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