srcVsSrcObject.js

const videoPlayer = document.getElementById("video-player") // this is a <video> element 

onSuccessWithSrcObject(stream) {
   videoPlayer.srcObject = stream;
}

onSuccessWithSrc(stream) {
   videoPlayer.src = window.URL.createObjectURL(stream);
}

navigator.mediaDevices.getUserMedia({ audio: true, video: true })
  .then(stream => {
    if ("srcObject" in videoPlayer) {
      onSuccessWithSrcObject(stream)
    } else {
      onSuccessWithSrc(stream)
    }
  })
  .catch( err => alert(`Bummer! ${err.name}.`))