The future

Once your codebase is using Promises you can start using co-routines, if you are using using a version of Node that supports generators. Read more about generators on promisejs.org or on MDN. Co-routines are an elegant way of handling callbacks using generators.

Generators are a feature of ES6/ES2015: the new version of JavaScript. Generators allow JavaScript code to block and are supported from NodeJS 4.0 onwards.

You can install NodeJS 4.0 using nvm, the Node Version Management utility.

To use co-routines look at the co npm module or bluebirds co-routine support.

This example using Promises

userService
    .findAllUsers()
    .then(function(users){
        res.render('users', users);
    })
    .catch(function(err){
        next(err);
    });

can refactored to use co-routines using co like this:

co(function* (){
    try{
        var users = yield userService.findAllUsers();
        res.render('users', user)
    }
    catch(err){
        next(err);
    }
});

Even cleaner database connections using Co-routines

Using the connection-provider module and co-routines together results in readable code. This almost doesn’t look asynchronous, but be careful as it is still asynchronous code.

exports.getProduct = function(req, res, next){

    co(function* (){
        var services = yield req.getServices();
        var productId = req.params.productId;
        var productsDataService = services.productDataService;
        var product = yield productsDataService.getProduct(productId);
        try{
            if (products && products.length > 0){
                res.render('product', {products : product});
            }
            else{
                res.render('product', {error : 'Product not found.'})
            }
        }
        catch(err){
            res.render('product', {error : err});
        }
    });

};

Multiple callbacks

Co-routines makes calling multiple asynchronous methods easier.

Call multiple asynchronous methods like this:

co(function* (){
    try{
        var users = userService.findAllUsers();
        var categories = userService.findAllCategories();

        var results = yield [users, categories];
        //[[user data], [category data]]
    }
    catch(err){
        next(err);
    }
});