Some things that can go wrong when setting up a camera with getUserMedia.

getUserMedia-setup.js

function cameraReady(videoElement) {
  console.log('cameraReady: checking...');
  if (videoElement) {
    if (videoElement.srcObject) {
      let tracks = videoElement.srcObject.getVideoTracks();
      if (tracks.length > 0) {
        if (tracks[0].readyState == 'live') {
          let w = videoElement.videoWidth, h = videoElement.videoHeight;
          if (w > 0 || h > 0) {
            console.log('cameraReady: live! ' + w + 'x' + h);
            return true;
          }
          console.error('cameraReady: live, but 0x0');
          return false;
        }
        console.error('cameraReady: readyState is ' + tracks[0].readyState);
        return false;
      }
      console.error('cameraReady: no tracks');
      return false;
    }
    console.error('cameraReady: no srcObject');
    return false;
  }
  console.error('cameraReady: no element');
  return false;
}

function initCamera(videoElement) {
  // these are essential on iOS
  videoElement.setAttributeNode(document.createAttribute('autoplay'));
  videoElement.setAttributeNode(document.createAttribute('playsinline'));
  
  // don't re-initialize the camera if it's already live
  if (cameraReady(videoElement)) {
    console.log('initCamera: done!');
    return Promise.resolve();
  }

  return navigator.mediaDevices
    .getUserMedia({
      audio: false,
      video: { facingMode: 'user' }
    })
    // stream is ready, set to the video element
    // don't return until the metadata is ready
    // https://stackoverflow.com/a/41866914/940196
    .then(stream => new Promise(resolve => {
      console.log('initCamera: setting srcObject...');
      videoElement.onloadedmetadata = resolve;
      videoElement.srcObject = stream;
    }))
    .then(() => console.log('initCamera: done!'))
    .catch(err => {
      console.error(err);
      alert(config.cameraErrorMessage);
    })
}