jQuery ajax调用没有设置变量的值

Question

I have this jquery call to ajax,

function findTaxForProcess(argPrc, argPrcAmount, argPrcDiscount) {
            if (argPrc == '') { return 0; };
            var _valToReturn;

            if ($('#hdnTaxBefore').val() == "true") {
                // if tax is calculated before discount
                $.ajax({
                    url: '/AutoComplete.asmx/FindProcessTax',
                    type: 'POST',
                    timeout: 5000,
                    datatype: 'xml',
                    cache: false,
                    data: 'argProcess=' + argPrc + '&argAmt=' + argPrcAmount,
                    success: function (response) {
                        _valToReturn = $(response).find('double').text();
                        alert(_valToReturn);
                    }
                });
            }

            else {
                // the tax is calculated after discount
                $.ajax({
                    url: '/AutoComplete.asmx/FindProcessTaxAter',
                    type: 'POST',
                    timeout: 5000,
                    datatype: 'xml',
                    cache: false,
                    data: 'argProcess=' + argPrc + '&argAmt=' + argPrcAmount + '&argDiscount=' + argPrcDiscount,
                    success: function (response) {
                        _valToReturn = $(response).find('double').text();
                        alert(_valToReturn);
                    }
                });
            }
            alert('came here ' + _valToReturn);
            return _valToReturn;
        };

The problem is first alert shows 2.873 (in the else case, it shows 2.178 ), but the problem is second alert, the second alert shows , came here undefined ??? FTW? what the hack is going wrong? I've been messing around this for 2 days but nothing!

Why is the value of _valToReturn undefined at second alert? and defined at first? any help?

EDIT : The alert came here _valToReturn is executing after the first alert. So, as (almost!) all answerer said that if its coming before, it would be undefined, I know that, but as I said it is executing after not before.

Answer 1

Ajax is an asynchronous request. By the time the success event executes, the alert "came here undefined" would have executed. And the variable has not been initialized by then.

Update: In answer to your question looking closely, I believe The anonymous function sees valToReturn as a local variable. Hence those variables are on a different "stack" if you will.

See this: Javascript anonymous function not updating global variable

Or

Using variable outside of ajax callback function

Similar to yours.

Answer 2

尝试初始化_valToReturn值以摆脱undefined

Answer 3

If the second alert comes first, that is if undefined comes first and then the value, then it is because of the async call of ajax. To overcome this you can make the ajax call synchronous.

If the second alert comes after the first one then it may the problem with the variable value.

Answer 4

您可以尝试使用console.log来查找首先执行的操作，在我看来，“来到这里”的警报应该先执行，然后去查看firebug或chrome console