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bitjs: Binary Tools for JavaScript


A set of dependency-free JavaScript modules to work with binary data in JS (using Typed Arrays). Includes:

  • bitjs/archive: Decompressing files (unzip, unrar, untar, gunzip) in JavaScript, implemented as Web Workers where supported, and allowing progressive unarchiving while streaming.
  • bitjs/codecs: Get the codec info of media containers in a ISO RFC6381 MIME type string.
  • bitjs/file: Detect the type of file from its binary signature.
  • bitjs/image: Parsing GIF, JPEG, PNG. Conversion of WebP to PNG or JPEG.
  • bitjs/io: Low-level classes for interpreting binary data (BitStream, ByteStream). For example, reading or peeking at N bits at a time.


Install it using your favourite package manager, the package is registered under @codedread/bitjs.

npm install @codedread/bitjs


yarn add @codedread/bitjs

CommonJS/ESM in Node

This module is an ES Module. If your project uses CommonJS modules, it's a little trickier to use. One example of this is if a TypeScript project compiles to CommonJS, it will try to turn imports into require() statements, which will break. The fix for this (unfortunately) is to update your tsconfig.json:

 "moduleResolution": "Node16",

and use a Dynamic Import:

const { getFullMIMEString } = await import('@codedread/bitjs');



This package includes objects for decompressing and compressing binary data in popular archive formats (zip, rar, tar, gzip). Here is a simple example of unrar:


import { Unrarrer } from './bitjs/archive/decompress.js';
const unrar = new Unrarrer(rarFileArrayBuffer);
unrar.addEventListener('extract', (e) => {
  const {filename, fileData} = e.unarchivedFile;
  console.log(`Extracted ${filename} (${fileData.byteLength} bytes)`);
  // Do something with fileData...
unrar.addEventListener('finish', () => console.log('Done'));

More details and examples are located on the API page.


This package includes code for dealing with media files (audio/video). It is useful for deriving ISO RFC6381 MIME type strings, including the codec information. Currently supports a limited subset of MP4 and WEBM.

How to use:

  • First, install ffprobe (ffmpeg) on your system.
  • Then:
import { getFullMIMEString } from 'bitjs/codecs/codecs.js';
 * @typedef {import('bitjs/codecs/codecs.js').ProbeInfo} ProbeInfo

const cmd = 'ffprobe -show_format -show_streams -print_format json -v quiet foo.mp4';
exec(cmd, (error, stdout) => {
  /** @type {ProbeInfo} */
  const info = JSON.parse(stdout);
  // 'video/mp4; codecs="avc1.4D4028, mp4a.40.2"'
  const contentType = getFullMIMEString(info);


This package includes code for dealing with files. It includes a sniffer which detects the type of file, given an ArrayBuffer.

import { findMimeType } from './bitjs/file/sniffer.js';
const mimeType = findMimeType(someArrayBuffer);


This package includes code for dealing with image files. It includes low-level, event-based parsers for GIF, JPEG, and PNG images.

It also includes a module for converting WebP images into alternative raster graphics formats (PNG/JPG), though this latter module is deprecated, now that WebP images are well-supported in all browsers.

GIF Parser

import { GifParser } from './bitjs/image/parsers/gif.js'

const parser = new GifParser(someArrayBuffer);
parser.onApplicationExtension(evt => {
  const appId = evt.detail.applicationIdentifier;
  const appAuthCode = new TextDecoder().decode(evt.detail.applicationAuthenticationCode);
  if (appId === 'XMP Data' && appAuthCode === 'XMP') {
    /** @type {Uint8Array} */
    const appData = evt.detail.applicationData;
    // Do something with appData (parse the XMP).

JPEG Parser

import { JpegParser } from './bitjs/image/parsers/jpeg.js'
import { ExifTagNumber } from './bitjs/image/parsers/exif.js';

const parser = new JpegParser(someArrayBuffer)
    .onApp1Exif(evt => console.log(evt.detail.get(ExifTagNumber.IMAGE_DESCRIPTION).stringValue));
await parser.start();

PNG Parser

import { PngParser } from './bitjs/image/parsers/png.js'
import { ExifTagNumber } from './bitjs/image/parsers/exif.js';

const parser = new PngParser(someArrayBuffer);
    .onExifProfile(evt => console.log(evt.detail.get(ExifTagNumber.IMAGE_DESCRIPTION).stringValue))
    .onTextualData(evt => console.dir(evt.detail));
await parser.start();

WebP Converter

import { convertWebPtoPNG, convertWebPtoJPG } from './bitjs/image/webp-shim/webp-shim.js';
// convertWebPtoPNG() takes in an ArrayBuffer containing the bytes of a WebP
// image and returns a Promise that resolves with an ArrayBuffer containing the
// bytes of an equivalent PNG image.
convertWebPtoPNG(webpBuffer).then(pngBuf => {
  const pngUrl = URL.createObjectURL(new Blob([pngBuf], {type: 'image/png'}));
  someImgElement.setAttribute(src, pngUrl);


This package includes stream objects for reading and writing binary data at the bit and byte level: BitStream, ByteStream.

import { BitStream } from './bitjs/io/bitstream.js';
const bstream = new BitStream(someArrayBuffer, true, offset, length);
const crc = bstream.readBits(12); // read in 12 bits as CRC, advancing the pointer
const flagbits = bstream.peekBits(6); // look ahead at next 6 bits, but do not advance the pointer

More details and examples are located on the API page.


  • UnRar: A work-in-progress description of the RAR file format.


This project grew out of another project of mine, kthoom (a comic book reader implemented in the browser). This repository was automatically exported from my original repository on GoogleCode and has undergone considerable changes and improvements since then.

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