Question

I have the following Javascript function that makes a call to an Asp.Net WebMethod that returns true if a username already exists and false if it does not.
The following will give me the correct answer in an alert box but if I change the
alert(result.d) to return result.d
the result of the function is always undefined.
How can I get the function that the Ajax call is contained in to return a value based on the response from the WebMethod?

function doesEnteredUserExist() {
        var valFromUsernameBox = $("#txtUserName").val();
        var jsonObj = '{username: "' + valFromUsernameBox + '"}'; 

        $.ajax({
        type: 'POST',
        contentType: 'application/json; charset=utf-8',
        data: jsonObj,
        dataType: 'json',
        async: false,
        url: 'AddNewUser.aspx/IsUserNameValid',
        success: function (result) {
            alert(result.d);
        },
        error: function (err) {
            alert(err);
        }
    });
}

Answer 1

The thing to note is that the ajax function is non blocking. You specify two callback functions, one for success and one for error, one of these functions will get executed when the underlying request is finished.

Your doesEnteredUserExist does not wait for the ajax call to complete, and thus, the return value of the function is undefined.

The proper way to continue execution based on the result of your call is to break your functionality out into a separate function, and call that from your success function.