fit the given number within the available indexes of an array of a given length and overflow around from the end or beginning where appropriate

fitIndex.js

/**
 * Fit a given index into a given array
 * length, while properly handling overflow
 *
 * so for eg:
 * - given length=10 & index=3  -> return 3
 * - given length=10 & index=14 -> return 4 (overflow)
 * - given length=10 & index=-2 -> return 8 (overflow)
 *
 * @param index
 * @param length
 * @returns {number}
 */
export default function fitIndex(index, length) {
	return ((index % length) + length) % length;
}