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));
// outputs: image/png

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));
// outputs: image/png

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));
// outputs: image/png

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));
// outputs: image/png; charset=binary

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 Uint8Arrays 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 Uint8Arrays, 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
`,
    ),
  ),
);

// outputs: foobarfiletype

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

• Async / Worker threads
• Stream mime type detection

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));
// outputs: image/png

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");

// Only read the first 1024 bytes of our large file
const { bytesRead, buffer } = await file.read({ buffer: Buffer.alloc(1024) });

// We're assuming that largeFile.mp4 is >= 1024 bytes in size and our buffer
// will only have the first 1024 bytes of largeFile.mp4 in it
console.log(magic.detect(buffer));
await file.close();

// outputs: video/mp4

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:
  • libmagic, 4 years old
  • mmmagic, 3 years old
  • mime-magic, 9 years old
  • @npcz/magic, 6 months old, but uses a broken version of libmagic. Misidentifies .mp3 files
• 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.