day10.js

const next = input => {
    let chars = input.match(/(\d)\1*/g);
    return chars.map(repetition => `${repetition.length}${repetition[0]}`).join('');
};

const nextN = (input, times) => {
    if (times === 0) {
        return input;
    }
    return nextN(next(input), times - 1);
};

console.log(nextN(input, 50).length);