makeFunctionSerial.js

// Ensures mutual exclusion of a single asynchronous function. Configurable callback and backlog.
function makeFunctionSerial(func, argNum, backlog) {
    if (arguments.length < 2) {
        argNum = func.length - 1;
    }
    if (arguments.length < 3) {
        backlog = Infinity;
    }
    var pending = [];
    var running = false;
    function next() {
        if (pending.length === 0 || running) {
            return;
        }
        running = true;
        var o = pending.shift();
        var callback = o.ARGS[argNum];
        o.ARGS[argNum] = function() {
            var result;
            if (typeof callback === "function") {
                result = callback.apply(this, arguments);
            }
            running = false;
            setTimeout(next, 0);
            return result;
        }
        return func.apply(o.THIS, o.ARGS);
    }
    return function() {
        if (pending.length < backlog) {
            pending.push({ THIS : this, ARGS : Array.prototype.slice.call(arguments) });
        }
        next();
    }
}

// Contrived example:

var serialSetTimeout = makeFunctionSerial(setTimeout, 0, 2);

serialSetTimeout(function() { console.log("a"); }, 2000); // t=2s
serialSetTimeout(function() { console.log("b"); }, 1000); // t=3s
serialSetTimeout(function() { console.log("c"); }, 1000); // t=4s
serialSetTimeout(function() { console.log("d"); }, 1000); // dropped!