Function group

group<In, Out, Group, Inner>(f, inner): UnregisteredIndex<In, Out, GroupedIndex<In, Out, Group, Inner>>
Groups input by a function and maintains a separate index for each group.
Type Parameters
- In
- Out
- Group extends string | number
- Inner extends Index<In, Out, Inner>
Parameters
- f: ((_) => Group)
  - - (_): Group
    - Parameters
      
      _: In
      
      Returns Group
- inner: UnregisteredIndex<In, Out, Inner>
Returns UnregisteredIndex<In, Out, GroupedIndex<In, Out, Group, Inner>>
Example
```
const collection = new Collection<Readonly<{ 
 name: string,
 team: string,
}>>();

const ixTeamSizes = collection.registerIndex(
  group(
    (p) => p.team,
    countIndex()
  )
);

console.log(ixTeamSizes.get("Team One")?.value)
```
- Defined in core/src/core/Index.ts:92

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