NODEJS PATTERNS

Design patterns we are going to investigate are:

• Dependency Injection
• Factories
• Middlewares
• Observers
• Singlestons
• Streams

Dependency Injection

Dependency injection is a software design pattern in which one or more dependencies (or services) are injected, or passed by reference, into a dependent object.

Let’s look at an example where we pass a database as a dependancy.

function employeeModel(opt) {
  var db;

  if (!opt.db) {
    throw new Error('opt.db is not defined');
  }

  db = opt.db;

  return {
    create: function (done) {
      db.query('SELECT ...', done);
    },
  };
}

module.exports = employeeModel;

Factories

This pattern provides us with a generic interface for creating object, so we don’t have to worry about the nitty-gritty of the complex constructors.

function Employee(opt) {
  this.opt = opt;
}

function create(opt) {
  // modify the options here
  return new Employee(opt);
}

module.exports.create = create;

Middlewares

Middlewares (or pipelines) are an essential part of Express, where the output of one unit is the input of the next.

Let’s look at an example

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

app.get('/', function (req, res) {
  res.send('Hello World!');
});

app.listen(3000);

Observers

The observers notify the owner object on state changes automatically. In Node.js we can use EventEmitter to implement this pattern.

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

function EmployeeObservable() {
  EventEmitter.call(this);
}

util.inherits(EmployeeObservable, EventEmitter);

Now we can use this observable by adding some functionality to it.

EmployeeObservable.prototype.login = function (name) {
  this.emit('Logged in as ', name);
};

Now our observable can emit event.

var EmployeeObservable = require('EmployeeObservable');
var observable = new EmployeeObservable();

observable.on('Logged in as ', function (name) {
  console.log(name);
});

observable.login('Sam');

Singlestons

By using singleton pattern we are making sure that the number of instances of a class is restricted to one.

var PI = Math.PI;

function circle(radius) {
  return radius * radius * PI;
}

module.exports.circle = circle;

Now let’s use this module to instantiate an object.

var area = require('./area');

console.log(area.circle(5));

Streams

Stream are like special pipelines, designed for complex operations like processing large amounts of flowing data.

process.stdin.on('readable', function () {
  var buffer = process.stdin.read(5);
  console.dir(buffer);
  process.stdin.read(0);
});

The console output will be like:

(echo abc; sleep 1; echo def; sleep 1; echo ghi) | node consume2.js

Output:

<Buffer 61 62 63>
<Buffer 0a 64 65>
<Buffer 66 0a 67>
<Buffer 68 69 0a>

Conclusion

It is totally up to you when to use patterns in Node.js, but having this knowledge in your toolbox will come handy.