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kaprekar_operation.js

const kaprekarNext = n => {
  const a = [...n + ''].map(n => +n);
	a.sort((a, b) => b - a);
  return +a.join('') - +a.reverse().join('');
};

const kaprekarLoop = n => {
  let last;
  while (last !== n) {
    last = n;
    n = kaprekarNext(n);
  }
  return n;
};

const blackHoles = new Map();

for (let i = 1111; i <= 9999; ++i) {
  const hole = kaprekarLoop(i);
  const cnt = blackHoles.get(hole) || 0;
  blackHoles.set(hole, cnt + 1);
}

console.log(
  [...blackHoles].map(([n , times]) => `${n} - ${times} times`)
);