在AJAX调用之后获取未定义

Question

我不是很喜欢Javascript。我主要用python做后端。这次我别无选择，只能做javascript：P

我遇到了XMMHttpRequest响应问题。 我有这个HTML

        <div>
        <h3>Drop in orders</h3>
        <hr>
        <button onclick="triggerScenario(this);"  class="btn btn-default" id="z-drop-sev1">
            <img src={{ url_for('static', filename='images/250.jpeg' ) }} class="img-responsive" alt="logo">
        </button>
        <button onclick="triggerScenario(this);"  class="btn btn-default" id="6-drop-sev1">
            <img src={{ url_for('static', filename='images/250.jpeg' ) }} class="img-responsive" alt="logo">
        </button>
        <button onclick="triggerScenario(this);"  class="btn btn-default" id="z-6-drop-sev1">
            <img src={{ url_for('static', filename='images/250.jpeg' ) }} class="img-responsive" alt="logo">
        </button>
    </div>

当用户单击其中一个按钮时，将调用此函数：

function triggerScenario(clickedButton) {
var jiraEndpoint = "/api/jira/create/"+clickedButton.id;
var xmattersEndpoint = "/api/dmx/trigger/";
var zitEndpoint = "/api/zit/"+clickedButton.id+"/";
var ticket;
var id;

function createTicket() {
    var httpRequest = new XMLHttpRequest();

    var data = "{"+
           "\"project\": {"+
           "\"id\": 10150"+
           "},"+
           "\"issuetype\": {"+
           "\"name\": \"Incident\""+
           "},"+
           "\"priority\": {"+
           "\"name\": \"high\""+
           "}"+
        "}"

    var jsonData = JSON.parse(data);
    httpRequest.open('POST',jiraEndpoint);
    httpRequest.onreadystatechange = function() {
        if (httpRequest.readyState === XMLHttpRequest.DONE) {
            if (httpRequest.status === 200) {
                var response = JSON.parse(this.responseText);
                ticket = response.number;
                //Here I see the value I need
                console.log(ticket)
                httpRequest = null;
                return ticket;
            }
        }
    };
    httpRequest.setRequestHeader('Content-Type', 'application/json');
    httpRequest.send(data);

};
ticket = createTicket();
//This gives me "undefined"
console.log("Ticket: "+ticket);
//sendZit(ticket);
//triggerCall();

}

问题是“票证”的值始终未定义。如果我在函数内部记录值，它会显示我需要的内容。有更多JS知识的人能解释我这里发生了什么吗？提前致谢！