js在前端获取在本地上传图片的尺寸

var MyTest = document.getElementById("upload_img").files[0];
var reader = new FileReader();
reader.readAsDataURL(MyTest);
reader.onload = function(theFile) {
　　var image = new Image();
image.src = theFile.target.result;
image.onload = function() {
　　alert("图片的宽度为"+this.width+",长度为"+this.height);
};
};

<input type="file" size="20" autocomplete="off" id="upload_img" name="Uploadfile">