RWC: Phone Number Keypad Combinations

phone-number.js

/**
 * @type {Object<String, String[]>}
 */
const keypad = {
	'2': ['a', 'b', 'c'],
	'3': ['d', 'e', 'f'],
	'4': ['g', 'h', 'i'],
	'5': ['j', 'k', 'l'],
	'6': ['m', 'n', 'o'],
	'7': ['p', 'q', 'r', 's'],
	'8': ['t', 'u', 'v'],
	'9': ['w', 'x', 'y', 'z']
};

/**
 * Recursively determines possible keypad combinations of a given string of digits
 * @param {string} digits - a string containing only digits from 2 to 9
 * @returns {string[]} all possible letters that could be made on the keypad with `digits`
 */
function phoneNumbers(digits) {
	// Base case
	if (digits.length === 1) {
		return keypad[digits];
	}

	// Recursive case
	const firstDigit = digits.charAt(0);
	const tail = digits.substr(1);
	const tailCombinations = phoneNumbers(tail);

	const combinations = [];
	for (const possibleCharacter of keypad[firstDigit]) {
		for (const tailCombination of tailCombinations) {
			combinations.push(possibleCharacter + tailCombination);
		}
	}

	return combinations;
}

console.log(phoneNumbers('23'));