无法改变（不完全）全局变量＆＃39;来自一个函数javascript

Question

我希望从这个函数返回teacherID，所以我在函数外定义了实际值的变量，但它似乎没有任何帮助吗？ （P.S.不确定这种变量叫什么）

function getTeacherID(courseID,block,roomID){
    //Get the rooms for the subjects the student does
    var teacherID;
    function doXHRPromise() {
        return new Promise(function(resolve, reject) {
            var xmlhttp = new XMLHttpRequest();
            xmlhttp.onreadystatechange = function() {
                if (this.readyState == 4 && this.status == 200) {
                    resolve(this.responseText);
                }
            }
            var everythingArray = [courseID,block,roomID];
            xmlhttp.open("GET", "getTeacherIDTimetable.php?q=" + JSON.stringify(everythingArray), true);
            xmlhttp.send(); 
        });
    }
    doXHRPromise().then(function(responseText) {
        teacherID = responseText;
    });
    return teacherID;
}