0.2.0 • Public • Published


JavaScript version of the Free Lossless Audio Codec (FLAC).

The core of the code here was compiled from version 1.3.1 of the reference implementation (written in C) using emscripten. So it's fully JavaScript, but with most of the features and quality you would expect from the C version.

Getting emflac

Emflac is available via npm. npm install emflac should install it into your current directory or project. This is the preferred means of obtaining a compiled source file even if you intend to use it outside Node. The source repository does not contain the compiled files.

The flac tool

The FLAC reference implementation comes with several components. One is the library libFLAC, another is the command line tool flac. While reasonable bindings to libFLAC are still under development, emflac does offer different kinds of access to the command-line tool.

As a command-line tool

Installing the package using npm install emflac will provide you with a script called node_modules/.bin/flac which resembles the command-line utility of the same name from the official sources.

export PATH=node_modules/.bin:$PATH # automatically done in npm scripts 
flac foo.wav                        # creates foo.flac 
flac -d -o foo2.wav foo.flac        # foo.wav and foo2.wav should sound the same 

As a node module

You can also use the module to call the command line utility from another piece of code, like this:

"use strict";
const fs = require("fs");
const flac = require("./");
const cmd = flac({
  TOTAL_MEMORY: 8*1024*1024, // 8 MiB
  TOTAL_STACK: 64*1024, // 64 kiB
let data = fs.readFileSync(process.argv[2]);
cmd.FS_createDataFile(".", "in.wav", data, true, false);
cmd.callMain(["-o", "out.flac", "in.wav"]);
if (cmd.getExitStatus() === 0) {
  data = cmd.FS_readFile("out.flac");
  data = Buffer.from(data);
  fs.writeFileSync(process.argv[3], data);

The flac function in the above example accepts an optional argument which is an emscripten module configuration object. This can be used to set a large variety of configuration settings for the underlying code, like the memory sizes given above.

There are some extra arguments:

  • standalone (boolean, default false): If set to true will allow the module to set up global exception handlers and also exit the node environment once the command is done. Mainly intended for standalone scripts which do nothing else.
  • mountNodeFS (boolean, default false): If set to true, the machine's file system will be mounted at /fs, so that a command-line argument /fs/home/john/in.wav refers to the file /home/john/in.wav on the local machine. The current working directory of the module will reflect that of the node process, so relative path names should work as expected.

The result is an emscripten-compiled module. By default the main method of that module has not been executed, so you may invoke callMain on that object. Usually after setting up some in-memory file as in the example above. The methods FS_readFile and getExitStatus are not commonly found in emscripten-compiled modules, but specific to this module here. Their use is demonstrated in the example above.

As a web worker

The file useWorker.html demonstrates how the codec can be used from within a browser. You can also visit https://gagern.github.io/emflac/ to see this in action without installing anything (apart from a decent and recent browser).

The worker expects messages of the form

  "id": ‹clonable value›,
  "arguments": ["‹arg1›", "‹arg2›", …],
  "data": ‹ArrayBuffer

If the first argument is -d, this will decode FLAC to WAV. Otherwise a WAV to FLAC encoding is performed. The given data is taken as the input of the operation. If the operation was successful, the reply is a message like

  "id": ‹copied from input›,
  "stderr": "",
  "stdout": "",
  "exitStatus": 0,
  "data": ‹ArrayBuffer

In case of an error, the resulting message is

  "id": ‹copied from input›,
  "exitStatus": ‹int or undefined,
  "stdout": "‹program output›",
  "stderr": "‹program messages›",
  "error": "‹message›",
  "stack": ‹string or undefined

Decompression tool

The file decompress.js provides more direct bindings to the stream decompression module of libFLAC. It does so using a fraction of the size of the full emflac.js script, and is also subject only to the MIT license of emflac and the BSD-style license of libFLAC, as opposed to the GPL license of the full flac tool.

These improvements come at a cost, though. It only supports WAV output, 16 bits per sample, one or two channels. And only on little-endian systems.

decodeSync(‹Uint8Array›[, ‹options›]) → ‹Uint8Array›

The script decompress.js exports a single symbol called emflac.decodeSync in the browser or decodeSync when used as a Node module. Input is a Uint8Array representing the contents of the compressed file. Output is a Uint8Array giving the corresponding WAV file. Possible options include TOTAL_MEMORY and similar.

Web worker decompression

The decompression tool can employ the same kind of web-worker protocol as the full emflac.js script, except it's restricted to those settings that actually make sense for its limited scope. This includes id, data, error and stack but excludes arguments, stdout, stderr and exitStatus.

Convenience functions

While decodeSync is a first step towards providing conventient-to-use functions for common tasks, more of these may be added to the project over time. It is unclear whether these would end up in one of the existing output files or in separate files tailored for their specific needs.

Library access

Currently the symbols from libFLAC are not exported by the generated modules. This may change in a future version. If you have an application where you would need some of these functions exported, feel free to file a ticket and request adding them to the code.


The sources of emflac itself are licensed under the MIT license. However, this does not apply to the FLAC sources I link against. Accoding to the FLAC README:

FLAC is comprised of several components distributed under different licenses. The codec libraries are distributed under Xiph.Org's BSD-like license (see the file COPYING.Xiph in this distribution). All other programs, libraries, and plugins are distributed under the LGPL or GPL (see COPYING.LGPL and COPYING.GPL).

In particular, most parts of the flac command line utility are covered by the GPL, and since that is the strictest of all these licenses, this makes the final binary (i.e. emscripten output) GPL-licensed. So while you may use part of this infrastructure under MIT license, you have to conform to GPL for the project as a whole.

The decoder has more relaxed licensing, as described above.

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