Function that continuously executes the function it has been most recently called with

single.js

const single = (max) => {

	let id
	let iterations

	const loop = (_id, fn) => {

		if (id!==_id) return
		if (max!==undefined && iterations>=max) return

		++iterations

		fn()
		requestAnimationFrame(() => loop(_id, fn))

	}

	return (fn) => {

		id = Symbol()
		iterations = 0

		loop(id, fn)

		return () => id = Symbol()

	}

}

Example:

const run = single()

run(() => console.log(1)) // Logs 1 till the next line has been executed
run(() => console.log(2)) // Logs 2 forever

const run = single()
let stop

stop = run(() => console.log(1)) // Logs 1 till the next line has been executed
stop = run(() => console.log(2)) // Logs 2 till the returned function has been executed

stop()

const run = single(5)

run(() => console.log(1)) // Never executes more than 5 times