Making object instances observable in Node.js

Question

Background

Consider the following:

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

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

util.inherits(MyClass, EventEmitter);

MyClass.prototype = {
  method1: function() {
    this.emit('method1');
  }
}

module.exports = MyClass;

The goal here is to require this module and create instances that are all 'observable' in that their events can pass data to other objects that will handle these events.

I use this module like so:

var MyClass = require('./my-module-index');
var instance = new MyClass();
instance.method1();

Here's the catch:

Calling method1 throws the error:

TypeError: undefined is not a function

whose stack trace points to the emit call.

Question

Should this be a singleton class to have the Observable behavior as implemented or is there something subtle to get instances to emit events that can be handled elsewhere?

I would normally think this was a scoping issue but since the method is on the Object's prototype, this should be referring to any instance (provided new is used).

mbcrute · Accepted Answer · 2015-03-11 13:32:19Z

3

You're completely blowing away the prototype for MyClass by assigning it to a brand new object so no wonder emit is undefined. Try this instead:

MyClass.prototype.method1 = function() {
  this.emit('method1');
};

That should work.

Here is a good write up on MDN about the prototype chain and how method lookup / inheritance works in JavaScript.

answered Mar 11, 2015 at 13:32

mbcrute

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1 Comment

theWanderer4865 Over a year ago

That's what I was thinking! I'm trying it out right now.

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