JavaScript使用FileReader实现图片上传预览效果

<div id="wrapper">  
 <input id="fileUpload" type="file" multiple /><br />
 <div id="image-holder"> </div>
</div>

$("#fileUpload").on('change', function () {
 
 //获取上传文件的数量
 var countFiles = $(this)[0].files.length;
 
 var imgPath = $(this)[0].value;
 var extn = imgPath.substring(imgPath.lastIndexOf('.') + 1).toLowerCase();
 var image_holder = $("#image-holder");
 image_holder.empty();
 
 if (extn == "gif" || extn == "png" || extn == "jpg" || extn == "jpeg") {
  if (typeof (FileReader) != "undefined") {
 
   // 循环所有要上传的图片
   for (var i = 0; i < countFiles; i++) {
 
    var reader = new FileReader();
    reader.onload = function (e) {
     $("<img />", {
      "src": e.target.result,
       "class": "thumb-image"
     }).appendTo(image_holder);
    }
 
    image_holder.show();
    reader.readAsDataURL($(this)[0].files[i]);
   }
 
  } else {
   alert("你的浏览器不支持FileReader！");
  }
 } else {
  alert("请选择图像文件。");
 }
});