convert png to jpeg using javascript and write back to original file

convertToJPG

function convertDataURIToBinary(dataURI) {
        var BASE64_MARKER = ';base64,';
        var base64Index = dataURI.indexOf(BASE64_MARKER) + BASE64_MARKER.length;
        var base64 = dataURI.substring(base64Index);
        var raw = window.atob(base64);
        var rawLength = raw.length;
        var array = new Uint8Array(new ArrayBuffer(rawLength));

        for (i = 0; i < rawLength; i++) {
          array[i] = raw.charCodeAt(i);
        }
        return array;
      }


var convertToJPG = function(file) {
        var image = new Image();

        image.src = file.nativeURL;
        var canvas = document.createElement("canvas");
        canvas.width = image.width;
        canvas.height = image.height;
        canvas.getContext("2d").drawImage(image, 0, 0);

        image.onload = function(){
          //save to temp location??

          file.createWriter(function(fileWriter) {

            file.onWriteEnd = function(e) {
              console.log('Write completed.');
            };

            file.onError = function(e) {
              console.log('Write failed: ' + e.toString());
            };

            // Create a new Blob and write it to log.txt.
            var ui8a = convertDataURIToBinary(image);

            var blob = new Blob(ui8a.buffer, {type: "image/jpeg"});

            fileWriter.write(blob);

          }, errorHandler);
        };

        image.src = canvas.toDataURL("image/jpg");
        return image;
      };