1. Why are you mutating the event you're passing in?
2. Why are you returning a string | event type?

I used the code from the official docs:
https://developer.mozilla.org/en-US/docs/Web/API/Window/beforeunload_event

Their example:

const beforeUnloadListener = (event) => {
  event.preventDefault();
  return (event.returnValue = "");
};

It is done that way due to the compatibility, quote from docs:

The HTML specification states that authors should use the Event.preventDefault() method instead of using Event.returnValue to prompt the user. However, this is not yet supported by all browsers.

Thanks for explaining. Can you clarify why you're returning event on line 108?

It's because of the eslint:

Arrow function expected no return value. eslint(consistent-return)

Does this function need to return something? Can't we just have two bare return;s?

Event | string is a very strange return type. Let's figure out what the return type should be. Is return undefined or return null possible?

Both works. Updated with return null.