derive

The Statety.derive function is used to create a derived state that automatically updates whenever its source state changes. Derived state is useful for values computed from a single piece of state, such as flags, transformations, or slices of larger objects.

TypeScript Signature

Statety.derive<T, U>(
  keyName: string,
  sourceKey: AnyStatetyKey<T>,
  fn: (state: T | null) => U
): DerivedStatetyKey<U>

Parameters

• keyName – A descriptive name for the derived key.
• sourceKey – The state key that this derived value depends on.
• computeFn – A function that receives the current value of the source key and returns the derived value.

Returns

• DerivedStatetyKey<U> – A new key representing the derived state.

Example

Creating a Derived State

import { Statety } from "statety";

const USER_KEY = Statety.create<{ username: string; role: string }>("user", {
  username: "guest",
  role: "visitor",
});

// Create a derived state for the user's role
const USER_ROLE_KEY = Statety.derive("user-role", USER_KEY, (user) => user?.role ?? "guest");

const role = Statety.read(USER_ROLE_KEY);
console.log(role); // Output: "visitor"

Notes

• Derived state cannot be set manually; it updates automatically when the source key changes.
• Derived state is ideal for transforming or selecting a portion of a single source state.
• You can derive from any type of state key.