Lets start talking about modularity, to grasp the differences between Node and code in the browser. I prepared a special something you can use as you read this article. You can find every example in this article on GitHub nicely packed for you to start playing right away.

Modularity in Node

Node implements CommonJS Modules/1.1, which allow you to keep files self-contained. You can learn all about Node Modules from their increasingly useful documentation.

Modules can expose an API through the module.exports convention.

// modules/math.js

var api = {
    sum: function(a, b){
        return a + b;
    }
};

module.exports = api;

Note the variable api won’t make it’s way to the global object. You can learn why from the docs. Globals work differently in Node. The top-level scope of a module is local to that module, but you can still access a few globals on your own, such as process, and console. Setting up your own globals on the global object is discouraged.

Consequently, module isn’t a global, but rather a local variable, private to the module we are currently working on.

Modules can be referenced using the require function. You can provide a package name (more on that later), or a physical path relative to the file you are invoking require from.

// modules/app.js

var math = require('./math.js');
var result = math.sum(1, 2);

console.log('I can\'t believe the result is:', result, '!');

Both files being on the same directory is assumed. The math variable will now equal the value of module.exports in math.js.

What does console.log do? It’s actually just syntactic sugar for process.stdout.write, and it will append a new line \n at the end. This is deliberately done all over Node to help ease your on-boarding onto the platform by leveraging the conventions and objects you are already used to from your experience in writing client-side JavaScript.

Sidebar. You might want to read the actual console API documentation.

Note that requiring a file multiple times in the same process will only execute the code in the module once. So if you were to require the app.js module several times, it would still be executed a single time. As a result, the output would only be buffered once.

// modules/several.js

require('./app.js');
require('./app.js');
require('./app.js');

Those are modules all right, but where’s the asynchronicity Node is supposedly so popular for?

Asynchronous Convention

Node is an event-based language, and most of the code written for Node follows a really simple convention that helps modules look inspiringly similar to each other, as far as coding conventions go.

Our math module would probably look more like this if we wanted to play nice with the Node community at large.

// async/math.js

var api = {
    sum: function(a, b, done){
        process.nextTick(function(){
            done(null, a + b);
        });
    }
};

module.exports = api;

process.nextTick is kind of hard to wrap our head around at first, but lets just imagine it’s setTimeout(fn,0), which we might have used while trying hacky fixes in the browser.

I’ve used process.nextTick to turn an otherwise synchronous function into an asynchronous one. When we are done processing, we’re going to pass the result as the second parameter of the done callback. The first parameter should always be err, if an error occurs, we are passing that as the first parameter, rather than throwing an exception. If no error occurs, we are fine passing any falsy value.

Consuming this module is still really easy.

// async/app.js

var math = require('./math.js');

math.sum(1, 2, function(err, result){
    if(err){
        throw err;
    }
    
    console.log('I can\'t believe the result is:', result, '!');
});

We are now waiting reactively for the sum function to let us know when it’s done. This is the basest of asynchronous examples in Node.js. Note how we changed modes, and use throw here; this is fine as long as we are in a synchronous path, throwing errors should always have the end result of process termination, so keep that in mind when you are dealing with this type of situations. This is acceptable for our console application, however in a web application we probably would prefer to just return an HTTP status code 500, internal server error, for the current request.

You’ll also have to consider the option of bubbling errors through multiple asynchronous calls. This, for example, might not be the best error-handling approach:

// async/wrong.js

var math = require('./math.js');

math.sum(1, 2, function(err, result){
    if(err){
        throw err;
    }

    math.sum(result, 3, function(err, result){
        if(err){
            throw err;
        }

        console.log('I can\'t believe the result is:', result, '!');
    });
});

A more sensible approach might be to avoid throwing errors all over the place, but handle those in a centralized location.

