Node.js Design Patterns: Master a series of patterns and techniques to create modular, scalable, and efficient applications

Losing control of asynchronous code in JavaScript is undoubtedly easy. Closures and in-place definition of anonymous functions allow a smooth programming experience that doesn't require the developer to jump to other points in the code base. This is perfectly in line with the KISS principle; it's simple, it keeps the code flowing, and we get it working in less time. Unfortunately, sacrificing qualities such as modularity, reusability, and maintainability will sooner or later lead to the uncontrolled proliferation of callback nesting, the growth in the size of functions, and will lead to poor code organization. Most of the time, creating closures is not functionally needed, so it's more a matter of discipline than a problem related to asynchronous programming. Recognizing that our code is becoming unwieldy—or even better, knowing in advance that it might become unwieldy—and then acting accordingly with the most adequate solution is what differentiates...

Now that we have met our first example of callback hell, we know what we should definitely avoid; however, that's not the only concern when writing asynchronous code. In fact, there are several situations where controlling the flow of a set of asynchronous tasks requires the use of specific patterns and techniques, especially if we are using only plain JavaScript without the aid of any external library. For example, iterating over a collection by applying an asynchronous operation in sequence is not as easy as invoking forEach() over an array, but it actually requires a technique similar to a recursion.

In this section, we will learn not only about how to avoid the callback hell but also about how to implement some of the most common control flow patterns using only simple and plain JavaScript.

If we take a look for a moment at every control flow pattern we have analyzed so far, we can see that they could be used as a base to build reusable and more generic solutions. For example, we could wrap the unlimited parallel execution algorithm into a function which accepts a list of tasks, runs them in parallel, and invokes the given callback when all of them are complete. This way of wrapping control flow algorithms into reusable functions can lead to a more declarative and expressive way to define asynchronous control flows, and that's exactly what async (https://npmjs.org/package/async) does. The async library is a very popular solution, in Node.js and JavaScript in general, to deal with asynchronous code. It offers a set of functions that greatly simplify the execution of a set of tasks in different configurations and it also provides useful helpers for dealing with collections asynchronously. Even though there are several other libraries with a similar goal, async...

We already mentioned at the beginning of the chapter that CPS is not the only way to write asynchronous code. In fact, the JavaScript ecosystem provides alternatives to the traditional callback pattern. One of these in particular is receiving a lot of momentum, especially now that it is going to be part of the ECMAScript 6 specification (also known as ES6 or Harmony), the upcoming version of the JavaScript language. We are talking, of course, about promises, and in particular about those implementations that follow the Promises/A+ specification (https://promisesaplus.com).

The ES6 specification introduces another mechanism that, besides other things, can be used to simplify the asynchronous control flow of our Node.js applications. We are talking about generators, also known as semi-coroutines. They are a generalization of subroutines, where there can be different entry points. In a normal function, in fact, we can have only one entry point, which corresponds to the invocation of the function itself. A generator is similar to a function, but in addition, it can be suspended (using the yield statement) and then resumed at a later time. Generators are particularly useful when implementing iterators, and this should ring a bell, as we have already seen how iterators can be used to implement important asynchronous control flow patterns such as sequential and limited parallel execution.

At this point of the chapter, we should have a better understanding of what options we have to tame the asynchronous nature of Node.js. Each one of the solutions presented has its own pros and cons. Let's summarize them in the following table:

At the beginning of this chapter, we said that Node.js programming can be tough because of its asynchronous nature, especially for people used to developing on other platforms. However, throughout this chapter we showed how asynchronous APIs can be bent to our will, starting with plain JavaScript, which provided us the foundation for the analysis of more sophisticated techniques. We then saw that the tools at our disposal are indeed variegated and provide good solutions to most of our problems, in addition to offering a programming style for every taste; for example, we may choose async to simplify the most common flows, or totally change paradigm by using promises with their fluent chaining and robust error management, or if we want to get fancy, we can always leverage generators and feel like we are programming with blocking APIs.

This chapter should not only have taught you how to choose between one or the other solutions but also how to use them together, even in the same project...