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:

``bash
npm install wasmagic
`

Detect the mime of something in Node, or Bun:
`javascript
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:
`javascript
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:
`javascript
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:
`shell
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.

$3

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

`typescript
type WASMagicOptions = {
flags?: WASMagicFlags;
loadDefaultMagicfile?: boolean;
magicFiles?: Uint8Array[];
stdio?: (stdioName: "stdout" | "stderr", text: string) => void;
locateFile?: (path: string, prefix: string) => string;
};
`

Default options:

`typescript
{
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:

`javascript
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:

`javascript
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)

###### locateFile

locateFile is a function that allows you to customize how the WASM module
locates the libmagic-wrapper.wasm file. This is particularly useful when
bundling with tools like Webpack or Rollup, or when deploying to environments
where the default file resolution doesn't work.

The function receives two parameters:
- path: The filename being requested (e.g., "libmagic-wrapper.wasm")
- prefix: The default prefix path

And should return the full URL or path to the WASM file.

Example using a custom path:

`javascript
import { WASMagic } from "wasmagic";

const magic = await WASMagic.create({
locateFile: (path, prefix) => {
if (path.endsWith(".wasm")) {
return /custom/path/to/${path};
}
return prefix + path;
},
});
`

Default: undefined (uses Emscripten's default file locator)

$3

- Async / Worker threads
- Stream mime type detection

$3

#### 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:

`javascript
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:

`javascript
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.

$3

WASMagic includes the default magic file which enables detection of any file
type detected by
libmagic, which (at time
of writing) 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:

`bash
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:

`bash
make clean docker-builder-run test
`

---

If you would like to build natively on your Mac, and have
Homebrew installed, install these additional packages:

`bash
brew install autoconf automake coreutils emscripten libtool
`

Then:

`bash
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

$3

- 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

$3

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.