Lame.js is an MP3 encoder that runs entirely in the web browser. It is built on an MP3 encoder written in C (Lame) converted into JavaScript using Emscripten.

A compiled version of lame.js is available in the dist folder.

You will need:

• emscripten (which implies also node.js and LLVM-fastcomp compiler, see emscripten docs for instructions on how to get it),
• CMake.

The build is a classic CMake cross-compilation, using the toolchain provided by emscripten:

$ cd .../lame.js # This folder
$ mkdir build
$ cd build
$ cmake -DEMSCRIPTEN=1 -DCMAKE_TOOLCHAIN_FILE=path_to_emscripten/cmake/Modules/Platform/Emscripten.cmake ..
$ make

This generates lame.js.

The API is rather simple:

• 3 objects must be created: an encoder, an input buffer, an output buffer.
• To start a new file, call Start.
• To process samples, use Encode, you provide input raw PCM samples, and receive encoded MP3 frames.
• To stop the file, call Stop, which will also give you the final remaining frames.
• To release the memory used by the structures, you must explicitely call delete() on them.

For instance:

var encoder = new Module.Encoder();
encoder.Start();

var inBuffer = new Module.AudioBuffer();
var outBuffer = new Module.MP3Buffer();
var encodedData = [];
/* ... */
inBuffer.push_back(sample) // Must be 16kHz, 16 bits audio sample

/* Looping for every recorded samples, putting them in an array 
that will contain the MP3 file */
outBuffer.clear();
encoder.Encode(inBuffer, outBuffer);
for (var i = 0; i < outBuffer.size(); i++) {
    encodedData.push(outBuffer.get(i));
}
outBuffer.clear();
encoder.Stop(outBuffer);
for (var i = 0; i < outBuffer.size(); i++) {
    encodedData.push(outBuffer.get(i));
}
/* ... */
// We finally create a binary blob that contains the MP3 file
var audioBlob = new Blob([new Uint8Array(encodedData)], { type: 'audio/mp3' });
outBuffer.delete();
inBuffer.delete();
encoder.delete();

There is a test suite in the tests folder. Since it used lame.js in the dist folder, you can start it from the root folder of this repository:

$ cd .../lame.js
$ python -m SimpleHTTPServer 8080

Then open http://localhost:8080/tests/index.html in your browser.