asar

What is asar?

The 'asar' npm package is used to create and manage ASAR (Atom Shell Archive) files, which are essentially archives used by Electron applications to package their resources. ASAR files are similar to tar files but are optimized for use with Electron.

What are asar's main functionalities?

Creating an ASAR archive

This feature allows you to create an ASAR archive from a source directory. The code sample demonstrates how to use the 'createPackage' method to create an ASAR archive from the contents of 'path/to/source' and save it to 'path/to/output.asar'.

const asar = require('asar');

asar.createPackage('path/to/source', 'path/to/output.asar').then(() => {
  console.log('ASAR archive created successfully');
}).catch(err => {
  console.error('Error creating ASAR archive:', err);
});

Extracting an ASAR archive

This feature allows you to extract the contents of an ASAR archive to a specified directory. The code sample demonstrates how to use the 'extractAll' method to extract the contents of 'path/to/archive.asar' to 'path/to/destination'.

const asar = require('asar');

asar.extractAll('path/to/archive.asar', 'path/to/destination').then(() => {
  console.log('ASAR archive extracted successfully');
}).catch(err => {
  console.error('Error extracting ASAR archive:', err);
});

Listing files in an ASAR archive

This feature allows you to list the files contained within an ASAR archive. The code sample demonstrates how to use the 'listPackage' method to list the files in 'path/to/archive.asar'.

const asar = require('asar');

asar.listPackage('path/to/archive.asar').then(files => {
  console.log('Files in ASAR archive:', files);
}).catch(err => {
  console.error('Error listing files in ASAR archive:', err);
});

Other packages similar to asar

tar

The 'tar' npm package is used for working with tar archives. It provides functionality to create, extract, and list files in tar archives. While 'tar' is a general-purpose tool for tar files, 'asar' is specifically optimized for Electron applications and their resource packaging needs.

zip

The 'zip' npm package allows for creating and extracting zip archives. Similar to 'asar', it provides methods to compress and decompress files, but it uses the zip format instead of the ASAR format. 'zip' is more widely used for general file compression and decompression tasks.

node-tar

The 'node-tar' package is another tool for handling tar archives in Node.js. It offers a range of features for creating, extracting, and manipulating tar files. Compared to 'asar', 'node-tar' is more versatile for general tar file operations but lacks the specific optimizations for Electron applications.

README

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

$ npm install 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('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('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, there is also a node.js binding of Pickle class.

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
               },
               "cd": {
                 "offset": "100",
                 "size": 100,
                 "executable": true
               }
             }
           }
         }
      },
      "etc": {
         "files": {
           "hosts": {
             "offset": "200",
             "size": 32
           }
         }
      }
   }
}

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.