Discussion on: The Happy Number Problem

[Duy K. Bui]

Your solution makes sense, mathematically.
However, I think it's a wasted chance to use and explain recursion with carryover.
The function can be something like this:

function isHappy(n, seenSums = []) {
  // ... same as above
  if (sum === 1) return true; // answer found
  if (seenSums.includes(sum)) return false; // generic infinite loop detection mechanism
  return isHappy(sum, seenSums.concat([sum])); // recursion with carryover
}

[Luis Perez Bautista]

You could use Set instead of an array for seenSums, that way your validation to check the infinite loop could be done in constant time by changing

  if (seenSums.includes(sum)) return false; // generic infinite loop detection mechanism

for

  if (seenSums.has(sum)) return false; // generic infinite loop detection mechanism