Function groupBy

groupBy<T, K>(arr: T[], keyFn: (item: T) => K): Map<K, T[]>
Group array elements by a key function.
Type Parameters
- T
- K
Parameters
- arr: T[]
- keyFn: (item: T) => K
Returns Map<K, T[]>
A Map from key to array of elements that share that key.
Example
```
const notes = line.notes ?? [];
const byType = groupBy(notes, n => n.type);
// byType.get(1) → all tap notes
// byType.get(2) → all hold notes
```
- Defined in math.ts:621