An EventEmitter lets code subscribe to named events and invokes the registered callbacks when those events are emitted.

An upload component, for example, can emit a progress event while a progress bar and a logger each react independently. The upload code does not need direct references to every consumer. Subscriptions also have a lifecycle: when a consumer no longer needs updates, it should be able to remove its callback without affecting other listeners.

Implement an EventEmitter class similar to the one in Node.js. Preserve listener order, duplicate registrations, and instance isolation.

Example usage of the EventEmitter class:

const emitter = new EventEmitter();

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

emitter.off('foo', addTwoNumbers);
emitter.emit('foo', -3, 9);
// > "The product is -27"

EventEmitter API

Implement the following class and APIs:

new EventEmitter()

Creates an instance of the EventEmitter class. Events and listeners are isolated within the EventEmitter instances they're added to; that is, listeners should not 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 the EventEmitter instance so that calls can be chained.

emitter.off(eventName, listener)

Removes the specified listener from the list of listeners for the event with the name eventName.

Returns the EventEmitter instance so that calls can be chained.

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

Invokes each listener registered for eventName with the supplied arguments in order.

Returns true if the event had listeners, false otherwise.

Notes

• Invoke listeners in registration order.
• Registering the same listener more than once creates separate subscriptions, so it is invoked once per registration. One off() call removes only one matching registration.
• Calling off() for a missing event or listener is a no-op.
• Event names are arbitrary strings and can match inherited object property names such as toString.