Function countBy

countBy<const Ar extends readonly unknown[], G extends Primitive>(
array: Ar,
grouper: (value: Ar[number], index: ArrayIndex<Ar>) => G,
): IMap<G, ArrayIndex<Ar>>
Groups elements by a key and counts the elements in each group.
Type Parameters
- const Ar extends readonly unknown[]
- G extends Primitive
Parameters
- array: Ar
- grouper: (value: Ar[number], index: ArrayIndex<Ar>) => G
Returns IMap<G, ArrayIndex<Ar>>
Example
```
const words = ['Ada', 'Grace', 'Alan', 'Barbara'] as const;

const counts = Arr.countBy(words, (word) => word[0]);

assert.deepStrictEqual(counts.get('A'), Optional.some(2));
assert.deepStrictEqual(counts.get('G'), Optional.some(1));
assert.deepStrictEqual(counts.get('B'), Optional.some(1));
```
- Defined in src/array/impl/array-utils-reducing-value.mts:263
countBy<E, G extends Primitive>(
grouper: (value: E, index: Uint32) => G,
): (array: readonly E[]) => IMap<G, Uint32>
Groups elements by a key and counts the elements in each group.
Type Parameters
- E
- G extends Primitive
Parameters
- grouper: (value: E, index: Uint32) => G
Returns (array: readonly E[]) => IMap<G, Uint32>
Example
```
const words = ['Ada', 'Grace', 'Alan', 'Barbara'] as const;

const counts = Arr.countBy(words, (word) => word[0]);

assert.deepStrictEqual(counts.get('A'), Optional.some(2));
assert.deepStrictEqual(counts.get('G'), Optional.some(1));
assert.deepStrictEqual(counts.get('B'), Optional.some(1));
```
- Defined in src/array/impl/array-utils-reducing-value.mts:271