Record youtube / any <video>/<audio> clip

record-audio.js

// paste the code below into browser console in any youtube page and call `record(from, to)` 
// from and to are in seconds

async function record(from, to) {
var mediaElement=document.querySelector('video')
var recordedChunks = [];
var mimeType = 'audio/webm;codecs="opus"'

var ac = new AudioContext();
var mediaSource = new MediaElementAudioSourceNode(ac, {mediaElement});
var recordingDest = ac.createMediaStreamDestination();
mediaSource.connect(recordingDest);
mediaSource.connect(ac.destination);
var mediaRecorder = new MediaRecorder(recordingDest.stream, {mimeType});

mediaRecorder.ondataavailable = event => {
  console.log("data-available");
  if (event.data.size > 0) {
    recordedChunks.push(event.data);
    download();
  } else {
    // ...
  }
};
mediaRecorder.start();

function download() {
  var blob = new Blob(recordedChunks, {
    type: mimeType
  });
  var url = URL.createObjectURL(blob);
  var a = document.createElement("a");
  document.body.appendChild(a);
  a.style = "display: none";
  a.href = url;
  a.download = "test.webm";
  a.click();
  window.URL.revokeObjectURL(url);
}

mediaElement.currentTime = from
mediaElement.play()
setTimeout(event => {
  console.log("stopping");
  mediaRecorder.stop();
  mediaElement.stop()
  mediaSource.disconnect();
}, (to-from)*1000);
}

record-video.js

// paste code below into browser console in any youtube page and call `record(from, to)` 

async function record(from, to) {
  var mediaElement=document.querySelector('video')
  var recordedChunks = [];
  var mimeType = 'video/webm'
  
  var stream = mediaElement.captureStream(25); //optional fps arg
  var mediaRecorder = new MediaRecorder(stream, {mimeType});

  mediaRecorder.ondataavailable = event => {
    if (event.data.size > 0) {
      recordedChunks.push(event.data);
      download();
    } else {
      // ...
    }
  };
  mediaRecorder.start();

  mediaElement.currentTime = from;  // from 2:36
  mediaElement.play();

  function download() {
    var blob = new Blob(recordedChunks, {
      type: mimeType
    });
    var url = URL.createObjectURL(blob);
    var a = document.createElement("a");
    document.body.appendChild(a);
    a.style = "display: none";
    a.href = url;
    a.download = "test.webm";
    a.click();
    window.URL.revokeObjectURL(url);
  }

  mediaElement.currentTime = from
  mediaElement.play()
  setTimeout(event => {
    console.log("stopping");
    mediaRecorder.stop();
    mediaElement.stop()
    mediaSource.disconnect();
  }, (to-from)*1000);
}