possibileStrings.js

const numMap = {
	'1': [],
	'2': ['a', 'b', 'c'],
	'3': ['d', 'e', 'f'],
	'4': ['g', 'h', 'i'],
	'6': ['m', 'n', 'o'],
	'7': ['p','q','r','s']
};

function possibleStrings(numPadStr) {

	if(numPadStr.length === 0) {
		return [];
	}

	if(numPadStr.length === 1) {
		return numMap[numPadStr];
	}

	const letters = [];
	const nums = numPadStr.split('');

	nums.forEach((val) => {
		letters.push(numMap[val]);
	});

	return buildStringPossibilities(letters.shift(), letters);
}

function buildStringPossibilities(left, right) {
	if (right.length === 0) {
		return left;
	}
	const rightToMove = right.shift();
	const newLeft = [];
	for(let i = 0; i < left.length; i++) {
		for(let j = 0; j < rightToMove.length; j++) {
			newLeft.push(left[i]+rightToMove[j]);
		}
	}
	return buildStringPossibilities(newLeft, right);
}
console.log(possibleStrings('237'));