Join 2 'threads' in javascript

Question

If I have an ajax call off fetching (with a callback) and then some other code running in the meantime. How can I have a third function that will be called when both of the first 2 are done. I'm sure it is easy with polling (setTimeout and then check some variables) but I'd rather a callback.

Is it possible?

Answer 1

You could just give the same callback to both your AJAX call and your other code running in the meantime, use a variable to track their combined progress, then link them to a callback like below:

// Each time you start a call, increment this by one
var counter = 0;

var callback = function() {
    counter--;
    if (counter == 0) {
        // Execute code you wanted to do once both threads are finished.
    }
}

Answer 2

Daniel's solution is the proper one. I took it and added some extra code so you don't have to think too much ;)

function createNotifier() {
    var counter = 2;
    return function() {
        if (--counter == 0) {
            // do stuff
        }
    };
}

var notify = createNotifier();
var later = function() {
    var done = false;
    // do stuff and set done to true if you're done
    if (done) {
        notify();
    }
};

function doAjaxCall(notify) {
    var ajaxCallback = function() {
        // Respond to the AJAX callback here
        // Notify that the Ajax callback is done
        notify();
    };
    // Here you perform the AJAX call action
}

setInterval(later, 200);
doAjaxCall(notify);