JavaScript异步上传图片文件的实例代码

<form action="url" enctype="multipart/form-data" id="myform" method="post">
    <input accept="image/*" id="addfile" type="file" />
</form> 

$("#addfile").on('change', function () {
  var f = $(this).get(0).files[0];
  var form = document.getelementbyid('myform');
  var formdata = new formdata(form);
  formdata.append('filedata', f);
  var xhr = new xmlhttprequest();
  xhr.addeventlistener("load", uploadcomplete, false);
  xhr.addeventlistener("error", uploadfailed, false);
  xhr.open('post', form.action);
  xhr.send(formdata);
});
function uploadcomplete(evt)
{
  var data=evt.target.responsetext
}
function uploadfailed()
{
  alert("上传失败！请重试！"); 
}