Promises (Q)

callbacksBad.js

function getAllUserDetails(id, cb){
    database.findUserByID(id, function(err,user){
        if(err) return cb(err);
        var roles,friends;
        user.getRoles(function(err,res){
            if(err) return cb(err);
            roles = res;
            next();
        });
        user.getFriends(function(err,res){
            if(err) return cb(err);
            friends = res;
            next();
        });
        function next(){
            if(roles&&friends){
                cb(null, {user:user, roles:roles, friends:friends});
            }
        }
    });
}

callbacksGood.js

database.findUserByID(1, function(err,user){
    if(err) return retry() || throw err;
    alert("hello " + user.name);
});

callpromised.js

getAllUserDetails(1).then(function(details){
    //this is where you can use the return value
    //of the function (details)
},function(err){
    //if there is an error, it can be caught in
    //this optional function
    //If you want to re-throw the error you should
    //Use:
    return Q.reject(err);
}).end();//always call end unless you're returning a promise.

events.js

$("#myButton").click(function(){
    alert("My button was clicked");
});

maybepromised.js

/**
 * Wrap a function so that it may be called by someone who uses 
 * promises, or someone who prefers to stick to plain callbacks
 * 
 * You can even write your function either way round too.  The 
 * only requirement is that you can't take a function as the
 * last argument to your function, other than a callback.
 * 
 * @param  {Function} fn       The function to wrap
 * @param  {Boolean}  promised Is the function being wrapped promised
 * @return {Function}          A wrapped function
 */
function(fn,promised){
    return function(){
        var args = Array.prototype.slice.call(arguments);
        if(promised){
            if(typeof args[args.length-1] === 'function'){
                var cb = args.pop();
                fn.apply(this, args).then(function(res){
                    cb(null,res);
                },function(err){
                    cb(err);
                }).end();
            } else {
                return fn.apply(this, args);
            }
        } else {
            if(typeof args[args.length-1] === 'function'){
                fn.apply(this, args);
            } else {
                var res = Q.defer();
                args.push(res.makeNodeResolver());
                fn.apply(this, args);
                return res.promise;
            }
        }
    }
}

noyield.js

var getAllUserDetails = function (id){

    return database.findUserByID(id)
        .then(function(user){

    var rolesP = user.getRoles();
    var friendsP = user.getFriends();

    return rolesP.then(function(roles){
        return friendsP.then(function(friends){
        
            return {user:user, roles:roles, friends:friends};
            });//friendsP
        });//rolesP
    });//userP
}

promised.js

var getAllUserDetails = function (id){

    return database.findUserByID(id)
        .then(function(user){

    return Q.all(user.getRoles(), user.getFriends()).spread(function(roles,friends){
        return {user:user, roles:roles, friends:friends};
    });//rolesP and friendsP

    });//userP
}

promisedtocb.js

//promised to cb
function getAllUserDetailsCB(id, cb){
    getAllUserDetails(id).then(function(details){
        cb(null, details);
    },function(err){
        cb(err);
    }).end();
}

//cb to promised
function getAllUserDetails(id){
    var res = Q.defer();
    getAllUserDetailsCB(id,function(err,data){
        if(err)res.reject(err);
        else res.resolve(data);
    });
    return res.promise;
}

promisedtocbgeneral.js

function(fn){
    return function (){
        var cb = arguments.pop();
        fn.apply(this, arguments).then(function(res){
            cb(null, res);
        },function(err){
            cb(err);
        }).end();
    }
}

yield.js

var getAllUserDetails = Q.async(function (id){

    var user = yield database.findUserByID(id);

    var rolesP = user.getRoles();
    var friendsP = user.getFriends();

    var roles = yield rolesP;
    var friends = yield friendsP;

    Q.return({user:user, roles:roles, friends:friends});
});