JSFiddle - React, Tailwind, and code Playground

HTML

<h1>Max sequence for 1 million</h1>
<input id="btnCalc" type="button" value="Calculate" onclick="solveCollatz()" />
<p id="pMaxNum">Number:</p>
<p id="pTotal">Total number of sequences:</p>

JavaScript

function solveCollatz() {
  let num = 2;
  let max = 1000000;
  let total = 0;
  let finish = 0;
  while (num < max) {
    let n = parseInt(num);
    let array = [];
    while (n > 1) {
      n = (n % 2 == 0) ? n = n / 2 : n = 3 * n + 1;
      array.push(n);
    }
    if (array.length > total) {
      total = array.length;
      finish = num;
    }
    num++;
  }
  document.getElementById("pMaxNum").innerHTML = `Number: ${finish}`;
  document.getElementById("pTotal").innerHTML = `Total number of sequences: ${total}`;
}