Function countBy

countBy<T, K extends PropertyKey>(
xs: readonly T[],
keyFn: (x: T) => K,
): Record<K, number>
Tally elements by key. Key collisions increment the same bucket.

Dual API — works data-first or curried for use in pipe.
Type Parameters
- T
- K extends PropertyKey
Parameters
- xs: readonly T[]
- keyFn: (x: T) => K
Returns Record<K, number>
Example
```
pipe(
    events,
    Arrays.countBy((e) => e.type),
  )
  // { click: 12, view: 48 }
```
- Defined in arrays/aggregate.ts:42
countBy<T, K extends PropertyKey>(
keyFn: (x: T) => K,
): (xs: readonly T[]) => Record<K, number>
Tally elements by key. Key collisions increment the same bucket.

Dual API — works data-first or curried for use in pipe.
Type Parameters
- T
- K extends PropertyKey
Parameters
- keyFn: (x: T) => K
Returns (xs: readonly T[]) => Record<K, number>
Example
```
pipe(
    events,
    Arrays.countBy((e) => e.type),
  )
  // { click: 12, view: 48 }
```
- Defined in arrays/aggregate.ts:46