This is a demuxer for Matroska (and thus WebM) files, designed to be exactly sufficient for demuxing the WebM files generated by MediaRecorder, and nothing else. As a consequence, it's capable of live-demuxing data fed to it and giving that information back in a semi-useful way, and nothing more.

To use it, create an MkvDemux object, then push ArrayBuffer frames, and call demux to demux them. For instance:

let mkvDemuxer = new mkvdemuxjs.MkvDemux();
let part = null;
mkvDemuxer.push(chunk);
while ((part = mkvDemuxer.demux()) !== null) {
    // Do something with part
}

Since it's intended to stream, you can push partial data and push more data whenever you have it. The demux function will return null when no new data is available.

The parts that demux returns are in various forms. If it encountered a track description, it returns an object like so:

{
    "track": {
        "number": 1,
        "type": "audio",
        "sampleRate": 48000,
        ... etc ...
    }
}

If it encountered a block of frames, it returns an object like so:

{
    "frames": [
        {
            timestamp: (in seconds, floating point),
            track: (track number),
            data: (ArrayBuffer)
        }
    ]
}

For everything else, it returns an internal representation of the EBML tag, which is likely not useful.