Count up and down between two numbers

counter.js

const counter = (min, max, initial) => {

	let index  = initial - min
	let length = max - min + 1

	return (modifier = 0) => {

		index = (index + modifier) % length
		
		if (index>=0) index = 0 + index
		if (index<0)  index = length + index

		return min + index

	}

}

Example:

let c = counter(2, 5, 5)

c(1) // 2
c(1) // 3
c(3) // 2
c(-2) // 4
c(0) // 4
c() // 4

Moved to count-between.