Function countBy

countBy<TElement, TKey>(
    source: Iterable<TElement>,
    keySelector: Selector<TElement, TKey>,
    comparator?: EqualityComparator<TKey, TKey>,
): IEnumerable<KeyValuePair<TKey, number>>
Counts the occurrences of elements grouped by a derived key.
Type Parameters
- TElement
  Type of elements within the source iterable.
- TKey
  Type produced by keySelector.
Parameters
- source: Iterable<TElement>
  The source iterable.
- keySelector: Selector<TElement, TKey>
  Selector used to derive the grouping key for each element.
- Optionalcomparator: EqualityComparator<TKey, TKey>
  Optional equality comparator used to match keys. Defaults to the library's standard equality comparison.
Returns IEnumerable<KeyValuePair<TKey, number>>
A sequence of key/count pairs describing how many elements share each key.
Remarks
Each key appears exactly once in the result with its associated occurrence count.
Example
```
const products = [
  { name: 'Apple', category: 'Fruit' },
  { name: 'Banana', category: 'Fruit' },
  { name: 'Carrot', category: 'Vegetable' },
];

const countByCategory = countBy(products, p => p.category).toArray();
console.log(countByCategory);
// [
//   { key: 'Fruit', value: 2 },
//   { key: 'Vegetable', value: 1 }
// ]
```
- Defined in src/enumerator/functions/countBy.ts:32