pass parameter from $.post() callback to outer variable - php

Question

basically i want to hold a parameter that retrieve value from $.post() call like this:
init = function(){
var lastpage = getLastPage();
}
function getLastPage(){
$.post("getInfo.php",{
last: "yes"
},
function(data){
setLast(data.last);
},'json');
return function setLast(data){
return data;
}
}
so when reach at last post (last page) i should check with lastpage variable that has a value returned from getLastPage() function.
I'm pretty blur with javascript pointer and all. Please help guys.
update (20/4/2010):
I've done the other way around, like this:
init = function(){
getLastPage();
if((page+1) == $("#lastpage").val()){
alert("this is last post");
}else{
page++;
//get info and display to the page here
}
}
function getLastPage(){
$.post("getInfo.php",{
last: "yes"
},
function(data){
$("#lastpage").val(data.last);
},'json');
}
first run the function to temporarily store the value in hidden input tag (lastpage) and then grab the value again to check it whenever i click forward button.
if you all have more appropriate way please tell me.

Answer 1

You should change your code around like this:
$.post("getInfo.php",{ last: "yes" },
function(data){
functionToRunAfterYouHaveDataSometimeLater(data.last);
}
,'json');
The problem with your overall approach is that with AJAX, you're dealing with an asynchronous operation. This means that the function(data) { } portion doesn't run then, it runs later, so your return doesn't actually return anything, it'll be undefined.
Instead of this approach, you need to call $.post() then call whatever function relies on this data to continue as part of $.post()'s callback. After doing that your code order looks like this:
$.post() executes, firing off a request to the server
The rest of your code after $.post() runs
Later when the response comes from the server and you have data, your callback executes
Now continue to do what you need with that data