I was recently looking for an easy way to call a specific function after two separate/unrelated AJAX calls to two remote endpoints have been completed. The naive method would be to make the first AJAX call -> callback to the second AJAX  call -> callback to doSomething, but we can use the fact that these two AJAX calls are not related and run them concurrently. An easy way to achieve that is to:

1. set flags, say initialising two global variables at the beginning of the file:

var callback_one_done = false;
var callback_two_done = false;

2. have each callbacks set its own flag to ‘true’ upon completion and call doSomething
3. check both flags in doSomething:

var doSomething = function() {
    if (callback_one_done && callback_two_done){
        // actually do something
    }
}

But this is a bit ugly, as it litters the global namespace with two flags that are only used there. Instead, thanks to Javascript’s lexical scope we can declare doSomething as a closure and have the flags live inside the function itself:

var doSomething = (function(){
        var callback_one_done = false;
        var callback_two_done = false;
        return function(source) {
            if (source === 'callback_one') { callback_one_done = true; }
            if (source === 'callback_two') { callback_two_done = true; }
            if (callback_one_done && callback_two_done) {
                // actually do something
            }
        };
    }());

What we’ve done here is declare an anonymous function that returns a function. This newly created function, that gets attributed to doSomething, is a closure that contains both the code needed to run and the flag variables. The state is set and kept inside the function itself, without leaking on the outside environment.

Now we just need to call doSomething(‘callback_one’) from the first AJAX call and doSomething(‘callback_two’) from the second AJAX call and doSomething will wait until both calls are complete to run the “actually do something” part.

Recently at work, we wanted to modify some js ad code to include weather data for better ad targeting. For certain caching reasons, weather data has to be fetched by an AJAX call, then fed to the ad code.

The existing ad code was (schematically) as follows.

My first idea was to replace those calls by a call to a single js method, which would stall the ad calls using, say, a setTimeout, until the weather data is retrieved. The js looked like this.

Before you go get the pitchforks, I knew that this code was crap, and asked one of my amazing colleagues for help. He advised me to get rid of the timeOuts and instead use a function that would either, depending if the data was retrieved or not, shelf the task in an array, or execute it.

Using an array of tasks (actually callbacks, or anonymous functions, or closures) allows us to have actual event-driven javascript. This means executing the ad loading code only once, not using resources for timeouts when the resource is not available yet, and maybe most importantly not looking stupid when code review time codes.

Additionally, my code littered the general namespace with his variables. We could instead create a module, and have only that module’s name public – we clean up the global namespace and also get private variables for free!
The module pattern is an amazingly useful javascript pattern, implemented as a self-executing anonymous function. If that sounds clear as mud, Ben Cherry has a better, in-depth explanation on his blog.

The js code now looked like this.

But the HTML still contained <script> tags, and separation of concerns taught us it’s better to keep HTML and JS as separate as possible. Cool beans, let’s just add a couple of line to our js file.

Result: a pure event-driven wait for a callback, a single variable in the global js namespace, and a cleaned-up HTML.

And of course… We ended up not using it. The ad code actually expects a document.write, making this delayed approach impossible (document.write appends data inline while the page is still loading, but completely rewrites the page if called after pageload). Good learning experience still!