Failed to execute 'setRequestHeader' on 'XMLHttpRequest': The object's state must be OPENED.

在设置请求头的时候报这个Failed to execute 'setRequestHeader' on 'XMLHttpRequest': The object's state must be OPENED.，是由于需要先打开XMLHttpRequest

 var xhr = new XMLHttpRequest();
//打开xhr
  xhr.open('GET',url );
//设置请求头，请求头的设置必须在xhr打开之后，并且在send之前
 xhr.setRequestHeader('user-sessionid', sessionId);