derive

Creates a derived store from an existing store.

A derived store computes its state based on the original store’s state using a derive function. The derived store’s value is memoized and only recomputed when necessary.

const derivedStore = derive(store, deriveFn, dependencyFn?);

Parameters

• store - The original store to derive from
• deriveFn - A function that computes the derived state from the original state
• dependencyFn? - Optional function that returns an array of dependencies. If provided, the derived state will only be recomputed when these dependencies change

Note

The dependencyFn introduces overhead and is only necessary for heavy computations and/or if you need granular control over when the derived state is recomputed. It is provided in the code example for demonstration purposes.

Returns

A new store containing the derived state.

Actions

• mount - Start listening to changes in the original store. Called automatically on the first subscription.
• unmount - Stop listening to changes in the original store. Called automatically when there are no more subscribers.

Usage

import { derive } from "dharma-core";
import { counterStore } from "./counterStore";

export const doubleCounterStore = derive(
  counterStore,
  (state) => state.count * 2,
  (state) => [state.count],
);