Encode microphone audio to flac format in React.js

FlacAudioRecorder.js

// To encode microphone audio to flac format in React.js
// put encoder.js, libflac3-1.3.2.min.js, libflac3-1.3.2.min.js.mem in public folder
// Get necessary files from https://github.com/mmig/speech-to-flac

export function FlacAudioRecorder({ onData }) {

    let recording = false;
    let stream;
    let input;
    let node;
    let encoder;

    function startRecording() {

        if (recording)
            return;

        console.log('start recording');//DEBUG

        encoder = new Worker('encoder.js');

        encoder.onmessage = function (e) {
            if (e.data.cmd === 'end') {
                console.log('e.data', e.data)
                onData(e.data.buf);
                encoder.terminate();
                encoder = null;
            }
        };

        function userMediaFailed(code) {
            console.log('grabbing microphone failed: ' + code);
        }

        function gotUserMedia(localMediaStream) {
            recording = true;
            console.log('success grabbing microphone');
            stream = localMediaStream;

            let audio_context;
            if (typeof AudioContext !== 'undefined') {
                audio_context = new AudioContext;
            } else {
                console.error('JavaScript execution environment (Browser) does not support AudioContext interface.');
                alert('Could not start recording audio:\n Web Audio is not supported by your browser!');

                return;
            }
            audio_context = audio_context;
            input = audio_context.createMediaStreamSource(stream);

            if (input.context.createJavaScriptNode)
                node = input.context.createJavaScriptNode(4096, 1, 1);
            else if (input.context.createScriptProcessor)
                node = input.context.createScriptProcessor(4096, 1, 1);
            else
                console.error('Could not create audio node for JavaScript-based Audio Processing.');

            encoder.postMessage({
                cmd: 'init',
                config: {
                    samplerate: 44100,
                    channels: 1,
                    bps: 16,
                    compression: 5
                }
            });
            node.onaudioprocess = function (e) {
                // see also: http://typedarray.org/from-microphone-to-wav-with-getusermedia-and-web-audio/
                let channelLeft = e.inputBuffer.getChannelData(0);
                encoder.postMessage({ cmd: 'encode', buf: channelLeft });
            };

            input.connect(node);
            node.connect(audio_context.destination);
        }

        if (navigator.webkitGetUserMedia)
            navigator.webkitGetUserMedia({ video: false, audio: true }, gotUserMedia, userMediaFailed);
        else if (navigator.mozGetUserMedia)
            navigator.mozGetUserMedia({ video: false, audio: true }, gotUserMedia, userMediaFailed);
        else
            navigator.getUserMedia({ video: false, audio: true }, gotUserMedia, userMediaFailed);
    }

    function stopRecording() {
        if (!recording) {
            return;
        }
        console.log('stop recording');
        const tracks = stream.getAudioTracks()
        for (let i = tracks.length - 1; i >= 0; --i) {
            tracks[i].stop();
        }
        recording = false;
        encoder.postMessage({ cmd: 'finish' });

        input.disconnect();
        node.disconnect();
        input = node = null;
    }

    return {
        start: startRecording,
        stop: stopRecording,
    }
}