Node.js Web Development - Fourth Edition

Node.js is a platform for writing JavaScript applications outside web browsers. This is not the JavaScript we are familiar with in web browsers! For example, there is no DOM built into Node.js, nor any other browser capability.

Beyond its native ability to execute JavaScript, the bundled modules provide capabilities of this sort:

The network layer of Node.js is low level while being simple to use. For example, the HTTP modules allow you to write an HTTP server (or client) using a few lines of code. This is powerful, but it puts you, the programmer, very close to the protocol requests and makes you implement precisely those HTTP headers that you should return in request responses.

Typical web application developers don't need to work at a low level of the HTTP or other protocols. Instead, we tend to be more productive, working with higher-level interfaces. For example, PHP coders assume that Apache (or other HTTP servers) is already there providing the HTTP protocol, and that they don't have to implement the HTTP server portion of the stack. By contrast, a Node.js programmer does implement an HTTP server to which their application code is attached.

To simplify the situation, the Node.js community has several web application frameworks, such as Express, providing the higher-level interfaces required by typical programmers. You can quickly configure an HTTP server with baked-in capabilities such as sessions, cookies, serving static files, and logging, letting developers focus on their business logic. Other frameworks provide OAuth 2 support, or focus on REST APIs, and so on.

Node.js is not limited to web service application development. The community around Node.js has taken it in many other directions,

Build tools: Node.js has become a popular choice for developing command-line tools used in software development, or communicating with service infrastructure. Grunt and Gulp are widely used by frontend developers to build assets for websites. Babel is widely used for transpiling modern ES-2016 code to run on older browsers. Popular CSS optimizers and processors, such as PostCSS, are written in Node.js. Static website generation systems such as Metalsmith, Punch, and AkashaCMS, run at the command line and generate website content that you upload to a web server.

Web UI testing: Puppeteer gives you control over a headless-Chrome web browser instance. With it, you can develop Node.js scripts controlling a modern full-featured web browser. Typical use cases involve web scraping and testing web applications.

Desktop applications: Both Electron and node-webkit (NW.js) are frameworks for developing desktop applications for Windows, macOS, and Linux. These frameworks utilize a large chunk of Chrome, wrapped by Node.js libraries, to develop desktop applications using web UI technologies. Applications are written with modern HTML5, CSS3, and JavaScript, and can utilize leading-edge web frameworks, such as Bootstrap, React, or AngularJS. Many popular applications have been built using Electron, including the Slack desktop client application, the Atom and Microsoft Visual Code programming editors, the Postman REST client, the GitKraken GIT client, and Etcher, which makes it incredibly easy to burn OS images to flash drives to run on single-board computers.

Mobile applications: The Node.js for Mobile Systems project lets you develop smartphone or tablet computer applications using Node.js, for both iOS and Android. Apple's App Store rules preclude incorporating a JavaScript engine with JIT capabilities, meaning that normal Node.js cannot be used in an iOS application. For iOS application development, the project uses Node.js-on-ChakraCore to skirt around the App Store rules. For Android application development the project uses regular Node.js on Android. At the time of writing, the project is in an early stage of development, but it looks promising.

Internet of Things (IoT): Reportedly, it is a very popular language for Internet-of-Things projects, and Node.js does run on most ARM-based single-board computers. The clearest example is the NodeRED project. It offers a graphical programming environment, letting you draw programs by connecting blocks together. It features hardware-oriented input and output mechanisms, for example, to interact with General Purpose I/O (GPIO) pins on Raspberry Pi or Beaglebone single-board computers.

Among the many available web application development platforms, why should you choose Node.js? There are many stacks to choose from; what is it about Node.js that makes it rise above the others? We will see in the following sections.

Node.js's blistering performance is said to be because of its asynchronous event-driven architecture, and its use of the V8 JavaScript engine. That's a nice thing to say, but what's the rationale for the statement?

The V8 JavaScript engine is among the fastest JavaScript implementations. As a result, Chrome is widely used not just to view website content, but to run complex applications. Examples include Gmail, the Google GSuite applications (Docs, Slides, and so on), image editors such as Pixlr, and drawing applications such as draw.io and Canva. Both Atom and Microsoft's Visual Studio Code are excellent IDE's that just happen to be implemented in Node.js and Chrome using Electron. That these applications exist and are happily used by a large number of people is testament to V8's performance. Node.js benefits from V8 performance improvements. 

