question for issue #224 of rendezvous

phoneLetters.js

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'],
};

function phoneLetters(numberString) {
    if (typeof numberString !== 'string' || !numberString.match(/^[2-9]+$/)) {
        throw new Error('input must be a string of only numbers between 2 and 9');
    }

    const numbersArray = numberString.split('');

    if (numbersArray.length === 1) {
        return keypad[numbersArray];
    }

    const lettersArray = numbersArray.map((number) => keypad[number]);

    const answer = lettersArray.reduce(
        (combinations, newLetters) => {
            return (
                combinations
                    .map((prev) => {
                        // for each new letter combine to each previous combination
                        return newLetters.map((letter) => {
                            return prev.concat(letter);
                        });
                    })
                    // combine all the new combinations into a single array
                    // for the next group of new letters
                    .reduce((a, b) => {
                        return a.concat(b);
                    }, [])
            );
        },
        [[]],
    );

    return answer.map((letters) => letters.join(''));
}

console.log(phoneLetters('9'));
console.log(phoneLetters('23'));