Hint
Set stores unique values in insertion order. Adding a duplicate is silently ignored.
const set = new Set([1, 2, 3, 2, 1, 3, 4]);
console.log(set.size);
set.add(5); set.add(1);
console.log(set.size);
console.log([...set][0]);4 5 1
Explanation: Duplicates removed: {1,2,3,4}=size 4. add(5)→5. add(1) already exists, no change. First element=1.
Key Insight: Set stores unique values in insertion order. Adding a duplicate is silently ignored.