let video = [];
this.service.nextVideo().subscribe( response =>
{
    response.array.forEach(element => {
    video.push(element.videoId);
  });
})
console.log(video[0]); //Output is undefined

let video = [];
this.service.nextVideo().subscribe( response =>
{
    response.array.forEach(element => {
    video.push(element.videoId);
    console.log(video[0]);
  });
})