JavaScript回调函数在我的情况下不起作用[重复]

Question

我有一个函数会返回一个 AJAX 响应并在另一个函数上使用它。这是我的代码：

function updateLoanApproval(answer){
    var password = document.getElementById("password").value;
    var usid; 
    getExistingId(function(success){
        if (success === 0){
            var usid = "No ID Available";
        } else {
            var usid = success; //To get the returning value
        }
    });

    alert(usid);

这里是 getExistingId() 的代码

function getExistingId(){
    var url =  "../../library/functions.php?action=getId";
    var uid;
    http.onreadystatechange = function(){
        if (http.status == 200 && (http.readyState === 4)){
            uid = http.responseText;
            if (uid == "No ID"){
                callback(0);
            } else {
                callback(id);
            }
        }
    }

    http.open("GET",url,true);
    http.send();
}

在我测试代码时，查询或 PHP 代码没有问题，所以我不会在此处包含它，但为什么 usid 总是返回未定义？

Answer 1

仔细看，您在 getEdiistingId() 函数中缺少回调参数。像下面这样：

function getExistingId(callback){
var url =  "../../library/functions.php?action=getId";
var uid;
http.onreadystatechange = function(){
    if (http.status == 200 && (http.readyState === 4)){
        uid = http.responseText;
        if (uid == "No ID"){
            callback(0);
        } else {
            callback(id);
        }
    }
}

http.open("GET",url,true);
http.send();

}

并将警报放入回调方法中。

function updateLoanApproval(answer){
    var password = document.getElementById("password").value;
    var usid; 
    getExistingId(function(success){
        if (success === 0){
            var usid = "No ID Available";
        } else {
            var usid = success; //To get the returning value
        }
        alert(usid);
    });

需要usid的方法：

function doMyTaskThatNeedUsid(usid)
{
    // do something.
}

并调用如下方法：

function updateLoanApproval(answer){
    var password = document.getElementById("password").value;
    var usid; 
    getExistingId(function(success){
        if (success === 0){
            var usid = "No ID Available";
        } else {
            var usid = success; //To get the returning value
        }

        // Calling method that need usid.
        doMyTaskTaskThatNeedUsid(usid);
    });