Why use event listeners over function calls?

Question

I\'ve been studying event listeners lately and I think I\'ve finally gotten them down. Basically, they are functions that are called on another object\'s method. My questi

Answer 1

I think the main reason for events vs function calls is that events are 'listened to' while calls are 'made'. So a function call is always made to another object whereas listeners 'choose' to listen for an event to be broadcast from your object. The observer pattern is a good study for this capability. Here is a brief node.js example which illustrates the concept:

var events = require('events');
var Person = function(pname) {
    var name = pname;
};


var james = new Person('james');
var mary = new Person('mary');
var loudmouth = new Person('blabberer');

loudmouth.mouth = new events.EventEmitter();

//jame's observer.
james.read_lips = function(msg){
    console.log("james found out: " + msg);
};

//james adds his event to the emitter's event listener.
james.enter_elevator = function(){
    console.log('james is in the elevator');
    //NOTE: james adds HIMSELF as a listener for the events that may
    //transpire while he is in the elevator.
    loudmouth.mouth.on('elevator gossip', james.read_lips)
};

//james removes his event from the emitter when he leaves the elevator.
james.leave_elevator = function(){
    // read lips is how james responds to the event.
    loudmouth.mouth.removeListener('elevator gossip', james.read_lips);
    console.log('james has left the elevator');
};

//mary's observer
mary.overhear = function(msg){
    console.log("mary heard: " + msg);
};

//mary adds her observer event to the emitter's event listeners
mary.enter_elevator = function(){
    // overhear is how mary responds to the event.
    console.log('mary is in the elevator');
    //NOTE: now mary adds HERSELF to the listeners in the elevator and 
    //she observes using a different method than james which suits her.
    loudmouth.mouth.on('elevator gossip', mary.overhear);
};

loudmouth.speaks = function(what_is_said){
    console.log('loudmouth: ' + what_is_said);
    this.mouth.emit('elevator gossip', what_is_said);
};

james.enter_elevator();
mary.enter_elevator();
loudmouth.speaks('boss is having an affair');
james.leave_elevator();
loudmouth.speaks('just kidding');
console.log('james did not hear the last line because he was not listening anymore =)');

so in this 'story' the actors choose to listen or when to not listen for events from a third party. I hope this helps.