happiness.js

function getSumOfSquaresOfDigits(n) {
  let _n = n;
  let result = 0;
  while (_n > 0) {
    result += (_n % 10) ** 2;
    _n = Math.floor(_n / 10);
  }
  return result;
}

function getConvergence(n) {
  let previous = [n];
  let _n = n;
  while (true) {
    let sumOfSquares = getSumOfSquaresOfDigits(_n);
    if (previous.includes(sumOfSquares)) {
      return sumOfSquares;
    }
    previous.push(sumOfSquares);
    _n = sumOfSquares;
  }
}

const convergences = new Set();
for (let i = 1; i < 1_000_000_000; i++) {
  if (i % 100_000_000 === 0) console.log(i);
  convergences.add(getConvergence(i));
}
console.log(convergences);

if (i % 100_000_000 === 0) console.log(i); Could've saved an operation for each number.