DropZone插件实现图片压缩

 transformFile: function(file, done) {
     //如果不超过2M直接上传
     if (file.size < 2048 * 1000) return done(file);
 
     //这里的File对象继承自Blob,
     //当执行到这里的时候File不一定加载完
     //所以需要再加载一次,回调中进行压缩
     var reader = new FileReader();
     reader.addEventListener("loadend",
     function() {
         // 压缩图片大小
         var img = new Image();
         img.onload = function() {
             var cvs = document.createElement('canvas');
             cvs.width = img.naturalWidth;
             cvs.height = img.naturalHeight;
 
             var ctx = cvs.getContext("2d").drawImage(img, 0, 0);
 
             var zipImg = cvs.toDataURL("image/jpeg", 70 / 100);
             console.log(zipImg);
             return done(Dropzone.dataURItoBlob(zipImg));
         };
         img.src = reader.result;
     });
     reader.readAsDataURL(file);
 }