Javascript function containing JQuery ajax - Function returns undefined

问题

Can any give me a clue why this isn't working? The function returns undefined. It alerts a boolean values but still return undefined?!

Thanks

    function IsUniqueEmail() {

        var email = $("#<%=EmailAddress.ClientID%>").val();

        if (email.length > 0) {

            $.ajax({
                url: 'handlers/validator.ashx',
                dataType: 'json',
                data: { "n": "email", "v": email },
                async: false,
                success: function(data) {
                    alert(eval(data.success));
                    return eval(data.success);
                },
                error: function(XMLHttpRequest, textStatus, errorThrown) {
                    console.log(textStatus, errorThrown);
                    return true;
                }
            });
        }            
    }

    $(document).ready(function() {

        var execScript = $(".buttonStep1").attr("href").replace("javascript:", "");

        $(".buttonStep1").attr("href", "#").click(function() {
            // Add before click logic here
            var IsOk = IsUniqueEmail();
            if (IsOk) {
                $("#EmailAddressInUseMessage").hide();
                eval(execScript);
            }
            else {
                $("#EmailAddressInUseMessage").show();
            }
        });
    });

this is the response of the ajax call

    { "success": false, "error" : "ERROR_EMAILINUSE" }

回答1:

The script execution path when using AJAX is not linear. Your IsUniqueEmail will exit before the AJAX request has received a response, returning nothing.

What you're returning is being sent to $.ajax, which is the method that is invoking success, and there it is probably being disregarded.

You could rewrite your code in this manner:

$(document).ready(function() {
    $(".buttonStep1").click(function() {

        IsUniqueEmail();

        // never follow this link
        return false;

    });
});

In this way, the button click just starts the request - it doesn't go ahead and do anything else. After that, you do all your magic in the AJAX response:

function IsUniqueEmail() {

    var email = $("#<%=EmailAddress.ClientID%>").val();

    if (email.length > 0) {

        $.ajax({
            url: 'handlers/validator.ashx',
            dataType: 'json',
            data: { "n": "email", "v": email },
            async: false,
            success: function(data) {
                alert(eval(data.success));

                if(eval(data.success)) {

                   // Execute code to continue after the click here

                }
            },
            error: function(XMLHttpRequest, textStatus, errorThrown) {
                console.log(textStatus, errorThrown);
                return true;
            }
        });
    }            
}

回答2:

You cannot do it that way. IsUniqueEmail(); will be return before the Ajax call finished. It is asynchronous! You have to put the logic into your callback function:

function IsUniqueEmail() {

    var email = $("#<%=EmailAddress.ClientID%>").val();

    if (email.length > 0) {

        $.ajax({
            url: 'handlers/validator.ashx',
            dataType: 'json',
            data: { "n": "email", "v": email },
            async: false,
            success: function(data) {
               if (data.success) {
                   $("#EmailAddressInUseMessage").hide();
                   eval(execScript);
               }
               else {
                   $("#EmailAddressInUseMessage").show();
               }
            },
            error: function(XMLHttpRequest, textStatus, errorThrown) {
                console.log(textStatus, errorThrown);
            }
        });
    }            
}

If you specify the data type as JSON, you don't have to use eval to parse it. data will already be an object. And even if data.success contains a string, compare it to a string instead of using eval: data.success == 'true'.

回答3:

Adding to other answers, what you can do is you need to store the result of ajax call in a variable which has the scope of your IsUniqueEmail function. Your Ajax success call back can access the variable and update it (with the help of closure). After the Ajax call completes, you can return the updated variable (since it is not async).

function IsUniqueEmail() { 

        var email = $("#<%=EmailAddress.ClientID%>").val(); 
        var isUnique = false;

        if (email.length > 0) { 

            $.ajax({ 
                url: 'handlers/validator.ashx', 
                dataType: 'json', 
                data: { "n": "email", "v": email }, 
                async: false, 
                success: function(data) { 
                    alert(eval(data.success)); 
                    isUnique = eval(data.success); 
                }, 
                error: function(XMLHttpRequest, textStatus, errorThrown) { 
                    console.log(textStatus, errorThrown); 
                    return true; 
                } 
            }); 
        }
        return isUnique;             
    } 

来源：https://stackoverflow.com/questions/3405635/javascript-function-containing-jquery-ajax-function-returns-undefined