Help with a javascript callback

So, all AJAX methods in jQuery are just wrappers around the core ajax() method jQuery provides, if I recall correctly: http://api.jquery.com/jQuery.ajax/ I recommend setting something that duplicates the post() functionality, but try setting 'async' to false. That is:$.ajax({ url: "getLatLong.php", type: "POST", async: false, dataType: 'json', data: { latUser: 10, longUser: 20 }, success: function(arrayData) { arrayValue = arrayData[2]; }});Think that's right, and basically duplicates your post() function using the ajax() function, but I haven't tested it. I'm not a genius at this stuff, I get confused pretty easily by asynchronicity and callbacks and whatnot. But the basic issue with a lot of AJAX things like what you're experiencing, I've found, is that things are happening at the wrong time, since fundamentally the default functions are all set to be asynchronous...of course, since that's the first A in AJAX. If you set 'async' to false in the above function, you'll see whether or not this is the case right away. Of course, then you'll have to deal with the resultant lag, and essentially forgo the point of AJAX—which is to make interactions with the server hidden for the end user—but there ya go. Hope this helps at least diagnose it.

(I tried in firebug but I have just started using it and can't figure out where it shows me that) So pop open your Firebug console, and you'll see a row of tabs on the top. You want the "Net" one. Once you open that, there will be some sub-tabs. "All" will show you all the traffic involved with the currently open site. "XHR" (for XMLHttpRequest) will just show you the AJAX requests, which is what you're interested in. Is there any way for me to check whether that value has arrived in my code? You could do this with Firebug's "Console" tab. Pop that open, and you'll get a prompt at the bottom. You can type variable names in there, and it'll tell you what they evaluate to. And to add to what dubitable said, we need callbacks for flow control. In synchronous code, we know that things will run one after another, but we don't have that assurance with asynchronous code. So the utility of the callback function is that when it runs, we know for sure that we've received some sort of response, and we can act accordingly. Without a callback you'd have to rig up some sort of polling function - basically continually checking "arrayValue" until it returns something other than undefined. Which would suck.

I think you want to simply pass the function as the third parameter to $.post, rather than assigning it to this.success.e.g. $.post("getLatLong.php", { latUser: 10, longUser: 20},function(arrayData){ arrayValue = arrayData[2]; }, 'json');

evisceratordeath, I think you are correct that it is the time delay for the POST request (I tried in firebug but I have just started using it and can't figure out where it shows me that). Is there any way for me to check whether that value has arrived in my code? In the first case it will be easy as I could check that the number is non-zero, as I initialize to be zero. After that I'm not sure how to test for the returned value.

Thanks dubitable, that's completely correct. I think I have a much better understanding of this whole system now. I tested out that code and it worked fine.

Great, glad it helped!

It's all a lot clearer to me now, evisceratordeath. I understand the difference between synchronous/asynchronous code; and I feel like callbacks are making more sense. woot! thank you, hivemind.

Another nice thing about the Firebug and Webkit debuggers -- you can use console.log to dump stuff right to the console. Makes it really easy to make sure you're getting the stuff back you expect...