work.js

module.exports = function(self) {
  self.addEventListener('message', function(ev) {
    console.log('worker', ev);
    var blob = exportWAV(ev.data.leftBuf, ev.data.rightBuf, ev.data.sampleRate);
    self.postMessage(blob);
  }.bind(self));
}

function exportWAV(leftBuffer, rightBuffer, sampleRate) {
  var interleaved = interleave(leftBuffer, rightBuffer);
  var dataview = encodeWAV(interleaved, sampleRate);
  var audioBlob = new Blob([dataview], {type: "audio/wav"});

  this.postMessage(audioBlob);
}

function mergeBuffers(recBuffers, recLength){
  var result = new Float32Array(recLength);
  var offset = 0;

  for (var i = 0; i < recBuffers.length; i++){
    result.set(recBuffers[i], offset);
    offset += recBuffers[i].length;
  }

  return result;
}

function interleave(leftBuffer, rightBuffer){
  var length = leftBuffer.length + rightBuffer.length;
  var result = new Float32Array(length);

  var idx = 0,
      bufIdx = 0;

  while (idx < length) {
    // idx++
    result[idx++] = leftBuffer[bufIdx];
    result[idx++] = rightBuffer[bufIdx];
    bufIdx++;
  }

  return result;
}

function writeString(view, offset, string) {
  for (var i = 0; i < string.length; i++){
    view.setUint8(offset + i, string.charCodeAt(i));
  }
}

function encodeWAV(samples, sampleRate){
  var buffer = new ArrayBuffer(44 + samples.length * 2);
  var view = new DataView(buffer);

  /* RIFF identifier */
  writeString(view, 0, 'RIFF');
  /* RIFF chunk length */
  view.setUint32(4, 36 + samples.length * 2, true);
  /* RIFF type */
  writeString(view, 8, 'WAVE');
  /* format chunk identifier */
  writeString(view, 12, 'fmt ');
  /* format chunk length */
  view.setUint32(16, 16, true);
  /* sample format (raw) */
  view.setUint16(20, 1, true);
  /* channel count */
  view.setUint16(22, 2, true);
  /* sample rate */
  view.setUint32(24, sampleRate, true);
  /* byte rate (sample rate * block align) */
  view.setUint32(28, sampleRate * 4, true);
  /* block align (channel count * bytes per sample) */
  view.setUint16(32, 4, true);
  /* bits per sample */
  view.setUint16(34, 16, true);
  /* data chunk identifier */
  writeString(view, 36, 'data');
  /* data chunk length */
  view.setUint32(40, samples.length * 2, true);

  floatTo16BitPCM(view, 44, samples);

  return view;
}

function floatTo16BitPCM(output, offset, input){
  for (var i = 0; i < input.length; i++, offset+=2){
    var s = Math.max(-1, Math.min(1, input[i]));
    output.setInt16(offset, s < 0 ? s * 0x8000 : s * 0x7FFF, true);
  }
}