# RefArray.groupBy

Group the values of a RefArray by a key.

## Signatures

```ts
export declare const groupBy: {
    <A>(f: (a: A) => string): <E, R>(ref: RefArray<A, E, R>) => RefSubject.Computed<R, E, Record<string, ReadonlyArray<A>>>;
    <A, E, R>(ref: RefArray<A, E, R>, f: (a: A) => string): RefSubject.Computed<R, E, Record<string, ReadonlyArray<A>>>;
};
```

## Why

Projects array state with group by for both current reads and future pushes, avoiding a second
mutable cache of the derived value.

## Ownership and lifetime

The group by view retains no independent state. An Effect read samples the source once; Fx
observation follows later pushes and its observing Scope owns subscription cleanup.