Promise.js

function Promise(promise) {
    if (promise instanceof Promise) {
        return promise;
    } else {
        // this is a new promise chain
        this.callbacks = [];
    }
}

Promise.prototype.then = function(callback_ok, callback_error) {
    this.callbacks.push({
        ok: callback_ok,
        error: callback_error
    });
    return this;
};

Promise.prototype.resolve = function() {
    var callback = this.callbacks.shift();
    if (callback && callback.ok) {
        callback.ok.apply(this, arguments);
    }
};

Promise.prototype.reject = function() {
    var callback = this.callbacks.shift();
    if (callback && callback.error) {
        callback.error.apply(this, arguments);
    }
};