index.js

function startRecording(stream, lengthInMS) {
  let recorder = new MediaRecorder(stream);
  let data = [];

  recorder.ondataavailable = event => data.push(event.data);
  recorder.start();

  let stopped = new Promise((resolve, reject) => {
    recorder.onstop = resolve;
    recorder.onerror = event => reject(event.name);
  });

  let recorded = new Promise((resolve, reject) => {
    setTimeout(() => {
      recorder.state == "recording" && recorder.stop()
      resolve();
    }, lengthInMS);
  });

  return Promise.all([
    stopped,
    recorded
  ])
  .then(() => data);
}