The normal application server model uses blocking I/O to retrieve data, and it uses threads for concurrency. Blocking I/O causes threads to wait on results.  That causes a churn between threads as the application server starts and stops the threads to handle requests. Each suspended thread (typically waiting on an I/O operation to finish) consumes a full stack trace of memory, increasing memory consumption overhead. Threads add complexity to the application server as well as server overhead.

Node.js has a single execution thread with no waiting on I/O or context switching. Instead, there is an event loop looking for events and dispatching them to handler functions. The paradigm is that any operation that would block or otherwise take time to complete must use the asynchronous model. These functions are to be given an anonymous function to act as a handler callback, or else (with the advent of ES2015 promises), the function would return a Promise. The handler function, or Promise, is invoked when the operation is complete. In the meantime, control returns to the event loop, which continues dispatching events.

At the Node.js interactive conference in 2017, IBM's Chris Bailey made a case for Node.js being an excellent choice for highly scalable microservices. Key performance characteristics are I/O performance, measured in transactions per second, startup time, because that limits how quickly your service can scale up to meet demand, and memory footprint, because that determines how many application instances can be deployed per server. Node.js excels on all those measures; with every subsequent release each, is either improving or remaining fairly steady. Bailey presented figures comparing Node.js to a similar benchmark written in Spring Boot showing Node.js to perform much better. To view his talk, see https://www.youtube.com/watch?v=Fbhhc4jtGW4.

To help us wrap our heads around why this would be, let's return to Ryan Dahl, the creator of Node.js, and the key inspiration leading him to create Node.js. In his Cinco de NodeJS presentation in May 2010, https://www.youtube.com/watch?v=M-sc73Y-zQA, Dahl asked us what happens while executing a line of code such as this:

result = query('SELECT * from db'); 
// operate on the result 

Of course, the program pauses at that point while the database layer sends the query to the database, which determines the result and returns the data. Depending on the query, that pause can be quite long; well, a few milliseconds, which is an eon in computer time. This pause is bad because that execution thread can do nothing while waiting for the result to arrive. If your software is running on a single-threaded platform, the entire server would be blocked and unresponsive. If instead, your application is running on a thread-based server platform, a thread context switch is required to satisfy any other requests that arrive. The greater the number of outstanding connections to the server, the greater the number of thread context switches. Context switching is not free because more threads require more memory per thread state and more time for the CPU to spend on thread management overhead.

Simply using an asynchronous, event-driven I/O, Node.js removes most of this overhead while introducing very little of its own.

Using threads to implement concurrency often comes with admonitions such as these: expensive and error-prone, the error-prone synchronization primitives of Java, or designing concurrent software can be complex and error prone. The complexity comes from the access to shared variables and various strategies to avoid deadlock and competition between threads. The synchronization primitives of Java are an example of such a strategy, and obviously many programmers find them difficult to use. There's the tendency to create frameworks such as java.util.concurrent to tame the complexity of threaded concurrency, but some might argue that papering over complexity does not make things simpler. 

Node.js asks us to think differently about concurrency. Callbacks fired asynchronously from an event loop are a much simpler concurrency model—simpler to understand, simpler to implement, simpler to reason about, and simpler to debug and maintain. 

Ryan Dahl points to the relative access time of objects to understand the need for asynchronous I/O. Objects in memory are more quickly accessed (in the order of nanoseconds) than objects on disk or objects retrieved over the network (milliseconds or seconds). The longer access time for external objects is measured in zillions of clock cycles, which can be an eternity when your customer is sitting at their web browser ready to move on if it takes longer than two seconds to load the page.

In Node.js, the query discussed previously will read as follows:

query('SELECT * from db', function (err, result) { 
    if (err) throw err; // handle errors 
    // operate on result 
}); 

The programmer supplies a function that is called (hence the name callback function) when the result (or error) is available. Instead of a thread context switch, this code returns almost immediately to the event loop. That event loop is free to handle other requests. The Node.js runtime keeps track of the stack context leading to this callback function, and eventually an event will fire causing this callback function to be called.

