WASMagic
About
A WebAssembly compiled version of
libmagic with a simple API for Node, Bun, or
Deno. WASMagic provides accurate filetype detection with zero
prod dependencies.
Usage
Install wasmagic
from npm:
npm install wasmagic
Detect the mime of something in Node, or Bun:
import { WASMagic } from "wasmagic";
const magic = await WASMagic.create();
const pngFile = Buffer.from("89504E470D0A1A0A0000000D49484452", "hex");
console.log(magic.detect(pngFile));
CommonJS version in Node, or Bun:
const { WASMagic } = require("wasmagic");
async function main() {
const magic = await WASMagic.create();
const pngFile = Buffer.from("89504E470D0A1A0A0000000D49484452", "hex");
console.log(magic.detect(pngFile));
}
main().catch((err) => console.error(err));
Deno:
import { WASMagic } from "npm:wasmagic";
const magic = await WASMagic.create();
const pngFile = Uint8Array.from([
0x89, 0x50, 0x4e, 0x47, 0x0d, 0x0a, 0x1a, 0x0a,
0x00, 0x00, 0x00, 0x0d, 0x49, 0x48, 0x44, 0x52
]);
console.log(magic.getMime(pngFile));
Run with permissions:
deno run \
--allow-read="myscript.js,$PWD/node_modules/.deno/wasmagic@0.2.0/node_modules/wasmagic/dist/libmagic-wrapper.wasm" \
myscript.js
WASMagic should also work in modern browsers by using a packaging utility like
Rollup or Webpack, however
this isn't tested, and requires downloading the WASM payload which is currently
1.6MB.
Options
The WASMagic.create()
method takes an optional options object with the type
WASMagicOptions
. These options can be used to customize file type detection by
either augmenting the default magic file or replacing it completely.
For examples on using these options, please look at the
tests.
WASMagicOptions
type WASMagicOptions = {
flags?: WASMagicFlags;
loadDefaultMagicfile?: boolean;
magicFiles?: Uint8Array[];
stdio?: (stdioName: "stdout" | "stderr", text: string) => void;
};
Default options:
{
flags: WASMagicFlags.MIME_TYPE,
loadDefaultMagicfile: true,
magicFiles: [],
stdio: (_stdioName: "stdout" | "stderr", _text: string) => {},
}
flags
flags
control libmagic
behavior. To use the flags, import the enum
from
the module, and pass the desired combination of flags:
import { WASMagic, WASMagicFlags } from "wasmagic";
const magic = await WASMagic.create({
flags: WASMagicFlags.MIME_TYPE | WASMagicFlags.MIME_ENCODING,
});
const pngFile = Buffer.from("89504E470D0A1A0A0000000D49484452", "hex");
console.log(magic.detect(pngFile));
Please refer to the code for the flag definitions
Default: WASMagicFlags.MIME_TYPE
loadDefaultMagicfile
loadDefaultMagicFile
is a boolean
dictates whether or not to load the
bundled magic file. The bundled magic file is the default magic.mgc
file
generated in the libmagic
build.
Default: true
magicFiles
magicFiles
is an Array
of Uint8Array
s representing magic definition files.
This option can be used to tell libmagic
to use custom file type detection.
See the test foobarfiletype
magic file definition as an example custom file
type magic definition.
As these are passed as Uint8Array
s, custom definitions can be loaded from
a file, or embedded directly in your code. For example:
import { WASMagic } from "wasmagic";
const foobarMagic = Buffer.from(
`
0 string FOOBARFILETYPE Made up filetype
!:mime foobarfiletype
`,
);
const magic = await WASMagic.create({
magicFiles: [foobarMagic],
});
console.log(
magic.detect(
Buffer.from(
`FOOBARFILETYPE
Some made up stuff
`,
),
),
);
Default: []
stdio
stdio
is a function
defined to override stdout / stderr output from
libmagic
. This option might not be particularly useful for normal usage, but
very useful for getting warnings about custom magic files and debugging the
development of this module.
