Reverse or rotate given string.

revrot.js

function revrot(str, sz) {

	/**
	 * If string (str), size (sz) or size is greater than string's
	 * length, return empty string.
	 */
	if (sz === 0 || str.length === 0 || sz > str.length) {
		return ''
	}

	const
	/**
	 * Splits string into groups of size provided
	 * @param  {String} string
	 * @param  {Number} size
	 * @return {Array}
	 */
		splitter = (string) => {
			/* Pattern to split string into groups, in our case provided size */
			let pattern = new RegExp(`.{1,${sz}}`, 'g')
			return string.match(pattern)
		},

		chunked = splitter(str),

		/**
		 * Get sum of the cubes of each elements
		 * @param  {String} element
		 * @return {Array}
		 */
		sumOfCubes = chunked.map(element => {
			return Array.prototype.reduce.call(element, (init, next) => {
				init = init + Math.pow(Number(next), 3);
				return init;
			}, 0);
		}),

		/**
		 * Check if number is divisble by 2
		 * @param  {Number} n
		 * @return {Boolean}
		 */
		isDivisibleByTwo = n => n % 2 === 0,

		/**
		 * Map sumOfCubes & chunked array together
		 * @return {Array} Array of objects
		 */
		mapElements = chunked.map((e, i) => {
			return {
				chunked: chunked[i],
				sumOfCubes: sumOfCubes[i]
			};
		}),

		/* Final maped array */
		finalMapedArray = mapElements.map(num => {

			/* Ignore chunk if it's size is less than sz */
			if (num.chunked.length === sz) {
				if (isDivisibleByTwo(num.sumOfCubes)) {
					/* Reversing the string */
					return num.chunked.split('').reverse().join('')
				} else {
					/* Rotating the string to the left by one position */
					return num.chunked[1] + num.chunked.slice(2) + num.chunked[0]
				}
			}
		})

	return finalMapedArray.join('')
}

// console.log('Final', revrot('123456779', 8) === '23456771')