Waiters (promises/deferreds/futures)

waiter.js

function Waiter(context) {
    if (context) { this.context = context; }
}

Waiter.prototype.failed = function (cb) {
    if (this.result) {
        if (!this.success) { cb.apply(this.context || this, this.result); }
    } else {
        if (!this.failedCbs) { this.failedCbs = []; }
        this.failedCbs.push(cb);
    }
    return this;
};

Waiter.prototype.then = function (cb) {
    if (this.result) {
        if (this.success) { cb.apply(this.context || this, this.result); }
    } else {
        if (!this.thenCbs) { this.thenCbs = []; }
        this.thenCbs.push(cb);
    }
    return this;
};

Waiter.prototype.fail = function () {
    if (this.result) { return; }

    this.result = arguments;
    this.success = false;

    var cbs = this.failedCbs;

    delete this.failedCbs;
    delete this.thenCbs;

    if (!cbs) { return; }

    var i, len = cbs.length;
    for (i = 0; i < len; i++) {
        cbs[i].apply(this.context || this, arguments);
    }
};

Waiter.prototype.failAfter = function (ms) {
    var waiter = this;
    setTimeout(function () {
        waiter.fail();
    }, ms);
};

Waiter.prototype.finish = function () {
    if (this.result) { return; }

    this.result = arguments;
    this.success = true;

    var cbs = this.thenCbs;

    delete this.failedCbs;
    delete this.thenCbs;

    if (!cbs) { return; }

    var i, len = cbs.length;
    for (i = 0; i < len; i++) {
        cbs[i].apply(this.context || this, arguments);
    }
};

Waiter.join = function (list) {
    var waiters = [].slice.call(list instanceof Array ? list : arguments);

    var count = waiters.length;

    var w = new Waiter();
    w.waiters = waiters;
    if (!count) {
        w.finish();
        return w;
    }

    w.remaining = count;

    var then = Waiter.joinThen.bind(w), failed = Waiter.joinFailed.bind(w);

    var i;
    for (i = 0; i < count; i++) {
        waiters[i].then(then).failed(failed);
    }
    return w;
};

Waiter.joinThen = function () {
    if (!this.remaining || --this.remaining) { return; }
    delete this.remaining;

    var result = [], i, len = this.waiters.length;
    for (i = 0; i < len; i++) {
        result.push.apply(result, this.waiters[i].result);
    }

    this.finish.apply(this, result);
};

Waiter.joinFailed = function () {
    if (!this.remaining) { return; }
    delete this.remaining;

    this.fail.apply(this, arguments);
};