Question

I would like to write a constructor function for some work I’m doing in node.js. Amongst other things, I’d like to log the number of instances it creates using an event emitter. At the moment I do this by giving the constructor object a method which is an event emitter. For example:

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

function MyObject() {
    ...
    MyObject.events.emit('createdMyObject')
}

MyObject.events = new EventEmitter();

/* somewhere else... */

MyObject.events.on('createdMyObject', function () {
    // Do something when a new instance is created.
});

What I’d really prefer is for the constructor to more directly emit events, so I could listen for them with something like:

MyObject.on('createdMyObject', function () {
    // Do something when a new instance is created.
});

Is there a way to do this?

Please note, I don’t need the instances to be event emitters (there are lots of examples of this when you google it). I want something equivalent to a class method in the parlance of some other OO languages.

Answer 1

This is solved using the non-standard (yet apparently quite commonly used) __proto__ property of the constructor. For example:

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

MyObj = function (a) {
    this.a = a;
    MyObj.emit('new')
}

MyObj.__proto__ = EventEmitter.prototype;

MyObj.on('new', function () {
    console.log('A new myObj was constructed.');
});

var myObj = new MyObj(1);

The expected { a: 1 } object is created, and the creation event triggers the console.log! If anyone can provide an alternative route to this behaviour using something like Object.create, then that is certainly preferable for compliance reasons.