32Implement an EventEmitter with on, off, emit, and once

Implement an EventEmitter class, similar to Node's:

• on(event, listener): add a listener. Returns the emitter, so calls can be chained.
• once(event, listener): add a listener that removes itself after its first call.
• off(event, listener): remove a listener. Works for listeners added with once too.
• emit(event, ...args): call every listener for event in the order they were added, with this set to the emitter. Return true if there were listeners, otherwise false.
• listenerCount(event): the number of listeners for event.

const chat = new EventEmitter();
const onMessage = (from, text) => console.log(`${from}: ${text}`);
chat.on("message", onMessage);
chat.once("message", () => console.log("first message!"));
chat.emit("message", "Asha", "Hi"); // both listeners run
chat.emit("message", "Ravi", "Hello"); // only onMessage runs
chat.off("message", onMessage);

Requirements

• The same listener added twice is called twice, and off removes one of them (the most recently added).
• A listener that removes itself, or another listener, during emit must not cause others to be skipped.
• A listener added during emit is not called in that same emit.
• on throws a TypeError when the listener is not a function.