https://repl.it/CQrh/92 created by sethopia

Shift Array (3.1 == AKA 2.5).js

/*
	SHIFT DECK
	Create a function, deckShift() that takes a number and a direction ('left' or 'right) and returns an array of cards shifted that many spaces in that direction. This function should not mutate the original deck variable.
	
	Examples
		deckShift(4, 'left') => [6,7,8,9,10,'J','Q','A',2,3];
		deckShift(2, 'right') => ['K','A',2,3,4,5,6,7,8,9,10,'J','Q'];
*/

var deck = [2,3,4,5,6,7,8,9,10, 'J', 'Q', 'K', 'A'];

var deckShift = function(index, direction) {

	var shiftedCards = deck.slice();
	
	if (direction === "left") {
		for (var i = 0; i < index; i++) {
			shiftedCards.push(shiftedCards.shift());
		}
		return shiftedCards;
	} else if (direction === "right") {
		for (var i = 0; i < index; i++) {
			shiftedCards.unshift(shiftedCards.pop());
		}
		return shiftedCards;
	} else {
		return deck;
	}
}

console.log(deckShift(4,"left"));
console.log(deckShift(2,"right"));

Shift Array (3.1 == AKA 2.5)_console_dump.txt

Native Browser JavaScript
>>> [ 6, 7, 8, 9, 10, 'J', 'Q', 'K', 'A', 2, 3, 4, 5 ]
[ 'K', 'A', 2, 3, 4, 5, 6, 7, 8, 9, 10, 'J', 'Q' ]