Function countBy

Alias

countBy<T, K>(arr, mapper): Record<K, number>
Count the occurrences of each item in an array based on a transformation function.

This function takes an array and a transformation function that converts each item in the array to a key. It then counts the occurrences of each transformed item and returns an object with the transformed items as keys and the counts as values.
Type Parameters
- T
  The type of the items in the input array.
- K extends PropertyKey
  The type of keys.
Parameters
- arr: readonly T[]
  The input array to count occurrences.
- mapper: ((item: T) => K)
  The transformation function that maps each item to a key.
  - - (item): K
    - Parameters
      item: T
      Returns K
Returns Record<K, number>
An object containing the transformed items as keys and the counts as values.
Example
```
const array = ['a', 'b', 'c', 'a', 'b', 'a'];
const result = countBy(array, x => x);
// result will be { a: 3, b: 2, c: 1 }
```
Example
```
const array = [1, 2, 3, 4, 5];
const result = countBy(array, item => item % 2 === 0 ? 'even' : 'odd');
// result will be { odd: 3, even: 2 }
```
- Defined in src/array.ts:396