# Derived State

Derived state, also called computed properties, are values that depend on other state properties. Let's continue with our counter store from the State section and add a derived state property that doubles count.

# Use

const state = {
  count: 1,
  doubled: ({ count }) => count * 2
};

Unifire expects that any functions present in the state object are computed properties. When Unifire encounters a derivation function, it determines that function's state dependencies (in this case count). When any of those dependencies change, Unifire executes all of the derivation's subscribers.

For example, the following subscriber will only run when count changes.

store.subscribe(({ doubled }) => console.log('doubled', doubled));

Derived state can depend on other derived state.

const state = {
  count: 1,
  doubled: ({ count }) => count * 2,
  quadrupled: ({ doubled }) => doubled * 2
}

Unifire does not support making assignments to derived state.

# Memoization

Derived state methods run every time the property they represent is accessed. For simple computations such as string concatenation or basic arithmetic, this is fine. However, for more expensive computations, you may want to memoize your derivation method. Unifire exports a memoize method written specifically for derivations.

import Unifire from 'unifire';
import { memoize } from 'unifire/utils';

const expensiveMethod = ({ some, state }) => { ... };

const state = {
  count: 1,
  memoized: memoize(expensiveMethod)
}

Now store.state.memoized will only run once when called with identical input.

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