Kaprekar's constant solver

kaprekar-constant.js

// https://en.wikipedia.org/wiki/6174_(number)

const kaprekar = 6174;

const routine = (input) => {
    const res = new Map();
    let i = 0;
    while (input !== kaprekar) {
        input = cycle(input);
        res.set(i++, input);
    }
    return res;
};

const cycle = (input) => {
    const a = numberToArray(input);
    const s1 = sortArray(a, 'desc');
    const s2 = sortArray(a, 'asc');
    const n1 = arrayToNumber(s1);
    const n2 = arrayToNumber(s2);
    return n1 - n2;
};

const numberToArray = (num) =>
    num
        .toString()
        .split('')
        .map(n => parseInt(n, 10));

const arrayToNumber = (arr) => parseInt(arr.join(''), 10);

const sortArray = (arr, direction) => {
    const c = [...arr];
    switch (direction) {
        case 'asc':
            return c.sort((a, b) => a - b);
        case 'desc':
            return c.sort((a, b) => b - a);
        default:
            return c;
    }
};

const res = routine(1234);
console.log(res);