// async/better.js

var math = require('./math.js');

math.sum(1, 2, function(err, result){
    if(err){
        return then(err);
    }

    math.sum(result, 3, function(err, result){
        if(err){
            return then(err);
        }

        then(err, result);
    });
});

function then(err, result){
    if(err){
        throw err;
    }

    console.log('I can\'t believe the result is:', result, '!');
}

This is however, getting pretty verbose. Let me skip to a module for a bit, and then come back and explain what’s going on. We’re going to use the control flow module called async, to improve the readability of our code.

// async/right.js

var async = require('async');
var math = require('./math.js');

async.waterfall([
    function(next){
        math.sum(1, 2, next);
    },
    function(result, next){
        math.sum(result, 3, next);
    }
], then);

function then(err, result){
    if(err){
        throw err;
    }

    console.log('I can\'t believe the result is:', result, '!');
}

That’s a little better. Since we’ve been following the right conventions, we can use async, which allows us to get rid of all those pesky if(err) statements, and flatten our callback hell while we’re at it. waterfall‘s API is pretty simple, we give it an array of functions, and these will be called in series, when our first math.sum completes, it will invoke the next callback with the (null, 3) arguments. If a function returns a truthy value in the first parameter, this will shortcut the waterfall and immediatly jumping to the then function, passing the error argument, still in the first position. If no error occurs, then the next function in the sequence is executed, passing any resulting arguments to it (in this case, just the 3).

This is the recommended way of doing things because it flattens the structure of our code, turning our codebase into something more readable, while at the same time following the same conventions and using the same API that is used everywhere else. You must check out the async module and its comprehensive API, toy with it for a while.

That’s great and all, but where did async come from? It sure as hell isn’t part of Node.

I’m glad you asked.

Node Packaged Modules

npm is a small treasure that comes bundled with Node, and helps you manage dependencies in your projects. There is a huge repository you can search, and most people include installation instructions in their GitHub repositories. Ultimately, npm is a CLI (command-line interface) tool.

If you have been following the instructions of the learn-nodejs repository I provided, then you already have dependencies installed in your project folder. If not, just run the following command in your terminal.

$ npm install

That’s it, now you have everything you need. How does that work? Some weird magic? No, just package.json. This file helps us define the dependencies in our project. When you ran npm install in your terminal, all it did was install the dependencies listed in the package.json file.

{
  "name": "learn-nodejs",
  "description": "Simple NodeJS Application Examples",
  "homepage": "https://github.com/ponyfoo/learn-nodejs",
  "author": {
    "name": "Nicolas Bevacqua",
    "email": "nicolasbevacqua@gmail.com",
    "url": "http://www.ponyfoo.com"
  },
  "version": "0.0.1",
  "repository": {
    "type": "git",
    "url": "https://github.com/ponyfoo/learn-nodejs.gitt"
  },
  "dependencies": {
    "async": "~0.2.9"
  }
}

I rarely add dependencies manually to this definition file, in the case of async, for example, all I did was run the following command:

$ npm install async --save

That’s it. async has been added it to the dependencies object. Installing a module basically just fetches it, and adds it to a node_modules folder, which you should always exclude in your .gitignore settings.

If you are interested in developing your own npm module, you’ll be shocked to learn how simple that is.

Before we jump into building a decent application, lets look at one of the most powerful constructs in Node.

Events API

Yes! Of course, I was talking about the event emitter API. What are events? Well, the documentation explains it like this:

Many objects in Node emit events: a net.Server emits an event each time a peer connects to it, a fs.readStream emits an event when the file is opened. All objects which emit events are instances of events.EventEmitter. You can access this module by doing: require('events');

Functions can then be attached to objects, to be executed when an event is emitted. These functions are called listeners. Inside a listener function, this refers to the EventEmitter that the listener was attached to.

Lets write our own event emitter and explain a few things along the way. Then, we’ll see how it can be used.

// events/implementation.js

var util = require('util');
var EventEmitter = require('events').EventEmitter;

function Heartbeat(interval){
    EventEmitter.call(this);

    var emitter = this;
    var beats = 0;

    setInterval(function(){
        emitter.emit('beat', ++beats);
    }, interval);
}

util.inherits(Heartbeat, EventEmitter);

module.exports = Heartbeat;

Don’t laugh, that’s the best I could come up with. Here I’m simply creating a constructor function for my custom EventEmitter implementation. I’m using util.inherits, as it’s the recommended way of performing prototypal inheritance in Node applications.

