Node event emitter in other modules

Question

I have 3 different javascript files, the smallest one is emitting an event, meanwhile the second (bigger one) file picks up the event and sends it further to the main file. This is what I have tried so far:

//mini.js
var EventEmitter = require('events').EventEmitter;
var ee = new EventEmitter;

console.log("Emitting event");
var message = "Hello world";
ee.emit('testing',message);


//second.js
var mini = require('./mini.js');
var EventEmitter = require('events').EventEmitter;
var ee = new EventEmitter;

mini.on('testing',function(message){
    console.log("Second file received a message:",message);

    console.log("Passing further");
    ee.emit('testing',message);
});

//main.js
var sec = require('./second.js');

sec.on('testing',function(message){
    console.log("Main file received the message",message);
});

However, I get

mini.on('testing',function(message){
        ^
TypeError: undefined is not a function

error when executing the file with node.

What am I doing wrong here? Thanks

score 3 · Answer 1 · answered Mar 28 '16 at 10:04

3

You're not exporting your EventEmitter instance in mini.js. Add this to mini.js:

module.exports = ee;

You'll also need to add a similar line in second.js if you want to export its EventEmitter instance in order to make it available to main.js.

Another problem you'll run into is that you're emitting testing in mini.js before second.js adds its testing event handler, so it will end up missing that event.

answered Mar 28 '16 at 10:04

mscdex

104,356
15
192
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Any possible fixes for this? – Ned Mar 28 '16 at 11:13

ismnoiet · Accepted Answer · 2016-03-28T11:14:25.393

2

This one should work :

This is the content to put inside first.js :

//first.js
var util = require('util'),
    EventEmitter = require('events');


function First () {
  EventEmitter.call(this)
}
util.inherits(First, EventEmitter);


First.prototype.sendMessage = function (msg) {    
  this.emit('message', {msg:msg});
};


module.exports = First;

This is the content to put inside second.js :

//second.js
var First = require('./first.js');
var firstEvents = new First();

// listen for the 'message event from first.js'
firstEvents.on('message',function(data){
    console.log('recieved data from first.js is : ',data);
});


// to emit message from inside first.js
firstEvents.sendMessage('first message from first.js');

Now run node second.js and you should have the 'message' event fired for you.

You can use this pattern to achieve any level of messaging between modules.

Hope this helps.

edited Mar 28 '16 at 11:14

answered Mar 28 '16 at 10:30

ismnoiet

4,129
24
30

@NedasMikoleika in this case rename **second.js** to **main.js** and that's it or there is something i'm missing here ? – ismnoiet Mar 28 '16 at 11:15
I actually have everything else (the rest of my server) in the **main.js** file. – Ned Mar 28 '16 at 11:22
@NedasMikoleika no problem, just add the content of **second.js** to your **main.js** file. – ismnoiet Mar 28 '16 at 11:46

Node event emitter in other modules

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