ASAR Archives
After creating an application distribution, the
app's source code are usually bundled into an ASAR archive,
which is a simple extensive archive format designed for Electron apps. By bundling the app
we can mitigate issues around long path names on Windows, speed up require
and conceal your source
code from cursory inspection.
The bundled app runs in a virtual file system and most APIs would just work normally, but for some cases you might want to work on ASAR archives explicitly due to a few caveats.
Using ASAR Archives
In Electron there are two sets of APIs: Node APIs provided by Node.js and Web APIs provided by Chromium. Both APIs support reading files from ASAR archives.
Node API
With special patches in Electron, Node APIs like fs.readFile
and require
treat ASAR archives as virtual directories, and the files in it as normal
files in the filesystem.
For example, suppose we have an example.asar
archive under /path/to
:
$ asar list /path/to/example.asar
/app.js
/file.txt
/dir/module.js
/static/index.html
/static/main.css
/static/jquery.min.js
Read a file in the ASAR archive:
const fs = require('node:fs')
fs.readFileSync('/path/to/example.asar/file.txt')
List all files under the root of the archive:
const fs = require('node:fs')
fs.readdirSync('/path/to/example.asar')
Use a module from the archive:
require('./path/to/example.asar/dir/module.js')
You can also display a web page in an ASAR archive with BrowserWindow
:
const { BrowserWindow } = require('electron')
const win = new BrowserWindow()
win.loadURL('file:///path/to/example.asar/static/index.html')
Web API
In a web page, files in an archive can be requested with the file:
protocol.
Like the Node API, ASAR archives are treated as directories.
For example, to get a file with $.get
:
<script>
let $ = require('./jquery.min.js')
$.get('file:///path/to/example.asar/file.txt', (data) => {
console.log(data)
})
</script>
Treating an ASAR archive as a Normal File
For some cases like verifying the ASAR archive's checksum, we need to read the
content of an ASAR archive as a file. For this purpose you can use the built-in
original-fs
module which provides original fs
APIs without asar
support:
const originalFs = require('original-fs')
originalFs.readFileSync('/path/to/example.asar')
You can also set process.noAsar
to true
to disable the support for asar
in
the fs
module:
const fs = require('node:fs')
process.noAsar = true
fs.readFileSync('/path/to/example.asar')
Limitations of the Node API
Even though we tried hard to make ASAR archives in the Node API work like directories as much as possible, there are still limitations due to the low-level nature of the Node API.
Archives Are Read-only
The archives can not be modified so all Node APIs that can modify files will not work with ASAR archives.
Working Directory Can Not Be Set to Directories in Archive
Though ASAR archives are treated as directories, there are no actual
directories in the filesystem, so you can never set the working directory to
directories in ASAR archives. Passing them as the cwd
option of some APIs
will also cause errors.
Extra Unpacking on Some APIs
Most fs
APIs can read a file or get a file's information from ASAR archives
without unpacking, but for some APIs that rely on passing the real file path to
underlying system calls, Electron will extract the needed file into a
temporary file and pass the path of the temporary file to the APIs to make them
work. This adds a little overhead for those APIs.
APIs that requires extra unpacking are:
child_process.execFile
child_process.execFileSync
fs.open
fs.openSync
process.dlopen
- Used byrequire
on native modules
Fake Stat Information of fs.stat
The Stats
object returned by fs.stat
and its friends on files in asar
archives is generated by guessing, because those files do not exist on the
filesystem. So you should not trust the Stats
object except for getting file
size and checking file type.
Executing Binaries Inside ASAR archive
There are Node APIs that can execute binaries like child_process.exec
,
child_process.spawn
and child_process.execFile
, but only execFile
is
supported to execute binaries inside ASAR archive.
This is because exec
and spawn
accept command
instead of file
as input,
and command
s are executed under shell. There is no reliable way to determine
whether a command uses a file in asar archive, and even if we do, we can not be
sure whether we can replace the path in command without side effects.
Adding Unpacked Files to ASAR archives
As stated above, some Node APIs will unpack the file to the filesystem when called. Apart from the performance issues, various anti-virus scanners might be triggered by this behavior.
As a workaround, you can leave various files unpacked using the --unpack
option.
In the following example, shared libraries of native Node.js modules will not be
packed:
$ asar pack app app.asar --unpack *.node
After running the command, you will notice that a folder named app.asar.unpacked
was created together with the app.asar
file. It contains the unpacked files
and should be shipped together with the app.asar
archive.