Build an EventEmitter

Problem

Implement a Node-style EventEmitter with `on`, `off`, `emit`, and `once`. Foundation of every pub/sub system.

Input

N/A — script demonstration.

Output

Lines printed by the listeners.

Constraints

up to 10^4 listeners

Sample input

e.on('tick', n => console.log('A' + n));
e.emit('tick', 1);
e.once('tick', n => console.log('B' + n));
e.emit('tick', 2);
e.emit('tick', 3);

Sample output

A1
A2
B2
A3

Explanation

once fires on `2` only; regular on fires on every emit.

designoopclosuresobserver@Netflix@Meta@Microsoft

Visible test cases

in: e.on('x', () => console.log('hi')); e.emit('x')

out: hi

Your solution — run it, use AI if stuck

javascript