Implement an event emitter

Most front-end code ends up with a small pub/sub: a store tells the UI something changed, a socket tells the app a message arrived. Build the core of one.

Implement Emitter, a class with these methods:

• on(event, fn) registers fn for event.
• once(event, fn) registers fn, but it runs at most one time.
• off(event, fn) removes that listener. Removing a listener that was never added does nothing.
• emit(event, ...args) calls every listener for event, in the order they were added, passing args to each.

A listener can call off while emit is running, even on itself. Every listener that was registered when emit started still runs, once, and none of them is skipped.

const bus = new Emitter()
bus.on("save", (name) => console.log("saved", name))
bus.once("save", () => console.log("first save only"))
bus.emit("save", "draft") // logs "saved draft", then "first save only"
bus.emit("save", "final") // logs "saved final"

Requirements:

• Listeners run in the order they were added.
• Every argument given to emit reaches the listener.
• Emitting an event nobody listens to does nothing and does not throw.
• A listener for one event is never called for another.
• off removes only the listener you name.

Write a top-level Emitter that works with new. A class or a constructor function both work. Do not use export or module.exports.