javascript getElementById const not defined

async function asyncCall() {
    const startBtn = document.getElementById('startbtn');
    const stopBtn = document.getElementById('stopbtn');
 
    const userMedia = await navigator.mediaDevices.getUserMedia({audio : true, video : true});

    const videoElement2 = document.getElementById("localVideo1");
    const videoElement1 = document.getElementById("localVideo2");

    const videoTrack = userMedia.getVideoTracks()[0];
    const audioTrack = userMedia.getAudioTracks()[0];
    const screenMedia = await navigator.mediaDevices.getDisplayMedia();

    videoTrack.enabled = false;
    audioTrack.enabled = false;
  
    videoElement2.srcObject = userMedia;
    videoElement1.srcObject = screenMedia;
}
asyncCall();
startBtn.addEventListener('click', () => {
    videoTrack.enabled = true;
    audioTrack.enabled = true;
    videoElement2.srcObject = userMedia;
    videoElement1.srcObject = screenMedia;
    videoElement1.play();
    videoElement2.play();
});
stopBtn.addEventListener('click', () => {
    videoTrack.enabled = false;
    audioTrack.enabled = false;
    videoElement1.stop();
    videoElement2.stop();
});