Implement groupBy(array, iteratee) — lodash style

Question

Accepted Answer

**Approach:** Iterate, compute key (call fn or read prop), push to `result[key]`.

```javascript
function groupBy(collection, iteratee) {
  const fn = typeof iteratee === 'function' ? iteratee : (v) => v[iteratee];
  const result = {};
  for (const item of collection) {
    const key = String(fn(item));
    if (!result[key]) result[key] = [];
    result[key].push(item);
  }
  return result;
}

// Demo
console.log(groupBy([6.1, 4.2, 6.3], Math.floor)); // { '4': [4.2], '6': [6.1, 6.3] }
console.log(groupBy(['one','two','three'], 'length')); // { '3': ['one','two'], '5': ['three'] }
console.log(groupBy([{a:1},{a:2},{a:1}], x => x.a)); // { '1': [{a:1},{a:1}], '2': [{a:2}] }
function groupByMap(arr, fn) {
  const m = new Map();
  for (const v of arr) { const k = fn(v); if (!m.has(k)) m.set(k, []); m.get(k).push(v); }
  return m;
}
console.log([...groupByMap([1,2,3,4], x => x%2)]); // [[1,[1,3]],[0,[2,4]]]
```
ES2024 has native `Object.groupBy` and `Map.groupBy` — polyfill should match.

Implement groupBy(array, iteratee) — lodash style

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