Implementing image upload local preview function based on jquery

When we upload files, if we have to upload them to the server every time before we can preview them, it seems reasonable but it is actually unreasonable. If the network speed is slow or there are problems with the pictures, this will not only waste customer time but also waste server resources. Okay, below we introduce the local preview when uploading images using js. I hope this method will be helpful to you.
1. Principle

is divided into two steps:

When the input for uploading an image is triggered and a local image is selected, obtain the URL of the object (object URL) of the image to be uploaded;

Assign the object URL to the src attribute of the pre-written img tag to display the image.

Here, we need to understand the File object, Blob object and window.URL.createObjectURL() method in Javascript.

1. File object

File object can be used to obtain information about a file, and can also be used to read the contents of this file. Usually, the File object is the FileList object returned after the user selects a file on an input element, or it can Is from the DataTransfer object generated by drag and drop operation.

Let’s look at getting the FileList object:

<script type="text/javascript" src="jquery.js"></script>

<input id="upload" type="file">
<img id="preview" src="">

<script type="text/javascript">
$('#upload').change(function(){
  // 获取FileList的第一个元素
  alert(document.getelementbyid('upload').files[0]);
});
</script>

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2. Blob object

A Blob object is a file-like object containing read-only raw data. The data in the Blob object does not necessarily have to be in the native form in JavaScript. The File interface is based on Blob, inherits the functions of Blob, and extends support A local file on the user's computer.

The object URL we want to get is actually obtained from the Blob object, because the File interface inherits Blob. Let’s convert the Blob object into a URL:

<script type="text/javascript">
var f = document.getelementbyid('upload').files[0];
var src = window.URL.createObjectURL(f);
document.getElementById('preview').src = src;
</script>

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A relatively complete example

<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<title>HTML5 Upload</title>
<style type="text/css">
    #destination{
      filter:progid:DXImageTransform.Microsoft.AlphaImageLoader(true,sizingMethod=scale);
    }
</style>

<!--<script type="text/javascript" src="http://code.jquery.com/jquery-1.8.2.min.js"></script>-->
<script type="text/javascript" src="http://localhost/jQuery/jquery.js"></script>
<script type="text/javascript">
//处理file input加载的图片文件
$(document).ready(function(e) {
 //判断浏览器是否有FileReader接口
 if(typeof FileReader =='undefined')
 {
  $("#destination").css({'background':'none'}).html('亲,您的浏览器还不支持HTML5的FileReader接口,无法使用图片本地预览,请更新浏览器获得最好体验');
 //如果浏览器是ie
 if($.browser.msie===true)
 {
  //ie6直接用file input的value值本地预览
  if($.browser.version==6)
  {
    $("#imgUpload").change(function(event){   
    //ie6下怎么做图片格式判断&#63;
    var src = event.target.value;
    //var src = document.selection.createRange().text; //选中后 selection对象就产生了 这个对象只适合ie
    var img = '<img src="'+src+'" width="200px" height="200px" />';
    $("#destination").empty().append(img);
   });
  }
  //ie7,8使用滤镜本地预览
  else if($.browser.version==7 || $.browser.version==8)
  {
  $("#imgUpload").change(function(event){
    $(event.target).select();
    var src = document.selection.createRange().text;
    var dom = document.getElementById('destination');
    //使用滤镜 成功率高
    dom.filters.item('DXImageTransform.Microsoft.AlphaImageLoader').src= src;
    dom.innerHTML = '';
    //使用和ie6相同的方式 设置src为绝对路径的方式 有些图片无法显示 效果没有使用滤镜好
    /*var img = '<img src="'+src+'" width="200px" height="200px" />';
    $("#destination").empty().append(img);*/
   });
  }
 }
 //如果是不支持FileReader接口的低版本firefox 可以用getAsDataURL接口
 else if($.browser.mozilla===true)
 {
  $("#imgUpload").change(function(event){
  //firefox2.0没有event.target.files这个属性 就像ie6那样使用value值 但是firefox2.0不支持绝对路径嵌入图片 放弃firefox2.0
  //firefox3.0开始具备event.target.files这个属性 并且开始支持getAsDataURL()这个接口 一直到firefox7.0结束 不过以后都可以用HTML5的FileReader接口了
  if(event.target.files)
  {
   //console.log(event.target.files);
   for(var i=0;i<event.target.files.length;i++)
   { 
     var img = '<img src="'+event.target.files.item(i).getAsDataURL()+'" width="200px" height="200px"/>';
    $("#destination").empty().append(img);
   }
  }
  else
  {
   //console.log(event.target.value);
   //$("#imgPreview").attr({'src':event.target.value});
  }
  });
 }
 }
 else
 {
 // version 1
 /*$("#imgUpload").change(function(e){
  var file = e.target.files[0];
  var fReader = new FileReader();
  //console.log(fReader);
  //console.log(file);
  fReader.onload=(function(var_file)
  {
   return function(e)
   {
   $("#imgPreview").attr({'src':e.target.result,'alt':var_file.name});
   }
  })(file);
  fReader.readAsDataURL(file);
  });*/
  
