duplicateCount.js

function duplicateCount(text) {
  if (!text) return 0; // no need moving forward if input is empty

  const tracker = new Set(); // keep track of all characters in the input
  const dupes = new Set(); // keep track of duplicate characters

  for (const char of text.toLowerCase()) {
    if(tracker.has(char)) {
      dupes.add(char);
    } else {
      tracker.add(char);
    }
  }
  
  return dupes.size;
}