Advances in the JavaScript language are giving us new options to implement this idea. The equivalent code looks like so when used with ES2015 Promise's:

query('SELECT * from db') 
.then(result => { 
    // operate on result 
}) 
.catch(err => { 
    // handle errors 
}); 

The following with an ES-2017 async function:

try {
    var result = await query('SELECT * from db');
    // operate on result
} catch (err) {
    // handle errors
}

All three of these code snippets perform the same query written earlier. The difference is that the query does not block the execution thread, because control passes back to the event loop. By returning almost immediately to the event loop, it is free to service other requests. Eventually, one of those events will be the response to the query shown previously, which will invoke the callback function.

With the callback or Promise approach, the result is not returned as the result of the function call, but is provided to a callback function that will be called later. The order of execution is not one line after another, as it is in synchronous programming languages. Instead, the order of execution is determined by the order of the callback function execution.

When using an async function, the coding style LOOKS like the original synchronous code example. The result is returned as the result of the function call, and errors are handled in a natural manner using try/catch. The await keyword integrates asynchronous results handling without blocking the execution thread. A lot is buried under the covers of the async/await feature, and we'll be covering this model extensively throughout the book.

Commonly, web pages bring together data from dozens of sources. Each one has a query and response as discussed earlier. Using asynchronous queries, each query can happen in parallel, where the page construction function can fire off dozens of queries—no waiting, each with their own callback—and then go back to the event loop, invoking the callbacks as each is done. Because it's in parallel, the data can be collected much more quickly than if these queries were done synchronously one at a time. Now, the reader on the web browser is happier because the page loads more quickly.

var http = require('http'); 
http.createServer(function (req, res) { 
  res.writeHead(200, {'Content-Type': 'text/plain'}); 
  res.end('Hello World\n'); 
}).listen(8124, "127.0.0.1"); 
console.log('Server running at http://127.0.0.1:8124/'); 

const fibonacci = exports.fibonacci = function(n) { 
    if (n === 1 || n === 2) return 1; 
    else return fibonacci(n-1) + fibonacci(n-2); 
}

const http = require('http'); 
const url  = require('url'); 
 
const fibonacci = // as above 
 
http.createServer(function (req, res) { 
  const urlP = url.parse(req.url, true); 
  let fibo; 
  res.writeHead(200, {'Content-Type': 'text/plain'}); 
  if (urlP.query['n']) { 
    fibo = fibonacci(urlP.query['n']); 
    res.end('Fibonacci '+ urlP.query['n'] +'='+ fibo); 
  } else { 
    res.end('USAGE: http://127.0.0.1:8124?n=## where ## is the Fibonacci number desired'); 
  } 
}).listen(8124, '127.0.0.1'); 
console.log('Server running at http://127.0.0.1:8124'); 

const fibonacciAsync = function(n, done) { 
    if (n === 0) return 0;
    else if (n === 1 || n === 2) done(1); 
    else if (n === 3) return 2;
    else { 
        process.nextTick(function() { 
            fibonacciAsync(n-1, function(val1) { 
                process.nextTick(function() { 
                    fibonacciAsync(n-2, function(val2) {
                    done(val1+val2); }); 
                }); 
            }); 
        }); 
    } 
} 

const http = require('http'); 
const url  = require('url'); 
 
const fibonacciAsync = // as above 
 
http.createServer(function (req, res) { 
  let urlP = url.parse(req.url, true);
  res.writeHead(200, {'Content-Type': 'text/plain'}); 
  if (urlP.query['n']) { 
    fibonacciAsync(urlP.query['n'], fibo => {
        res.end('Fibonacci '+ urlP.query['n'] +'='+ fibo);
    });
  } else { 
    res.end('USAGE: http://127.0.0.1:8124?n=## where ## is the Fibonacci number desired');
  }
}).listen(8124, '127.0.0.1'); console.log('Server running at http://127.0.0.1:8124');

The last couple of years have been an exciting time for JavaScript programmers. The TC-39 committee that oversees the ECMAScript standard has added many new features, some of which are syntactic sugar, but several of which have propelled us into a whole new era of JavaScript programming. By itself, the async/await feature promises us a way out of what's called Callback Hell, or the situation we find ourselves in when nesting callbacks within callbacks. It's such an important feature that it should necessitate a broad rethinking of the prevailing callback-oriented paradigm in Node.js and the rest of the JavaScript ecosystem.

