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

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 computed from "zustand-computed"

const computeState = (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)) })),
    }),
    computeState
  )
)

With types, the previous example would look like this:

import computed from "zustand-computed"

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

type ComputedStore = {
  countSq: number
}

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

// use curried create
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)) })),
    }),
    computeState
  )
)

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.

[!WARNING]
Types may not be as you expect when using Immer, as it derives the SetState type from the output of GetState, where zustand-computed makes SetState only allow the regular Store and the GetState return both the store and the computed store. To access the ComputedStore inside Immer, you will need to assert the Store type as Store & ComputedStore.

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

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 third opts variable to the computed constructor, e.g.

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 })),
    }),
    computeState,
    { disableProxy: true }
  )
)

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.