//input type设为file

     <input type="file" name="file" id="fileInputElement" />

     <button id="submit">提交</button>

   var btn = document.getElementById('submit');

         var fileInputElement = document.getElementById('fileInputElement');

         btn.onclick = function () {

             var oMyForm = new FormData();

            //append来添加文件,

             oMyForm.append("afile", fileInputElement.files[]);

             var oReq = new XMLHttpRequest();

             oReq.open("POST", "/test");

          //用FormData生成的对象的好处是,send不需要setRequestHeader

             oReq.send(oMyForm);

         }

const app = express();

const multer = require('multer');

var upload = multer({dest:'file/'})//multer的参数dest表示文件接受后存放的地址

app.post('/test',upload.single('afile'),(req,res)=>{

})