EventEmitter

// Much of the Node.js core API is built around an idiomatic asynchronous event-driven architecture 
// in which certain kinds of objects (called "emitters") emit named events that cause 
// Function objects ("listeners") to be called.
Class: EventEmitter
    // Synchronously calls each of the listeners registered for the event named eventName, 
    // in the order they were registered, passing the supplied arguments to each.
    emit(eventName[, ...args])

    // Removes the specified listener from the listener array for the event named eventName.
    off(eventName, listener)
    
    // Adds the listener function to the end of the listeners array for the event named eventName. 
    // No checks are made to see if the listener has already been added. 
    // Multiple calls passing the same combination of eventName and listener will result in the listener being added, 
    // and called, multiple times.
    on(eventName, listener)

    // Adds a one-time listener function for the event named eventName. 
    // The next time eventName is triggered, this listener is removed and then invoked.
    once(eventName, listener)
    

Example:

const myEmitter = new EventEmitter();
myEmitter.on('event', () => {
  console.log('an event occurred!');
});
myEmitter.emit('event');


let superbowl = new EventEmitter()

const cheer = function (eventData) {
  console.log('RAAAAAHHHH!!!! Go ' + eventData.scoringTeam)
}

const jeer = function (eventData) {
  console.log('BOOOOOO ' + eventData.scoringTeam)
}

const gameIsOne = ()=> {
  console.log("game is on");
}

superbowl.on('touchdown', cheer);
superbowl.on('touchdown', jeer);
superbowl.once('game', gameIsOne);

superbowl.emit('game');
superbowl.emit('game');

superbowl.emit('touchdown', { scoringTeam: 'Patriots' });

superbowl.removeListener('touchdown', jeer);

superbowl.emit('touchdown', { scoringTeam: 'Seahawks' });