Array in success function undefined

Why does this code

for(var i = 0; i < array.length; ++i) {
    array[i]["bla"] = "check";
}

work perfectly, whereas the array is here, according to firebug, undefinied:

for(var i = 0; i < array.length; ++i) {
    $.ajax({
       开发者_如何学Python type: "POST",
        url: "my url",
        data: "data here",
        success: function() {
            array[i]["bla"] = "check";
        }
    });
}

How can I fix that issue?

---

Due to how closures work, the value of i is always going to be equal to array.length in the callback, because that's what it equals after the loop is done (after all, i < array.length is false). And that position is always undefined. You need to re-bind i inside the loop to make the current value "stick". Unfortunately the only way to do this in standard JS is to use yet another function, for instance:

for (...; i++) {
    (function(boundI) {
        // boundI is now bound to the current value of i in the current scope
        // If you want, you can call boundI just "i", but make sure you understand
        // how scopes work in JS before you do.
        $.ajax({
            ...
            success: function() {
                array[boundI]["bla"] = "check";
            }
        });
    })(i); // pass in the current value of the loop index which gets bound to boundI
}

---

You could do a synchronous call:

for(var i = 0; i < array.length; ++i) {
    $.ajax({
        type: "POST",
        async: false, //This does the magic
        url: "my url",
        data: "data here",
        success: function() {
            array[i]["bla"] = "check";
        }
    });
}

Or, to keep syncronicity

    var array = ...    
    function loadElement(i){
       if(i == array.length) return;
       $.ajax({
            type: "POST",
            url: "my url",
            data: "data here",
            success: function() {
                array[i]["bla"] = "check";
                loadElement(i+1); 
            }
        });
    }
    loadElement(0);

Hope this helps. Cheers