kaprekartest.js

const maxNum = num => +[...`${num}`].sort((a, b) => b - a).join('');
const minNum = num => +[...`${num}`].sort((a, b) => a - b).join('');


const reachedNums = new Set([]);
const seen = {};

const findKaprekar = (i, _min, _max, prev, step) => {
    let diff = _max-_min;
    while (diff) {
        if (step > 1000) {
            seen[diff] = [-i];
            break;
        }
        if (diff in seen) {
            break;
        }
        if (prev===diff) {
            seen[diff] = [i]
            reachedNums.add(prev);
            break;
        }
        prev = diff;
        diff = maxNum(diff)- minNum(diff);
        step++;
    }
}

for (let i = 100; i< 100_000; i++) {
    findKaprekar(i, minNum(i), maxNum(i), null, 1);
}

console.log(seen);
console.log(reachedNums);