Question

I’m tinkering with achieving threading in Javascript by sending bogus Ajax requests. However I’m surprised at the behavior of my code below.

With xhr.abort() commented out, both “quarter” and “half” get interspersed to the console as I expect. But if xhr.abort() is called, only “quarter” gets sent to the console, and not “half”.

Anybody with insight into how XMLHttpRequest’s abort method works or situations where it would be appreciated. I’ve looked at documentation at https://developer.mozilla.org/en/DOM/XMLHttpRequest and it says “abort()….aborts the request if it has already been sent.” The key being the *singular* request, not all others, as though XMLHttpRequest is a singleton.

function parallelize(bogusUrl, parallelFns) {
    for (var i=0, n=parallelFns.length; i<n; i++) {
        var fn = parallelFns[i]
            ,xhr;

        try {xhr = new XMLHttpRequest()}
        catch (e) {xhr = new ActiveXObject('Microsoft.XMLHTTP')}

        xhr.onreadystatechange = function() {
            if (xhr.readyState == 1) {
                fn();
                //xhr.abort();
            }
        };

        xhr.open('GET', bogusUrl);
        xhr.send(null);
    }
}

parallelize('bogusUrl', [
    function(){setInterval(function(){console.log('quarter')}, 250)},
    function(){setInterval(function(){console.log('half')}, 500)}
]);

Answer 1

Blocks do not create a scope in JavaScript; only functions do. Thus, every iteration of your loop is sharing the same “xhr” variable, and its value is overwritten on each iteration.

To fix the problem, you can create a separate function to do the xhr work:

function doXHR(fn) {
    var xhr = null;

    try {xhr = new XMLHttpRequest()}
    catch (e) {xhr = new ActiveXObject('Microsoft.XMLHTTP')}

    xhr.onreadystatechange = function() {
        if (xhr.readyState == 1) {
            fn();
            //xhr.abort();
        }
    };

    xhr.open('GET', bogusUrl);
    xhr.send(null);
}

Then your loop would look like this:

for (var i=0, n=parallelFns.length; i<n; i++)
    doXHR( parallelFns[i] );