Default: (_stdioName: "stdout" | "stderr", _text: string) => {}
(No
output)
Examples
Performance considerations
WASMagic instantiation
You should instantiate as few copies of WASMagic
as you can get away with for
your use case. Each instantiation loads the magic database, which is around 8MB.
One instance per process / worker thread should be enough as the main api
(WASMagic.detect
) is synchronous.
If you want to offload processing to another thread (and in production workloads
you probably should be), take a look at the Async / Worker
threads example.
If you aren't passing your instantiated dependencies down in your application,
and you aren't using Javascript modules (or Typescript), and are trying to use
this as a global, try something like the following for a CommonJS style module:
const { WASMagic } = require("wasmagic");
let magicGlobal = null;
const magicPromise = WASMagic.create().then((instance) => {
magicGlobal = instance;
return magicGlobal;
});
async function main() {
const magic = magicGlobal || (await magicPromise);
const pngFile = Buffer.from("89504E470D0A1A0A0000000D49484452", "hex");
console.log(magic.detect(pngFile));
}
main().catch((err) => console.error(err));
Large files
WASMagic will copy the entire provided buffer into the wasm heap for processing.
This can cause significant performance penalties if the files being processed
are exceptionally large (ex. video files, image files).
To ensure that your application remains performant, either load only the head
of the file you want to detect, or slice the head off of a file buffer:
import { WASMagic } from "wasmagic";
import * as fs from "fs/promises";
const magic = await WASMagic.create();
const file = await fs.open("largeFile.mp4");
const { bytesRead, buffer } = await file.read({ buffer: Buffer.alloc(1024) });
console.log(magic.detect(buffer));
await file.close();
Alternatively, if you are streaming a large file, look at the stream mime type
detection example. When you're dealing with
streams you can attempt detection of the mime type when the first chunks are
loaded.
However, this strategy falls apart depending on the type of file that you are
trying to detect and the accuracy you are looking for. For example; pre 2007
Microsoft Office documents will only be accurately detected if the entire file
is available to be parsed. If you use the above strategy to check the head of
the file, you will get application/CDFV2
instead of application/msword
.
To make sure you're getting accurate results for the files that you're trying
to detect, be sure to test example files.
Detected filetypes
WASMagic includes the default magic file which enables detection any file type
detected by libmagic,
which is over 1500 mime types. For comparison; the
file-type library supports 138 types.
Specifically, WASMagic will accurately detect all types of Microsoft Office
files, as well as many plain text file formats where file-type does not.
Development
Install Node and dependencies:
nvm install && nvm use
npm ci
Building requires the Emscripten sdk, autoconf,
automake, and libtool.
The easiest way to build is to use the bundled Docker builder image based on the
official Emscripten image. Simply
run:
make clean docker-builder-run test
If you would like to build natively on your Mac, and have
Homebrew installed, install these additional packages:
brew install autoconf automake coreutils emscripten libtool
Then:
make
Will build and test the module.
Why WASMagic?
Like many open source projects, WASMagic was created to solve my problems:
- I need to detect a diverse set of file types
- Recent versions of libmagic work for all the file types I need
- I need to detect the mime type of a
Buffer
in Node
- Incurring the performance penalty of disk I/O to detect a file I already had
in memory was unacceptable
Why existing libraries didn't meet these needs
- Non libmagic based libraries do not properly detect the filetypes I need. This
includes the popular file-type
library.
- All libmagic Node libraries I found
are using a very old or broken version of libmagic. At time of writing, these
were the libraries available:
- In the case of @npcz/magic, it's
API required reading the mime of a file on disk
Why WASM instead of using libmagic
natively?
When benchmarking against native modules like mmmagic
I found my WASM based
proof of concept to be faster. It also seems prudent to run libmagic within the
WASM sandbox, as it bypasses security concerns about libmagic itself.