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ASAR Archiv

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.

Beispiel: Angenommen es existiert ein example.asar-Archiv unter /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')

Alle Dateien unterhalb der Archivwurzel auflisten:

const fs = require('node:fs')
fs.readdirSync('/path/to/example.asar')

Ein Modul aus dem Archiv nutzen:

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.

Zum Beispiel, Holen einer Datei mit $.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. Zu diesem Zweck können Sie das integrierte original-fs-Modul nutzen. Dieses stellt originale fs APIs ohne asar-Unterstützung bereit:

const originalFs = require('original-fs')
originalFs.readFileSync('/path/to/example.asar')

Sie können zudem den process.noAsar auf true setzen, um die Unterstützung für asar im fs-Modul zu deaktivieren:

const fs = require('node:fs')
process.noAsar = true
fs.readFileSync('/path/to/example.asar')

Einschränkungen der 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.

Archive sind Read-only​

The archives can not be modified so all Node APIs that can modify files will not work with ASAR archives.

Festlegen von Arbeitsverzeichnissen innerhalb von Archiven nicht möglich​

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. Das Übergeben als cwd-Option einiger APIs wird ebenfalls Fehler hervorbringen.

Zusätzliches Entpacken einiger 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. Dies erhöht den Aufwand für diese APIs leicht.

APIs, die zusätzliches Entpacken benötigen sind:

  • child_process.execFile
  • child_process.execFileSync
  • fs.open
  • fs.openSync
  • process.dlopen - verwendet von require bei nativen Modulen

Falsche Statusmeldung von fs.stat​

Das Stats Object, welches von fs.stat zurückgegeben wird, und seine Freunde in Dateien in asar-Archiven, wird durch Raten generiert, da diese Dateien nicht im Dateisystem existieren. Deshalb sollten Sie dem Stats Objekt nicht trauen außer um die Dateigröße und Dateiart zu bestimmen.

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.

Das rührt daher, dass exec und spawn command an Stelle von file als Eingabe akzeptieren, und commands in der Shell ausgeführt werden. Es existiert kein verlässlicher Weg herauszufinden, ob ein Command eine Datei in einem asar-Archiv nutzt. Und selbst wenn, kann man nicht sicher sein, ob man den Pfad im Command ohne Nebeneffekte austauschen kann.

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.