Convert .mov URL to a blob and convert blog to mp4 using javascript and in a website?

How do I convert .mov URL to a blob and convert this blob to an .mp4 using Javascript and in a website?

But here is my attempt:

function convertVideo(givenURL) {

  var file = new Blob(
    [givenURL],
    {"type" : "video/mp4"});

  var mp4VideoFile = new File(
      [file],
      { "type" : 'video/mp4' });

  return mp4VideoFile
}