input file转base64

不想用canvas转

  <input type="file" id="file" accept="image/*" onchange="changeImg()"/>
  <script>

function blobToDataURL(blob,cb) {
    let reader = new FileReader();
    reader.onload = function (evt) {
      var base64 = evt.target.result
      cb(base64)
    };
    reader.readAsDataURL(blob);
}

  var file = document.getElementById("file");
  function changeImg() {
    var img = file.files[0]
    if(img){
      var url = URL.createObjectURL(img);
      var base64 = blobToDataURL(img,function(base64Url) {
        console.log(base64Url)
      })
    }
  }