on first call in JS FileReader is null, and base64string is empty, but on second call code working... why?

var base64String = "";
function imageUploaded(input) {
  var reader = new FileReader();
  reader.onloadend = function () {
    base64String = reader.result.replace("data:", "").replace(/^.+,/, "");
    console.log(base64String);
  }
  reader.readAsDataURL(input);
  return base64String;
}