javascript function return not working

Question

I have a problem returning a variable in my function, the below script works fine:

function sessionStatus(){
    $(document).ready(function(){
        $.getJSON(scriptRoot+"sessionStatus.php",function(status){
            alert(status);
        });
    });
}

sessionStatus();

Bet when I try the following I get a message box with the message "undefined":

function sessionStatus(){
    $(document).ready(function(){
        $.getJSON(scriptRoot+"sessionStatus.php",function(status){
            return status;
        });
    });
}
alert(sessionStatus());

This is really bugging me, I just can't seem to see what I've done wrong.

Answer 1

There are two things you should know:

1: the JSON thing is asynchronous, so the function call to sessionStatus could already be done when the JSON is still being fetched. The following would work:

function sessionStatus(callback){
    $(document).ready(function(){
        $.getJSON(scriptRoot + "sessionStatus.php", function(status){
                callback(status);
        });
    });
}
sessionStatus(function(s){alert(s);});

or rather:

function sessionStatus(callback){
    $(document).ready(function(){
        $.getJSON(scriptRoot + "sessionStatus.php", callback);
    });
}
sessionStatus(function(s){alert(s);});

2: even when it would be synchronous, you are only giving a return value from the inner function, so the sessionStatus would return nothing. Check out this code (not related to your JSON thing):

function do() {
    var x = 0;
    (function(){
       x = 2;
    })();
    return x;
}

or:

function do() {
    var x = (function(){
       return 2;
    })();
    return x;
}

Both return 2. Hopefully this explains a bit.

Answer 2

Your function sessionStatus() doesn't return anything, hence the alert says undefined.

All the function does is set thing up for the AJAX call to happen once the page loads - nothing actually happens within sessionStatus() that could be returned.

The code in function(status) { ...} doesn't get run until the AJAX call to the server returns a value, and that AJAX call doesn't get sent until the page loads.

You ought to read up on what $.getJSON() and $(document).ready() actually do, because you're going to continue to get confusing behaviour until you understand them properly.