How to use events in Node.js

In this blog post I will explain how to create and use Events in Node.Js The main purpose is to demonstrate the following :

I have a module (a) that will do some operations, and I want that module to emit an event when some operations are done. Then I would like other modules (such as b module) to be able to subscribe to those operations.

In order to solve this I will create an EventManager object based on node events.

Create the files :

a.js

var operations = {
  add:function(){
    // operation for add
  },
  remove:function(){
    // operation for remove
  },
  update:function(){
    // operation for update
  }
}
module.exports = operations;

b.js

var b = {
  someCodeWhenAddInA:function(){
    // this function should be executed after a.add is executed
  }
}
module.exports = b;

So the purpose is more than simple, when a.add is called, we want the function b.someCodeWhenAddInA to be called, and we also want to easylly add the ability to add modules that will execute code on a.add called.

Let’s create the EventManager.

EventManager.js

var util         = require("util");
var EventEmitter = require("events").EventEmitter;
function EventManager(){
    EventEmitter.call(this);
}

util.inherits(EventManager, EventEmitter);

module.exports = new EventManager();

refer to the documentation to know more about node js events

Edit the code in a and b modules :

a.js

var EventManager = require('./EventManager');
var operations = {
  add:function(){
    // operation for add
    EventManager.emit('addedToA');
  },
  remove:function(){
    // operation for remove
  },
  update:function(){
    // operation for update
  }
}
module.exports = operations;

b.js

var EventManager = require('./EventManager');
var b = {
  someCodeWhenAddInA:function(){
    // this function should be executed after a.add is executed
    console.log('someCodeWhenAddInA has been executed in b');
  }
}
EventManager.on('addedToA', b.someCodeWhenAddInA);
module.exports = b;

And now let’s simply create a runner file to try our code

main.js

var a = require('./a');
var b = require('./b');

a.add();

This should print someCodeWhenAddInA has been executed in b in the console.

And that’s all !

Now if you want to create a c module which will listen to the same event, it’s easy, and no need to edit the a module anymore :

c.js

var EventManager = require('./EventManager');
var c = {
  anotherFonction:function(){
    // this function should be executed after a.add is executed
    console.log('anotherFonction has been executed in c');
  }
}
EventManager.on('addedToA', c.anotherFonction);
module.exports = b;

main.js

var a = require('./a');
var b = require('./b'); (1)
var c = require('./c'); (2)

a.add();

This will print :

someCodeWhenAddInA has been executed in b
anotherFonction has been executed in c

If you inverse the 1 and 2 the log will also be impacted.

Here we are, comments are appreciated