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

1# one of the following
2npm i zustand-computed
3pnpm i zustand-computed
4bun add zustand-computed
5yarn 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.

1import { createComputed } from "zustand-computed"
2
3const computed = createComputed((state) => ({
4  countSq: state.count ** 2,
5}))
6
7const useStore = create(
8  computed(
9    (set, get) => ({
10      count: 1,
11      inc: () => set((state) => ({ count: state.count + 1 })),
12      dec: () => set((state) => ({ count: state.count - 1 })),
13      // get() function has access to ComputedStore
14      square: () => set(() => ({ count: get().countSq })),
15      root: () => set((state) => ({ count: Math.floor(Math.sqrt(state.count)) })),
16    })
17  )
18)

With types, the previous example would look like this:

1import { createComputed } from "zustand-computed"
2
3type Store = {
4  count: number
5  inc: () => void
6  dec: () => void
7}
8
9type ComputedStore = {
10  countSq: number
11}
12
13const computed = createComputed((state: Store): ComputedStore => ({
14  countSq: state.count ** 2,
15}))
16
17const useStore = create<Store>()(
18  computed(
19    (set) => ({
20      count: 1,
21      inc: () => set((state) => ({ count: state.count + 1 })),
22      dec: () => set((state) => ({ count: state.count - 1 })),
23      // get() function has access to ComputedStore
24      square: () => set(() => ({ count: get().countSq })),
25      root: () => set((state) => ({ count: Math.floor(Math.sqrt(state.count)) })),
26    })
27  )
28)

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

1function Counter() {
2  const { count, countSq, inc, dec } = useStore()
3  return (
4    <div>
5      <span>{count}</span>
6      <br />
7      <span>{countSq}</span>
8      <br />
9      <button onClick={inc}>+1</button>
10      <button onClick={dec}>-1</button>
11    </div>
12  )
13}

With Middleware

Here's an example with the Immer middleware.

1const computed = createComputed((state: Store) => { /* ... */ })
2const useStore = create<Store>()(
3  devtools(
4    immer(
5      computed(
6        (set) => ({
7          count: 1,
8          inc: () =>
9            set((state) => {
10              // example with Immer middleware
11              state.count += 1
12            }),
13          dec: () => set((state) => ({ count: state.count - 1 })),
14        })
15      )
16    )
17  )
18)

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.

1const computed = createComputed((state: Store) => { /* ... */ }, { disableProxy: true })
2const useStore = create<Store, [["chrisvander/zustand-computed", ComputedStore]]>(
3  computed(
4    (set) => ({
5      count: 1,
6      inc: () => set((state) => ({ count: state.count + 1 })),
7      dec: () => set((state) => ({ count: state.count - 1 })),
8    })
9  )
10)

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.