Implement dedupe / unique — remove duplicates preserving order

Question

Accepted Answer

**Approach:** `Set` for primitives; `Map` with key function for objects.

```javascript
function uniq(array) { return [...new Set(array)]; }
function uniqBy(array, iteratee) {
  const fn = typeof iteratee === 'function' ? iteratee : (v) => v[iteratee];
  const seen = new Set(); const res = [];
  for (const item of array) { const key = fn(item); if (!seen.has(key)) { seen.add(key); res.push(item); } }
  return res;
}

// Demo
console.log(uniq([1,2,2,3,1])); // [1,2,3]
console.log(uniqBy([{x:1},{x:2},{x:1}], x=>x.x)); // [{x:1},{x:2}]
console.log(uniqBy(['a','A','b'], x=>x.toLowerCase())); // ['a','b']
const o = {};
console.log(uniq([o, o, {}])); // [o, {}] (two distinct {} remain)
console.log(uniqBy([{id:1},{id:1},{id:2}], 'id')); // [{id:1},{id:2}]
```
NaN: `Set` treats `NaN` as equal (SameValueZero) — `uniq([NaN, NaN])` → `[NaN]`.

Implement dedupe / unique — remove duplicates preserving order

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