Javascript Variable Not Declaring in Function's Scope

Question

I've been working on some simple AJAX functions recently, using MooTools. My problem lies in the code immediately following:

function changePage(id, url) {
    var c = false,
        r = null;

    for (i = 0; i < history.length; i++) {
        if(history[i]['id'] === id) {
            console.log('cached');
            r = history[i].response;
            c = true;
        }
    }
    if(!c) {
        makeRequest(url, function(response) {
            r = response;
            history.push({id: id, response: response});
        });
    }

    changeBackground('_background', {color: r.bgColor, image: r.bgImage});
    lightboxContents(generateArticle(r.article.id, r.article.title, r.article.body, r.article.timestamp));
    return true;
}

Here, whenever the code is executed (it's routed via a click), I'm sent the error "'r' is not defined" - a statement that, in my opinion, is woefully incorrect.

I have, for that matter, also tried replacing 'r' with a global variable using the 'window' object - same problem.

I'm ever puzzled by this simple problem, and would be worlds grateful if someone with fresh eyes could point out my error.

Thanks for your time! Timon

If you do console.log(r), what do you get? Sounds like r is not getting its value assigned (maybe due to some error). — Mrchief
– Mrchief, Commented Jul 11, 2011 at 3:05

Anurag · Accepted Answer · 2011-07-11 03:05:21Z

3

r is most likely undefined because it looks like makeRequest is an asynchronous AJAX call, meaning that its callback function is only invoked once the request completes. Since the call is asynchronous, the JS runtime does not wait there, and continues executing the statements after it, which in your case are the calls to changeBackground, and generateArticle. However, as the AJAX call has not returned by this time and the callback hasn't been invoked, r is still null. So trying to access any property on r like r.bgColor will throw a ReferenceError.

answered Jul 11, 2011 at 3:05

Anurag

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3 Comments

im.nomit Over a year ago

I had a feeling that might be the problem. How do you think I'd best to go about this? Just stuff the callback function with the code from each function, or is there a more elegant manner for this?

Anurag Over a year ago

moving code to inside the ajax callback is the way to go. organize code info functions, or modules, as you want. but the launching point will be the ajax callback.

im.nomit Over a year ago

Alright - thanks so much for your help. Saved me some days of agonizing tinkering!

Darko · Accepted Answer · 2011-07-11 03:05:38Z

0

It seems like you are trying to use r before you give it a value, while it is still null. When you load your page, there is no history so r wont get a value. And then you have an async function assigning a value to r but the code that needs r to have a value will run before the async code has executed.

answered Jul 11, 2011 at 3:05

Darko

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