Adding event listener on instance, not function

Question

See example below. I am trying to trigger an event on a.

var events = require("events");

function foo() {
    this.on("ping", function() {
        console.log("bar!");
    })
}

foo.prototype = new events.EventEmitter();
var a = new foo();
var b = new foo();
a.emit("ping");

This prints "bar!" two times, so I am assuming I am adding the event listener on "all" the functions, not the specific instance. Since I am running foo two times, I am adding two event listeners, nothing strange really, althought not so intuitive.

Is there a way to add the event listener only on a?

(Please edit this question if I am using the wrong terminology, used to class-based programming)

edit: So I suspect it is because I am using the same event emitter as prototype.. But in that case, how can I create one for each new foo()?

Maybe this article on the EventEmitter will help you?

Brendan
– Brendan

2014-10-18 22:08:54 +00:00
Commented Oct 18, 2014 at 22:08 — Brendan
– Brendan, Commented Oct 18, 2014 at 22:08

mscdex · Accepted Answer · 2014-10-18 23:24:26Z

1

Typically you inherit (from EventEmitter) like this:

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

function Foo() {
  if (!(this instanceof Foo))
    return new Foo();

  EventEmitter.call(this);

  this.on('ping', function() {
    console.log('bar!');
  });
}
inherits(Foo, EventEmitter);

// add Foo-specific prototype functions *after* `inherits()`
Foo.prototype.myfunc = function(a, b) {
  return a + b;
});

var a = new Foo();
var b = new Foo();
a.emit('ping');

This is the setup that most modules on npm and objects in node core use to inherit from another object.

edited Oct 18, 2014 at 23:24

answered Oct 18, 2014 at 19:46

mscdex

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2 Comments

jfriend00 Over a year ago

Rather than just a block of code with no description, can you describe what is operationally different between your code and the OP's code? The OP is setting the prototype to be an EventEmitter.

mscdex Over a year ago

The OP already pointed out the main problem with their code:

So I suspect it is because I am using the same event emitter as prototype.. But in that case, how can I create one for each new foo()

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