Function groupBy

groupBy<T, K extends string | number | symbol>(
array: T[],
keyFn: (item: T) => undefined | null | K,
): Record<string, T[]>
Groups elements of an array by a key derived from each element.

The grouping key can be returned from a callback function. Handles undefined and null keys safely by grouping them under "__undefined__".
Type Parameters
- T
- K extends string | number | symbol
Parameters
- array: T[]
  The array to group.
- keyFn: (item: T) => undefined | null | K
  A function that returns the key used for grouping.
Returns Record<string, T[]>
An object where keys represent group identifiers and values are arrays of grouped elements.
Example
```
groupBy(
  [{ id: 1, type: "a" }, { id: 2, type: "b" }, { id: 3, type: "a" }],
  (item) => item.type
);
// → { a: [{ id: 1, type: "a" }, { id: 3, type: "a" }], b: [{ id: 2, type: "b" }] }

groupBy([1, 2, null, 3], (n) => (n ?? "missing").toString());
// → { "1": [1], "2": [2], "missing": [null], "3": [3] }
```
Since
1.2.0
- Defined in array/groupBy.ts:26