Promise

var Promise = function() {
    this.state = 'pending';
    this.thenables = [];
};

Promise.prototype.resolve = function(value) {
    if (this.state != 'pending') { return; };

    this.state = 'fulfilled';
    this.value = value;
    this._handleThen();
    return this;
};

Promise.prototype.reject = function(reason) {
    if (this.state != 'pending') { return; };

    this.state = 'rejected';
    this.reason = reason;
    this._handleThen();
    return this;
};

Promise.prototype.then = function(onFulfilled, onRejected) {
    var thenable = {};

    if (typeof onFulfilled == 'function') {
        thenable.fulfill = onFulfilled;
    };

    if (typeof onRejected == 'function') {
        thenable.reject = onRejected;
    };

    if (this.state != 'pending') {
        setImmediate(function () {
            this._handleThen();
        }.bind(this));
    };

    thenable.promise = new Promise();
    this.thenables.push(thenable);

    return thenable.promise;
};


Promise.prototype._handleThen = function() {
    if (this.state === 'pending') { return; };

    if (this.thenables.length) {
        for (var i = 0; i < this.thenables.length; i++) {
            var thenPromise = this.thenables[i].promise;
            var returnedVal;
            try {
                switch (this.state) {
                    case 'fulfilled':
                        if (this.thenables[i].fulfill) {
                            returnedVal = this.thenables[i].fulfill(this.value)
                        } else {
                            thenPromise.resolve(this.value);
                        }
                        break;
                    case 'rejected':
                        if (this.thenables[i].reject) {
                            returnedVal = this.thenables[i].reject(this.reason);
                        } else {
                            thenPromise.reject(this.reason);
                        }
                        break;
                }

                if (returnedVal === null) { 
                    this.thenables[i].promise.resolve(returnedVal);
                }
                else if (returnedVal instanceof Promise || typeof returnedVal.then === 'function') {
                    returnedVal.then(thenPromise.resolve.bind(thenPromise), thenPromise.reject.bind(thenPromise));
                }
                else {
                    this.thenables[i].promise.resolve(returnedVal);
                }
            } catch (e) {
                thenPromise.reject(e);
            }
            this.thenables = [];
        };
    };
};