[javascript]Failed to execute 'send' on 'XMLHttpRequest': The object's state must be OPENED.

//1. 创建xhr对象 
var httpRequest;
    if(window.XMLHttpRequest){
        httpRequest = new XMLHttpRequest(); //DOM
    }else if(window.ActiveXObject){
        httpRequest = new ActiveXObject("Microsoft.XMLHTTP");//IE
    }
    //2. 请求设置(打开请求)
    //url后面的参数
    //let param ="txtUsername="+$("#txtUser").val()+"&txtPwd="+$("#txtPwd").val();
   // console.log(param);
    //三个参数，1. 请求的方式，get,post 2. 请求的地址
    httpRequest.open(methed,url,aync);
    //3. 绑定事件，处理服务端响应的数据
    httpRequest.onreadystatechange=function(){
        if(httpRequest.readyState==4&&httpRequest.status==200){
             callback(httpRequest.responseText);
        }
    }
    //4.发送请求
    //get 请求时，send(),发送null
    // 如果是post请求，需要设置请求的编码
     //  httpRequest.setRequestHeader("Content-Type","application/x-www-form-urlencoded");
     httpRequest.send(null); //发送的数据