Nodejs EventEmitter - Define scope for listener function

Question

I'd like to have something like this work:

var Events=require('events'),
    test=new Events.EventEmitter,
    scope={
        prop:true
    };

test.on('event',function() {
   console.log(this.prop===true);//would log true
});
test.emit.call(scope,'event');

But, unfortunately, the listener doesn't even get called. Is there any way to do this w/ EventEmitter? I could Function.bind to the listener, but, I'm really hoping EventEmitter has some special (or obvious ;) way to do this...

Thanks for the help!

score 11 · Accepted Answer · answered Nov 04 '11 at 22:35

11

No, because the this value in the listener is the event emitter object.

However what you can do is this

var scope = {
  ...
};
scope._events = test._events;
test.emit.call(scope, ...);

The reason your event handler did not get called is because all the handlers are stored in ._events so if you copy ._events over it should work.

answered Nov 04 '11 at 22:35

Raynos

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Doh, seems so obvious now. Thanks for shining the light :). – Lite Byte Nov 07 '11 at 21:06
Since the _events property created on objects extended from EventEmitter isn't part of its documented public interface, is this wise advice? – Rob Raisch Nov 06 '12 at 21:03
@RobRaisch No, but it's unlikely to change. – Raynos Nov 06 '12 at 22:27
As of today, with Node 0.10.38, 0.12.4 and IO.JS 2.2.1, this approach still works without hiccup. Proven by time :) Great answer! Thank you! – vitaly-t Jun 07 '15 at 01:54

score 2 · Answer 2 · answered Nov 04 '11 at 22:25

2

That won't work, and emit only has a convenient way to pass parameters, but none for setting this. It seems like you'll have to do the binding stuff yourself. However, you could just pass it as a parameter:

test.on('event',function(self) {
   console.log(self.prop===true);//would log true
});
test.emit('event', scope);

answered Nov 04 '11 at 22:25

thejh

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Like this for simplicity! However, if the emit is not under your control but the framework's that won't fix the problem, unfortunately. – Thomas Fankhauser Jul 01 '13 at 10:16
All evidence to the contrary. 4 years later, the accepted answer still proves to be valid in every version of Node JS: 0.10.38, 0.12.4, IO.JS 2.2.1. Works like a charm :) – vitaly-t Jun 07 '15 at 01:57

score 0 · Answer 3 · answered Oct 02 '14 at 00:28

0

I came across this post when Google searching for a package in NPM which handles this:

var ScopedEventEmitter = require("scoped-event-emitter"),
    myScope = {},
    emitter = new ScopedEventEmitter(myScope);

emitter.on("foo", function() {
    assert(this === myScope);
});

emitter.emit("foo");

Full disclosure, this is a package I wrote. I needed it so I could could have an object with an EventEmitter property which emits for the containing object. NPM package page: https://www.npmjs.org/package/scoped-event-emitter

answered Oct 02 '14 at 00:28

Rich Remer

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The accepted answer works perfectly, there is no need for a whole extra library :) – vitaly-t Jun 07 '15 at 01:59
It's called code re-use. The "whole extra library" is a 37 line Javascript file, but because it's a published package, you can use it in many projects without looking for that one project you added the accepted answer to. In addition, the accepted answer did not exist when this was posted. – Rich Remer Nov 29 '17 at 05:49

Nodejs EventEmitter - Define scope for listener function

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