Chev Chev - 1 year ago 120
Node.js Question

How should you inherit from EventEmitter in node?

I was reading over this small article to understand inheriting from

, but I'm a little confused.

He does this:

function Door() {; = function() {
Door.prototype.__proto__ = events.EventEmitter.prototype;

Why does he manually invoke the EventEmitter constructor with his own constructor's
? Also, why does he set the prototype of his contsructor's prototype to the prototype of
? That's super confusing to me.

Then someone in the comments suggested he do this instead, which seemed more elegant:

function Door() {; = function () {
util.inherits(Door, events.EventEmitter);

This seems WAY cleaner than the other way, though that's probably just because I fail to understand what's going on in the first instance. I would not be surprised if
is doing the same thing as the first example.

The second one at least makes a little sense to me, but I still don't understand why they don't just do this:

function Door() { = function () {
Door.prototype = new events.EventEmitter();

Can anyone explain to me what the differences between all of these approaches is and why in the first two they invoke
on the
constructor? I omitted that line while trying out the examples and they still worked.

Answer Source

The third example is not generally correct: that creates one single EventEmitter instance for all door instances.

Let's imagine a simple case:

var Foo = function() {
    // each Foo instance has a unique id = Math.random();
Foo.prototype.doFoo = function() { console.log("Foo!"); }

Suppose we want to create a Bar constructor that inherits from Foo and adds some new properties. If you follow your final example:

var Bar = function() {
    this.something = 5;
Bar.prototype = new Foo();

This is wrong because all Bar instance will have the same id property. Instead, we must call the parent constructor for each instance:

var Bar = function() {;  // set unique `id` on `this`
    this.something = 5;
Bar.prototype = Object.create(Foo.prototype);

Note that the final line here is the same as Bar.prototype.__proto__ = Foo.prototype; because Object.create creates a new object whose __proto__ is set to the Object.create argument.

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