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       function changehandler(){

             console.log("change");
           var file = document.getElementById('fileupload').files[0];
           var reader=new FileReader();
           reader.onload = (function(theFile) {
               return function(e) {

                   console.log(e.target.result);
                   // img 元素
                   document.getElementById("img").src = e.target.result;
               };
           })(file);
           reader.readAsDataURL(file);

       }

    document.getElementById("fileupload").addEventListener("change",changehandler,false);