Build an event emitter

Easy20 minFree, no account

Twelve lines that work, and one edge case that almost everybody ships broken.

The question

Implement on, off, once and emit.

Then handle the case where a listener removes another listener, or itself, during emit.

const bus = new Emitter();
const un = bus.on("tick", n => console.log(n));
bus.emit("tick", 1);   // logs 1
un();                  // on() returns an unsubscribe
bus.once("tick", f);   // runs at most once

20:00Commit to an answer before you open the solution. Reading it first teaches you to recognise good answers, which is not the skill being tested.

Stuck?

0 of 3 hints taken

The worked solution

written by a person · not a grade

Score yourself

0 of 4 marked

Correct on/off/once/emit30
Handled removal during emit and explained why30
off removes a once-wrapped handler by its original reference20
Had a position on throwing listeners and stated complexity20

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