practice promise implementation (from http://dailyjs.com/2014/02/20/promises-in-detail/)

Promise.js

module.exports = function(fn) {

    var state = "pending";
    var value;
    var deferred = null;

    function resolve(newValue) {
        try {
            // checks if a newValue is a promise
            if (newValue && typeof newValue.then === "function") {
                newValue.then(resolve);
                return;
            }
            state = "resolved";
            value = newValue;
            if (deferred) {
                handle(deferred);
            }
        } catch(e) {
            reject(e);
        }
    }

    function reject(reason) {
        state = "rejected";
        value = reason;

        if (deferred) {
            handle(deferred);
        }
    }
    
    function handle(handler) {
        if (state === "pending") {
            deferred = handler;
            return;
        }

        var handlerCallback;
        if (state === "resolved") {
            handlerCallback = handler.onResolved;
        } else {
            handlerCallback = handler.onRejected;
        }

        if (!handlerCallback) {
            if (state === "resolved") {
                handler.resolve(value);
            } else {
                handler.reject(value);
            }
            return;
        }

        var ret;
        try {
            ret = handlerCallback(value);
        } catch(e) {
            handler.reject(e);
            return;
        }
        handler.resolve(ret);
    }

    this.then = function (onResolved, onRejected) {
        return new Promise(function (resolve, reject) {
            handle({
                onResolved: onResolved,
                onRejected: onRejected,
                resolve: resolve,
                reject: reject
            });
        });
    };

    fn(resolve, reject);
};