a 3D model is extremely annoying. You have first to declare your file structure, and then go over every structure again and make a proper code to parse it. This is mainly caused because the lack of introspection of C/C++ and for performance reasons.

jParser is available on Github. It works on both NodeJS and Browser.

In Javascript, it does not have to be that way! jParser is a class that only asks you to write a JSON version of the structure. It will parse the file automatically for you.

Here is an example of what you could write with jParser:

var description = {
  header: {
    magic: ['string', 4],
    version: 'uint32'
  model: {
    header: 'header'
var model = new jParser(file, description).parse('model');
// {
//   header: {
//     magic: 'MD20',
//     version: 272
//   }
// }


Standard Structure

The description object defines blocks that needs to be parsed. In the previous example, we define two blocks header and model where each block is a list of labelled sub-blocks.

Default blocks such as int32, char, double are provided by jDataView.

This organization makes it easy to reproduce C structures. Let's see the description of the BMP format.

// Javascript Description
header: {
  header_sz: 	'uint32',
  width: 	'int32',
  height: 	'int32',
  nplanes: 	'uint16',
  bitspp: 	'uint16',
  bmp_bytesz: 	'uint32',
  hres: 	'int32',
  vres: 	'int32',
  ncolors: 	'uint32',
  nimpcolors: 	'uint32'
// C Structure
typedef struct {
  uint32_t header_sz;
  int32_t  width;
  int32_t  height;
  uint16_t nplanes;
  uint16_t bitspp;
  uint32_t compress_type;
  uint32_t bmp_bytesz;
  int32_t  hres;
  int32_t  vres;
  uint32_t ncolors;
  uint32_t nimpcolors;

Reference Structures

As you already noticed, instead of using basic blocks, we can use our own blocks. In the following example, uvAnimation uses animationBlock that uses nofs:

nofs: {
  count: 'uint32',
  offset: 'uint32'
animationBlock: {
  interpolationType: 'uint16',
  globalSequenceID: 'int16',
  timestamps: 'nofs',
  keyFrame: 'nofs'
uvAnimation: {
  translation: 'animationBlock',
  rotation: 'animationBlock',
  scaling: 'animationBlock'


At this point, it is possible to express any C structure and parse files that could be loaded using a simple read. We now need to integrate a logic within our parser using anonymous functions.

Recursive Parsing

It is a common operation to read consecutive blocks. It is possible to make an array block that takes a block name and a count. It parses all theses blocks and aggregates them into a Javascript array.

array: function (type, length) {
  var array = [];
  for (var i = 0; i < length; ++i) {
  return array;

In order to call a function, we use an array literal where the first element is the block name and the rest are the arguments. We can easily define float[234].

float2: ['array', 'float', 2],
float3: ['array', 'float', 3],
float4: ['array', 'float', 4]

We can use the array block to build a string block. We parse an array of char and join it.

string: function (length) {
  return this.parse(['array', 'char', length]).join('');
filename: ['string', 32]

Seek & Tell

In the World of Warcraft models, there is a small structure called nofs that tells us "There are [count] consecutive [type] at [offset]". We build a struct block in order to parse this pattern. It will use seek and tell to navigate through the file.

nofs: {
  count: 'uint32',
  offset: 'uint32'
struct: function (type) {
  // Read the count & offset
  var nofs = this.parse('nofs');
  // Save the current offset & Seek to the new one
  var pos = this.tell();
  // Read the array
  var result = this.parse(['array', type, nofs.count]);
  // Seek back & Return the result
  return result;
triangles: ['struct', 'uint16'],
properties: ['struct', 'boneIndices']


The code that powers this is only 30 lines long (70 including the standard integral types). It just handles each possible data type.

parse: function (description, param) {
  var type = typeof description;
  // Function
  if (type === 'function') {
    return description.apply(this, [this.param].concat(param));
  // Shortcut: 'string' == ['string']
  if (type === 'string') {
    description = [description];
  // Array: Function Call
  if (description instanceof Array) {
    return this.parse(this.description[description[0]], description.slice(1));
  // Object: Structure
  if (type === 'object') {
    var output = {};
    for (var key in description) {
      if (description.hasOwnProperty(key)) {
        output[key] = this.parse(description[key]);
    return output;
  throw new Error('Unknown description type ' + description);


This little parser is an example of how to extensively use all the dynamic characteristics of Javascript such as Object Literals, Anonymous Functions and Dynamic Typing in order to build a powerful and easy to use tool.

I don't want to release the library just yet as I need to explore more use cases and find elegant solution for them too. But I hope it will give you inspiration to use full Javascript power.


You can see it in action in my 0.1% completed Javascript WoW Model Viewer demo. The two following files are important:

If you liked this article, you might be interested in my Twitter feed as well.

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