promise用于ajax

var ajax = function (token) {
    return new Promise(function(resolve,reject){
    var xhr = window.XMLHttpRequest ? new XMLHttpRequest : new ActiveXObject('Microsoft.XMLHTTP');
    var url = '';
    var obj = {'accessToken':token}
    var params = "request="+JSON.stringify(obj); 
    xhr.onreadystatechange = function() {
    if (xhr.readyState == 4) {
         if (xhr.status >= 200 && xhr.status < 300) {
             var res = JSON.parse(xhr.responseText);
             if (res.code == 0) {
                 resolve(res.response.userId)
              }else {
                  reject(res.msg)
               }
           } else {
                alert('error')
           }
        }
     }
    xhr.open('POST', url, true);
    xhr.setRequestHeader('Content-type', "application/x-www-form-urlencoded");
    xhr.send(params);
    })
},
ajax(token).then(function(res){
    sessionStorage.setItem('userid',res);
}).catch(function(res){
    console.log(res)
})