JSFiddle - React, Tailwind, and code Playground

HTML

<body>
Hello
</body>

CSS

body {
  size: 900px;
  background-color: gray;
  color: white;
}

JavaScript

function nextInSequence(n) {
  if (n === 1) return null;
  if (n % 2 === 0) return n / 2;
  return 3 * n + 1;
}

function sequence(n) {
  let sequenceArray = [];

  while (n) {
    sequenceArray.push(n);
    n = nextInSequence(n);
  }
  return sequenceArray;
}

function findLargestSequence(max) {
  var largestSequence = [1]
  var n = 2;

  while (n <= max) {
    var seq = sequence(n);

    if (seq.length > largestSequence.length) largestSequence = seq
    n++;
  }

  return largestSequence;
}

document.body(findLargestSequence(1000000)[0]);