我想上传图像文件，将它们绘制到画布中，进行更改并将其保存在数据库中。我尝试测试画布图像（Pic）返回的base64值，该值为空白。但是，当我将canvas（Pic）附加到文档时，会看到结果。我在这里做错了什么？

function handleFileSelect(evt) {
  var files = evt.target.files; // FileList object                
  for (var i = 0, f; f = files[i]; i++) {

    if (!f.type.match('image.*')) {
      continue;
    }
    // read contents of files asynchronously
    var reader = new FileReader();

    // Closure to capture the file information.
    reader.onload = (function(theFile) {
      return function(e) {

        var canvas = document.createElement("canvas");

        var datauri = event.target.result,
          ctx = canvas.getContext("2d"),
          img = new Image();

        img.onload = function() {

          canvas.width = width;
          canvas.height = height;
          ctx.drawImage(img, 0, 0, width, height);
        };
        img.src = datauri;
        var imageData = ctx.getImageData(0, 0, canvas.width, canvas.height);

        document.body.appendChild(canvas); //picture gets uploaded

        // Generate the image data

        var Pic = canvas.toDataURL("image/png");

        console.log(Pic); // => returns base64 value which when tested equivalent to blank                              
        Pic = Pic.replace(/^data:image\/(png|jpg);base64,/, "")

        // Sending image to Server
        $.ajax({
          // …
        });

      };
    })(f);
    reader.readAsDataURL(f);
  }
}