javascript函数没有返回正确的值

Question

我有检查英国邮政编码的功能。问题是它从不返回 true 或 false - 仅返回 undefined。

function PostcodeAnywhere_Interactive_FindByPostcode_v1_00(Key, Postcode, UserName) {

    var retval;

    $.getJSON("https://services.postcodeanywhere.co.uk/PostcodeAnywhere/Interactive/FindByPostcode/v1.00/json3.ws?",
    {
        Key: Key,
        Postcode: Postcode,
        UserName: UserName
    },
    function (response) {
        // Test for an error
        if (response.Items.length == 1 && typeof(response.Items[0].Error) != "undefined") {
            // Show the error message
            retval = false;
        } else {
            // Check if there were any items found
            if (response.Items.length == 0){
                retval = false;
            } else {
                retval = true;
            }
        }
    });

return retval;

}

对我来说，它看起来应该总是返回 true 或 false，所以我不明白哪里出错了。有人可以帮忙吗？是getJSON函数需要时间执行吗？

Answer 1

它返回 undefined 因为 $.getJSON 是异步运行的，因此 retval 在 $.getJSON 的成功函数执行之前返回。如果您需要使用 retval，您必须在回调中调用使用它的函数

   $.getJSON("https://services.postcodeanywhere.co.uk/PostcodeAnywhere/Interactive/FindByPostcode/v1.00/json3.ws?",
    {
        Key: Key,
        Postcode: Postcode,
        UserName: UserName
    },
    function (response) {
        // Test for an error
        if (response.Items.length == 1 && typeof(response.Items[0].Error) != "undefined") {
            // Show the error message
            retval = false;
        } else {
            // Check if there were any items found
            if (response.Items.length == 0){
                retval = false;
            } else {
                retval = true;
            }
            //use retval
            do_something_with_retval(retval);
        }
    });