Whenever our emitter .emits an event, all subscribers to that event will be notified, and receive the arguments which where provided when the event was emitted.

Remember what I mentioned about leveraging your API knowledge about the browser with that in Node? setInterval is one of those cases.

Fine, how do we use our newly born event emitter? It’s simple, really:

// events/usage.js

var Heartbeat = require('./implementation.js');
var a = new Heartbeat(400);
var b = new Heartbeat(1000);

a.on('beat', function(beats){
    console.log('Heart A beat n times:', beats);
});

b.on('beat', function(beats){
    console.log('Heart B beat n times:', beats);
});

I’m not even sure I need to explain this, but whenever the emitter invokes .emit, every listener for that event, added by .on, will have its callback triggered. This seemingly innocent API powers a lot of what Node does.

One last thing, read this quote from the documentation:

When an EventEmitter instance experiences an error, the typical action is to emit an 'error' event. Error events are treated as a special case in node. If there is no listener for it, then the default action is to print a stack trace and exit the program.

What this means is that if there is no .on('error', fn) listener, and your emitter emits an 'error' event, then your application will die a tragic death.

HTTP Server

Enough blabbering, here is an HTTP server in Node.

// http/server.js

var http = require('http');

http.createServer(function(req, res) {
    res.end('Hello Node', 200);
    console.log('I think I\'ve heard something!');
}).listen(8000);

console.log('Listening!');

That wasn’t so amusing, it was very simple and self-describing, though! Lets try something different, serving an HTML file from disk.

// http/html.js

var http = require('http');
var fs = require('fs');
var path = require('path');
var index = path.resolve(__dirname, './index.html');

http.createServer(function(req, res) {
    var stream = fs.createReadStream(index);

    stream.on('open', function(){
        res.writeHead(200, { 'Content-Type': 'text/html' });
    });

    stream.on('error', function(){
        res.writeHead(404);
        res.end();
    });

    stream.pipe(res);
}).listen(8000);

console.log('Listening!');

Couple of things. First of all, __dirname is a special local variable that contains the absolute path to the directory for our currently executing module. We just learned what events are, the fs.createReadStream method will provide us with an event emitter we can use to stream data to the response. The file will be piped straight into a chunked response, this can be achieved using the readable.pipe method. If the file isn’t found, the stream will emit an 'error' event; we can take advantage of that and respond with a 404 status code instead.

This is, however, a very convoluted thing to do to just serve a file. Enter Express.

Express Application Framework

Express is built on Connect, which expands on Node’s HTTP server. There’s also Socket.IO for implementing web socket communications, but I won’t be getting into realtime for now.

Connect just provides middleware, a nice abstraction over what the native HTTP module offers. Express builds on that, adding a lot of awesome features, and making your life more bearable. Here is a small sample application built on Express:

// http/express.js

var express = require('express');
var app = express();

app.get('/', function(req, res){
    res.send('hello world');
});

app.listen(8000);

The API is incredibly self-documenting, I wish more projects had an API as clean as Express does.

Enough already. You are mean for laughing at all of my stupid examples. You know what else is mean?

MongoDB, ExpressJS, AngularJS and NodeJS

The MEAN Stack is not a hipster thing, as delusional people try to assertain with no real reasoning behind their empty statements. The MEAN stack is a very real thing. Here’s a slideshare for you to look at.

• MongoDB as a database
• ExpressJS as your web framework
• AngularJS as your client-side framework
• NodeJS as the platform

The glaring benefit of using a stack such as this is the ease with which you can transfer objects through your application without having to resort to different interfaces, data presentation alternatives, and programming languages. You can really get away with just using JavaScript everywhere.

from:https://ponyfoo.com/articles/teach-yourself-nodejs-in-10-steps