zustand-computed

zustand-computed is a lightweight, TypeScript-friendly middleware for the state management system Zustand. It's a simple layer which adds a transformation function after any state change in your store.

Install

# one of the following
npm i zustand-computed
pnpm i zustand-computed
bun add zustand-computed
yarn add zustand-computed

Usage

The middleware layer takes in your store creation function and a compute function, which transforms your state into a computed state. It does not need to handle merging states.

import { createComputed } from "zustand-computed"

const computed = createComputed((state) => ({
  countSq: state.count ** 2,
}))

const useStore = create(
  computed(
    (set, get) => ({
      count: 1,
      inc: () => set((state) => ({ count: state.count + 1 })),
      dec: () => set((state) => ({ count: state.count - 1 })),
      // get() function has access to ComputedStore
      square: () => set(() => ({ count: get().countSq })),
      root: () => set((state) => ({ count: Math.floor(Math.sqrt(state.count)) })),
    })
  )
)

With types, the previous example would look like this:

import { createComputed } from "zustand-computed"

type Store = {
  count: number
  inc: () => void
  dec: () => void
}

type ComputedStore = {
  countSq: number
}

const computed = createComputed((state: Store): ComputedStore => ({
  countSq: state.count ** 2,
}))

const useStore = create<Store>()(
  computed(
    (set) => ({
      count: 1,
      inc: () => set((state) => ({ count: state.count + 1 })),
      dec: () => set((state) => ({ count: state.count - 1 })),
      // get() function has access to ComputedStore
      square: () => set(() => ({ count: get().countSq })),
      root: () => set((state) => ({ count: Math.floor(Math.sqrt(state.count)) })),
    })
  )
)

The store can then be used as normal in a React component or via the Zustand API.

function Counter() {
  const { count, countSq, inc, dec } = useStore()
  return (
    <div>
      <span>{count}</span>
      <br />
      <span>{countSq}</span>
      <br />
      <button onClick={inc}>+1</button>
      <button onClick={dec}>-1</button>
    </div>
  )
}

A fully-featured example can be found under the "example" directory.

With Middleware

Here's an example with the Immer middleware.

const computed = createComputed((state: Store) => { /* ... */ })
const useStore = create<Store>()(
  devtools(
    immer(
      computed(
        (set) => ({
          count: 1,
          inc: () =>
            set((state) => {
              // example with Immer middleware
              state.count += 1
            }),
          dec: () => set((state) => ({ count: state.count - 1 })),
        })
      )
    )
  )
)

Selectors

By default, when zustand-computed runs your computeState function, it tracks accessed variables and does not trigger a computation if one of those variables do not change. This could potentially be problematic if you have nested control flow inside of computeState, or perhaps you want it to run on all changes regardless of use inside of computeState. To disable automatic selector detection, you can pass a second opts variable to the createComputed function, e.g.

const computed = createComputed((state: Store) => { /* ... */ }, { disableProxy: true })
const useStore = create<Store, [["chrisvander/zustand-computed", ComputedStore]]>(
  computed(
    (set) => ({
      count: 1,
      inc: () => set((state) => ({ count: state.count + 1 })),
      dec: () => set((state) => ({ count: state.count - 1 })),
    })
  )
)

Other options include passing a keys array, which explicitly spell out the selectors which trigger re-computation. You can also pass a custom equalityFn, such as fast-deep-equal instead of the default zustand/shallow.