const EventEmitter = require('events').EventEmitter;
const Counter = function(i) {
    this.increment = function() {
        i++;
        this.emit('incremented', i);
    }
}

Counter.prototype = new EventEmitter();
const counter = new Counter(10);
const callback = function(n) {
    console.log(n);
}

counter.addListener('incremented', callback);
counter.increment();
counter.increment();