简单实现js上传文件功能

<form id="upload" enctype="multipart/form-data" method="post">  <input type="file" name="file" id="pic"/>  <input type="button" value="提交" onclick="uploadPic();"/>  <span class="showUrl"></span>  <img src="" class="showPic" alt=""> </form> 

function uploadPic() {   var form = document.getElementById('upload'),     formData = new FormData(form);   $.ajax({    url:"https://sscpre.boe.com/v1/medical-console/medical/file/upload",    type:"post",    data:formData,    processData:false,    contentType:false,    success:function(res){     if(res){      alert("上传成功！");     }     console.log(res);     $("#pic").val("");     $(".showUrl").html(res);     $(".showPic").attr("src",res);    },    error:function(err){     alert("网络连接失败,稍后重试",err);    }    })   }