@electron/asar

What is @electron/asar?

@electron/asar is a Node.js library used to create, manipulate, and extract ASAR (Atom Shell Archive) files. ASAR files are essentially archives used by Electron applications to package their resources. This package provides functionalities to pack files into an ASAR archive, extract files from an ASAR archive, and manipulate the contents of an ASAR archive.

What are @electron/asar's main functionalities?

Packing files into an ASAR archive

This feature allows you to pack a directory of files into an ASAR archive. The `createPackage` function takes the source directory and the output file path as arguments and creates the ASAR archive.

const asar = require('@electron/asar');

asar.createPackage('path/to/source', 'path/to/output.asar', function() {
  console.log('ASAR archive created successfully!');
});

Extracting files from an ASAR archive

This feature allows you to extract all files from an ASAR archive to a specified directory. The `extractAll` function takes the ASAR archive path and the destination directory as arguments and extracts the contents.

const asar = require('@electron/asar');

asar.extractAll('path/to/archive.asar', 'path/to/destination', function() {
  console.log('ASAR archive extracted successfully!');
});

Listing files in an ASAR archive

This feature allows you to list all files contained in an ASAR archive. The `listPackage` function takes the ASAR archive path as an argument and returns an array of file paths contained in the archive.

const asar = require('@electron/asar');

const files = asar.listPackage('path/to/archive.asar');
console.log(files);

Other packages similar to @electron/asar

tar

The `tar` package is used for reading and writing tar archives. It provides functionalities to create, extract, and list files in tar archives. Unlike ASAR, tar archives are more commonly used and supported across different platforms and tools.

zip

The `zip` package is used for creating and extracting zip archives. It provides functionalities to compress files into a zip archive and extract files from a zip archive. Zip archives are widely used and supported by many applications and operating systems.

node-7z

The `node-7z` package is a Node.js wrapper for the 7-Zip compression tool. It allows you to create and extract 7z archives, which offer high compression ratios. This package provides similar functionalities to ASAR but uses the 7z format instead.

README

@electron/asar - Electron Archive

Asar is a simple extensive archive format, it works like tar that concatenates all files together without compression, while having random access support.

Features

• Support random access
• Use JSON to store files' information
• Very easy to write a parser

Command line utility

Install

This module requires Node 10 or later.

$ npm install --engine-strict @electron/asar

Usage

$ asar --help

  Usage: asar [options] [command]

  Commands:

    pack|p <dir> <output>
       create asar archive

    list|l <archive>
       list files of asar archive

    extract-file|ef <archive> <filename>
       extract one file from archive

    extract|e <archive> <dest>
       extract archive


  Options:

    -h, --help     output usage information
    -V, --version  output the version number

Excluding multiple resources from being packed

Given:

    app
(a) ├── x1
(b) ├── x2
(c) ├── y3
(d) │   ├── x1
(e) │   └── z1
(f) │       └── x2
(g) └── z4
(h)     └── w1

Exclude: a, b

$ asar pack app app.asar --unpack-dir "{x1,x2}"

Exclude: a, b, d, f

$ asar pack app app.asar --unpack-dir "**/{x1,x2}"

Exclude: a, b, d, f, h

$ asar pack app app.asar --unpack-dir "{**/x1,**/x2,z4/w1}"

Using programatically

Example

const asar = require('@electron/asar');

const src = 'some/path/';
const dest = 'name.asar';

await asar.createPackage(src, dest);
console.log('done.');

Please note that there is currently no error handling provided!

Transform

You can pass in a transform option, that is a function, which either returns nothing, or a stream.Transform. The latter will be used on files that will be in the .asar file to transform them (e.g. compress).

const asar = require('@electron/asar');

const src = 'some/path/';
const dest = 'name.asar';

function transform (filename) {
  return new CustomTransformStream()
}

await asar.createPackageWithOptions(src, dest, { transform: transform });
console.log('done.');

Using with grunt

There is also an unofficial grunt plugin to generate asar archives at bwin/grunt-asar.

Format

Asar uses Pickle to safely serialize binary value to file.

The format of asar is very flat:

| UInt32: header_size | String: header | Bytes: file1 | ... | Bytes: file42 |

The header_size and header are serialized with Pickle class, and header_size's Pickle object is 8 bytes.

The header is a JSON string, and the header_size is the size of header's Pickle object.

Structure of header is something like this:

{
   "files": {
      "tmp": {
         "files": {}
      },
      "usr" : {
         "files": {
           "bin": {
             "files": {
               "ls": {
                 "offset": "0",
                 "size": 100,
                 "executable": true,
                 "integrity": {
                   "algorithm": "SHA256",
                   "hash": "...",
                   "blockSize": 1024,
                   "blocks": ["...", "..."]
                 }
               },
               "cd": {
                 "offset": "100",
                 "size": 100,
                 "executable": true,
                 "integrity": {
                   "algorithm": "SHA256",
                   "hash": "...",
                   "blockSize": 1024,
                   "blocks": ["...", "..."]
                 }
               }
             }
           }
         }
      },
      "etc": {
         "files": {
           "hosts": {
             "offset": "200",
             "size": 32,
             "integrity": {
                "algorithm": "SHA256",
                "hash": "...",
                "blockSize": 1024,
                "blocks": ["...", "..."]
              }
           }
         }
      }
   }
}

offset and size records the information to read the file from archive, the offset starts from 0 so you have to manually add the size of header_size and header to the offset to get the real offset of the file.

offset is a UINT64 number represented in string, because there is no way to precisely represent UINT64 in JavaScript Number. size is a JavaScript Number that is no larger than Number.MAX_SAFE_INTEGER, which has a value of 9007199254740991 and is about 8PB in size. We didn't store size in UINT64 because file size in Node.js is represented as Number and it is not safe to convert Number to UINT64.

integrity is an object consisting of a few keys:

• A hashing algorithm, currently only SHA256 is supported.
• A hex encoded hash value representing the hash of the entire file.
• An array of hex encoded hashes for the blocks of the file. i.e. for a blockSize of 4KB this array contains the hash of every block if you split the file into N 4KB blocks.
• A integer value blockSize representing the size in bytes of each block in the blocks hashes above