Read file as bytes and base64 encode before uploading to server in javascript

Question

I want to read a file as a list of bytes and encode it as base64 so I can upload it to a server. I have the code in dart (Flutter) which is this:

String fileString = base64Encode(file.readAsBytesSync());

I want to do this in javascript. I have used this but it doesn't work

  const form = document.getElementById('form');
  const file = document.getElementById('file');
  const courseId = "<?php print($courseId) ?>";
  form.addEventListener('submit', e => {
    e.preventDefault();
    // get file name and type
    var filename = file.files[0].name;
    var filetype = filename.split(".").pop();
    var reader = new FileReader();
    reader.readAsBinaryString(file.files[0]);

    reader.onload = function (){
      // filestring = btoa(reader.result);
      console.log(reader.result);
    }
    //base64 encode
    var filestring = btoa(reader.result);


    const formData = new FormData();
    formData.append("action", "uploadFile");
    formData.append("file", filestring.toString());
    formData.append("name", filename.toString());
    formData.append("type", filetype.toString());
    formData.append("courseId", courseId);

     fetch('upload.php', {
       method: 'POST',
       body: formData
     }).then(response => {
       console.log(response);
     }).catch(console.error);

  });