<div>
  <form>
    <input type="file" id="file-input" name="fileContent">
  </form>
</div>

var fileInput = document.querySelector('#file-input');
fileInput.onchange = function(){
  console.log('文件名:',this.value)
  var formData = new FormData(this.form);
  console.log(formData)
}
//打印出的结果是文件名: C:\fakepath\css.jpg然后在是一个空对象

var fileInput = document.querySelector('#file-input');
fileInput.onchange = function(){
 var filereader = new FileReader();
 var fileType = this.files[0].type;
 filereader.onload = function(){
   if(/^image\[jpeg|png|gif]/.test(fileType)){
     console.log(this.result);
   }
 }
 console.log(this.files[0]);
 filereader.readAsDataURL(this.files[0]);
}
console.dir(fileInput);

var files = document.getElementById('pic').files; 
//files是文件选择框选择的文件对象数组

if(files.length == 0) return; 

var form = new FormData(), 
  url = 'http://.......', //服务器上传地址
  file = files[0];
form.append('file', file);

var xhr = new XMLHttpRequest();
xhr.open("post", url, true);

//上传进度事件
xhr.upload.addEventListener("progress", function(result) {
  if (result.lengthComputable) {
    //上传进度
    var percent = (result.loaded / result.total * 100).toFixed(2); 
  }
}, false);

xhr.addEventListener("readystatechange", function() {
  var result = xhr;
  if (result.status != 200) { //error
    console.log('上传失败', result.status, result.statusText, result.response);
  } 
  else if (result.readyState == 4) { //finished
    console.log('上传成功', result);
  }
});
xhr.send(form); //开始上传