Nodejs发布不发送响应

Question

我试图理解为什么在函数外部发送ajax响应，但不在函数内部。响应对象可在函数内访问，但不会发送实际响应。什么会导致这种行为？

app.post('/login', urlencodedParser, function(req, res){
   if (!req.body) return res.sendStatus(400);
   var password = JSON.stringify(req.body.password);
   var username = JSON.stringify(req.body.username);
   //res.send('Invalid');  //This sends a response

   login.login(username, password,function(err,content) {
       if(err){res.send('Invalid');} //This does not send a response
       else {
          agent_valid = content[0].valid; 
          if (agent_valid === 0)
          {
              console.log(agent_valid);  //agent_valid is appearing as 1 or 0
              console.log(res);  //res exists and is written to console
              res.send('Invalid'); //This does not send a response
          }
          if (agent_valid === 1)
          {
              res.send('Valid'); //This does not send a response
          }
       }
    });
 });

这是发送帖子请求的javascript：

ajaxRequest.send(JSON.stringify({username:username, password:password})); 
ajaxRequest.onreadystatechange = function() {
    if(this.readyState == 4 && this.responseText == 'Invalid') {
        window.alert(this.responseText)    
    };
    if(this.readyState == 4 && this.responseText !='Invalid'){
        window.alert(this.responseText)

    };
}