How to implement inheritance in Node.JS

Although there are various ways of performing inheritance and OO in javascript, in Node.js you would typically use the built in util.inherits function to create a constructor which inherits from another.

See http://book.mixu.net/ch6.html for a good discussion on this subject.

for example:

var util = require("util");
var events = require("events");

function MyOwnClass() {
    // ... your code.
}

util.inherits(MyOwnClass, events.EventEmitter);

Creating an object constructor in pure JS:

They're just functions like any other JS function but invoked with the new keyword.

function Constructor(){ //constructors are typically capitalized

    this.public = function(){ alert(private); }

    var private = "Untouchable outside of this func scope.";

}

Constructor.static = function(){ alert('Callable as "Constructor.static()"'); }

var instance = new Constructor();

Inheritance:

function SubConstructor(){
    this.anotherMethod(){ alert('nothing special'); }
}

function SubConstructor.prototype = new Constructor();

var instance = new SubConstructor();

instance.public(); //alerts that private string

The key difference is that prototypal inheritance comes from objects, rather than the things that build them.

One disadvantage is that there's no pretty way to write something that makes inheritance of instance vars like private possible.

The whopping gigantor mega-advantage, however, is that we can mess with the prototype without impacting the super constructor, changing a method or property for every object even after they've been built. This is rarely done in practice in higher-level code since it would make for an awfully confusing API but it can be handy for under-the-hood type stuff where you might want to share a changing value across a set of instances without just making it global.

The reason we get this post-instantiated behavior is because JS inheritance actually operates on a lookup process where any method call runs up the chain of instances and their constructor prototype properties until it finds the method called or quits. This can actually get slow if you go absolutely insane with cascading inheritance (which is widely regarded as an anti-pattern anyway).

I don't actually hit prototype specifically for inheritacne a lot myself, instead preferring to build up objects via a more composited approach but it's very handy when you need it and offers a lot of less obvious utility. For instance when you have an object that would be useful to you if only one property were different, but you don't want to touch the original.

var originInstance = {
    originValue:'only on origin',
    theOneProperty:'that would make this old object useful if it were different'
}

function Pseudoclone(){
    this.theOneProperty = "which is now this value";
}
Pseudoclone.prototype = originInstance;

var newInstance = new Psuedoclone();
//accesses originInstance.originValue but its own theOneProperty

There are more modern convenience methods like Object.create but only function constructors give you the option to encapsulate private/instance vars so I tend to favor them since 9 times out of 10 anything not requiring encapsulation will just be an object literal anyway.

Overriding and Call Object Order:

( function Constructor(){
    var private = "public referencing private";
    this.myMethod = function(){ alert(private); }
} ).prototype = { myMethod:function(){ alert('prototype'); };

var instance = new Constructor();

instance.myMethod = function(){ alert(private); }

instance.myMethod();//"undefined"

Note: the parens around the constructor allow it to be defined and evaluated in one spot so I could treat it like an object on the same line.

myMethod is alerting "undefined" because an externally overwritten method is defined outside of the constructor's closure which is what effective makes internal vars private-like. So you can replace the method but you won't have access to what it did.

Now let's do some commenting.

( function Constructor(){
    var private = "public referencing private";
    this.myMethod = function(){ alert(private); }
} ).prototype = { myMethod:function(){ alert('prototype'); };

var instance = new Constructor();

//instance.myMethod = function(){ alert(private); }

instance.myMethod();//"public referencing private"

and...

( function Constructor(){
    var private = "public referencing private";
    //this.myMethod = function(){ alert(private); }
} ).prototype = { myMethod:function(){ alert('prototype'); };

var instance = new Constructor();

//instance.myMethod = function(){ alert(private); }

instance.myMethod();//"prototype"

Note that prototype methods also don't have access to that internal private var for the same reason. It's all about whether something was defined in the constructor itself. Note that params passed to the constructor will also effectively be private instance vars which can be handy for doing things like overriding a set of default options.

Couple More Details

It's actually not necessary to use parens when invoking with new unless you have required parameters but I tend to leave them in out of habit (it works to think of them as functions that fire and then leave an object representing the scope of that firing behind) and figured it would be less alien to a Java dev than new Constructor;

Also, with any constructor that requires params, I like to add default values internally with something like:

var param = param || '';

That way you can pass the constructor into convenience methods like Node's util.inherit without undefined values breaking things for you.

Params are also effectively private persistent instance vars just like any var defined in a constructor.

Oh and object literals (objects defined with { key:'value' }) are probably best thought of as roughly equivalent to this:

var instance = new Object();
instance.key = 'value';

With a little help from Coffeescript, we can achieve it much easier.

For e.g.: to extend a class:

class Animal
  constructor: (@name) ->

  alive: ->
    false

class Parrot extends Animal
  constructor: ->
    super("Parrot")

  dead: ->
    not @alive()

---

Static property:

class Animal
  @find: (name) ->

Animal.find("Parrot")

---

Instance property:

class Animal
  price: 5

  sell: (customer) ->

animal = new Animal
animal.sell(new Customer)

I just take the sample code Classes in CoffeeScript. You can learn more about CoffeeScript at its official site: http://coffeescript.org/