The network

I/O in the browser is mercilessly hobbled, for very good reasons - if the JavaScript on any given website could access your filesystem, for instance, users could only click links to new sites they trusted, rather than ones they simply wanted to try out. Keeping pages in a limited sandbox, the design of the web made navigating from thing1.com to thing2.com not have the consequences of double-clicking thing1.exe and thing2.exe.

Node, of course, recasts JavaScript in the role of a systems language, giving it direct and unfettered access to operating system kernel objects such as files, sockets, and processes. This lets Node create scalable systems with high I/O requirements. It's likely the first thing you coded in Node was a HTTP server.

Node supports standard network protocols in addition to HTTP, such as TLS/SSL, and UDP. With these tools we can easily build scalable network programs, moving far beyond the comparatively limited AJAX solutions JavaScript developers know from the browser.

Let's write a simple program that sends a UDP packet to another node:

const dgram = require('dgram');
let client = dgram.createSocket("udp4");
let server = dgram.createSocket("udp4");
let message = process.argv[2] || "message";
message = Buffer.from(message);
server
.on('message', msg => {
  process.stdout.write(`Got message: ${msg}\n`);
  process.exit();
})
.bind(41234);
client.send(message, 0, message.length, 41234, "localhost");

Go ahead and open two terminal windows and navigate each to your code bundle for Chapter 8, Scaling Your Application, under the /udp folder. We're now going to run a UDP server in one window, and a UDP client in another.

In the right window, run receive.js with a command like the following:

$ node receive.js

On the left, run send.js with a command, as follows:

$ node send.js

Executing that command will cause the message to appear on the right:

$ node receive.js
Message received!

A UDP server is an instance of EventEmitter, emitting a message event when messages are received on the bound port. With Node, you can use JavaScript to write your application at the I/O level, moving packets and streams of binary data with ease.

Let's continue to explore I/O, the process object, and events. First, let's dig into the machine powering Node's core, V8.