Refer back a few pages to this:

query('SELECT * from db', function (err, result) { 
    if (err) throw err; // handle errors 
    // operate on result 
});

This was an important insight on Ryan Dahl's part, and is what propelled Node.js's popularity. Certain actions take a long time to run, such as database queries, and should not be treated the same as operations that quickly retrieve data from memory. Because of the nature of the JavaScript language, Node.js had to express this asynchronous coding construct in an unnatural way. The results do not appear at the next line of code, but instead appear within this callback function. Further, errors have to be handled in an unnatural way, inside that callback function. 

The convention in Node.js is that the first parameter to a callback function is an error indicator, and the subsequent parameters are the results. This is a useful convention that you'll find all across the Node.js landscape. However, it complicates working with results and errors because both land in an inconvenient location — that callback function. The natural place for errors and results to land is on the subsequent line(s) of code.

We descend further into callback hell with each layer of callback function nesting. The seventh layer of callback nesting is more complex than the sixth layer of callback nesting. Why? If nothing else, it's that the special considerations for error handling become ever more complex as callbacks are nested more deeply.

var results = await query('SELECT * from db');

Instead, ES2017 async functions return us to this very natural expression of programming intent. Results and errors land in the correct location, while preserving the excellent event-driven asynchronous programming model that made Node.js great. We'll see later in the book how this works.

The TC-39 committee added many more new features to JavaScript, such as:

You may see the new JavaScript described as ES6 or ES2017. What's the preferred name to describe the version of JavaScript that is being used?

ES1 through ES5 marked various phases of JavaScript's development. ES5 was released in 2009, and is widely implemented in modern browsers. Starting with ES6, the TC-39 committee decided to change the naming convention because of their intention to add new language features every year. Therefore, the language version name now includes the year, hence ES2015 was released in 2015, ES2016 was released in 2016, and ES2017 was released in 2017.

New capabilities, such as cloud deployment systems and Docker, make it possible to implement a new kind of service architecture. Docker makes it possible to define server process configuration in a repeatable container that's easy to deploy by the millions into a cloud hosting system. It lends itself best to small single-purpose service instances that can be connected together to make a complete system. Docker isn't the only tool to help simplify cloud deployments; however, its features are well attuned to modern application deployment needs.

Some have popularized the microservice concept as a way to describe this kind of system. According to the microservices.io website, a microservice consists of a set of narrowly focused, independently deployable services. They contrast this with the monolithic application deployment pattern where every aspect of the system is integrated into one bundle (such as a single WAR file for a Java EE app server). The microservice model gives developers much needed flexibility.

Some advantages of microservices are as follows:

Where does Node.js fit in with this? Its design fits the microservice model like a glove:

Throughout this book, we'll call out aspects of the Twelve-Factor App model, and ways to implement those ideas in Node.js. This model is published on http://12factor.net, and is a set of guidelines for application deployment in the modern cloud computing era. It's not that the  Twelve-Factor App model is the be-all and end-all of application architecture paradigms. It's a set of useful ideas, clearly birthed after many late nights spent debugging complex applications, which offer useful ideas that could save us all a lot of effort by having easier-to-maintain and more reliable systems.

The guidelines are straightforward, and once you read them, they will seem like pure common sense. As a best practice, the Twelve-Factor App model is a compelling strategy for delivering the kind of fluid self-contained cloud-deployed applications called for by our current computing environment.

You learned a lot in this chapter. Specifically, you saw that JavaScript has a life outside web browsers and you learned about the difference between asynchronous and blocking I/O. We then covered the attributes of Node.js and where it fits in the overall web application platform market and threaded versus asynchronous software. Lastly, we saw the advantages of fast event-driven asynchronous I/O, coupled with a language with great support for anonymous closures.

Our focus in this book is real-world considerations of developing and deploying Node.js applications. We'll cover as many aspects as we can of developing, refining, testing, and deploying Node.js applications.

Now that we've had this introduction to Node.js, we're ready to dive in and start using it. In Chapter 2, Setting up Node.js, we'll go over setting up a Node.js environment, so let's get started.