collatz.js

const MAX = 1000000;
const cache = new Array(MAX);

function nextFor(n) {
  if (n == 1) {
    return 1;
  }
  else if (n <= MAX && cache[n - 1] !== undefined) {
    return cache[n - 1];
  }
  else if (n % 2 == 0) {
    return 1 + nextFor(n / 2);
  }
  else {
    return 1 + nextFor(n * 3 + 1);
  }
}

function longestLength() {
  let n = 1;
  let maxN = 0;
  let maxLength = 0;

  while (n <= MAX) {
    const length = nextFor(n);

    cache[n - 1] = length;

    if (length > maxLength) {
      maxN = n;
      maxLength = length;
    }

    n++;
  }

  return [maxN, maxLength];
}

console.log(longestLength());