In the observer pattern (also commonly known as the publish-subscribe model), we can observe/subscribe to events emitted by publishers and execute code whenever an event happens.

Implement an EventEmitter class similar to the one in Node.js that follows such an observer pattern. The difference between this question and the first Event Emitter question is the way listeners are unsubscribed. In this version, there's no emitter.off() method available on the EventEmitter instance. Instead, emitter.on() returns an object that has an off() method.

EventEmitter API

Implement the following classes and APIs:

new EventEmitter()

Creates an instance of the EventEmitter class. Events and listeners are isolated within the EventEmitter instances they're added to, aka listeners shouldn't react to events emitted by other EventEmitter instances.

emitter.on(eventName, listener)

Adds a callback function (listener) that will be invoked when an event with the name eventName is emitted.

Returns a subscription object that has an off() method that unsubscribes this listener from the event. More details below.

emitter.emit(eventName[, ...args])

Invokes each of the listeners listening to eventName with the supplied arguments in order.

Returns true if the event had listeners, false otherwise.

sub.off()

This object is returned from emitter.on(). Calling sub.off() unsubscribes the respective listener from the event.

Examples

const emitter = new EventEmitter();

function addTwoNumbers(a, b) {
  console.log(`The sum is ${a + b}`);
}

// Returns a subscription object that has an .off() method.
const sub = emitter.on('foo', addTwoNumbers);
emitter.emit('foo', 2, 5);
// > "The sum is 7"

emitter.on('foo', (a, b) => {
  console.log(`The product is ${a * b}`);
});
emitter.emit('foo', 4, 5);
// > "The sum is 9"
// > "The product is 20"

sub.off(); // This unsubscribes the callback that logs the sum of the numbers.
emitter.emit('foo', -3, 9);
// > "The product is -27"
// (Only the multiply callback is triggered, the first one was unsubscribed.)