  //单图上传 version 2 
  /*$("#imgUpload").change(function(e){
    var file = e.target.files[0];
    var reader = new FileReader(); 
  reader.onload = function(e){
   //displayImage($('bd'),e.target.result);
   //alert('load');
   $("#imgPreview").attr({'src':e.target.result});
  }
  reader.readAsDataURL(file);
   });*/
  //多图上传 input file控件里指定multiple属性 e.target是dom类型
   $("#imgUpload").change(function(e){ 
    for(var i=0;i<e.target.files.length;i++)
    {
     var file = e.target.files.item(i);
   //允许文件MIME类型 也可以在input标签中指定accept属性
   //console.log(/^image/.*$/i.test(file.type));
   if(!(/^image/.*$/i.test(file.type)))
   {
    continue;  //不是图片 就跳出这一次循环
   }
   
   //实例化FileReader API
   var freader = new FileReader();
   freader.readAsDataURL(file);
   freader.onload=function(e)
   {
    var img = '<img src="'+e.target.result+'" width="200px" height="200px"/>';
    $("#destination").empty().append(img);
   }
    }
   });
   
  //处理图片拖拽的代码
  var destDom = document.getElementById('destination');
  destDom.addEventListener('dragover',function(event){
   event.stopPropagation();
   event.preventDefault();
   },false);
   
  destDom.addEventListener('drop',function(event){
   event.stopPropagation();
   event.preventDefault();
   var img_file = event.dataTransfer.files.item(0);  //获取拖拽过来的文件信息 暂时取一个
   //console.log(event.dataTransfer.files.item(0).type);
   if(!(/^image/.*$/.test(img_file.type)))
   {
   alert('您还未拖拽任何图片过来,或者您拖拽的不是图片文件');
   return false;
   }
   fReader = new FileReader();
   fReader.readAsDataURL(img_file);
   fReader.onload = function(event){
   destDom.innerHTML='';
   destDom.innerHTML = '<img src="'+event.target.result+'" width="200px" height="200px"/>'; 
   };
  },false);
 }
});
</script>
</head>

<body>
<input type="file" id="imgUpload" name="imgUpload" draggable="true" single/> <!--允许file控件接受的文件类型-->
<!--<input type="file" id="imgUpload" name="imgUpload" accept="image/*" multiple/>-->
<div id="destination" style="width:200px;height:200px;border:1px solid #000000;"><img src="nopic.jpg" /></div>
</body>
</html>

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2. Compatibility

•The above method is suitable for chrome browser
•If it is an IE browser, you can directly use the input value instead of src
• When viewing information online, you can directly use the getAsDataURL() method of the File object to obtain the URL. Now this method has been abolished. Similar methods include getAsText() and getAsBinary();

The above is the entire content of this article, I hope it will be helpful to everyone’s study.

Implementing image upload local preview function based on jquery_jquery