zustand/docs/typescript.md

TypeScript Guide

Basic usage

When using TypeScript you just have to make a tiny change that instead of writing create(...) you'll have to write create<T>()(...) where T would be type of the state so as to annotate it. Example...

import create from "zustand"

interface BearState {
  bears: number
  increase: (by: number) => void
}

const useStore = create<BearState>()((set) => ({
  bears: 0,
  increase: (by) => set((state) => ({ bears: state.bears + by })),
}))

Why can't we just simply infer the type from initial state?

TLDR: Because state generic T is invariant.

Consider this minimal version create...

declare const create: <T>(f: (get: () => T) => T) => T

const x = create((get) => ({
  foo: 0,
  bar: () => get()
}))
// `x` is inferred as `unknown` instead of
// interface X {
//   foo: number,
//   bar: () => X
// }

Here if you look at the type of f in create ie (get: () => T) => T it "gives" T as it returns T but then it also "takes" T via get so where does T come from TypeScript thinks... It's a like that chicken or egg problem. At the end TypeScript gives up and infers T as unknown.

So as long as the generic to be inferred is invariant TypeScript won't be able to infer it. Another simple example would be this...

declare const createFoo: <T>(f: (t: T) => T) => T
const x = createFoo(_ => "hello")

Here again x is unknown instead of string.

Now one can argue it's impossible to write an implementation for createFoo, and that's true. But then it's also impossible to write Zustand's create... Wait but Zustand exists? So what do I mean by that?

The thing is Zustand is lying in it's type, the simplest way to prove it by showing unsoundness. Consider this example...

import create from "zustand/vanilla"

const useStore = create<{ foo: number }>()((_, get) => ({
  foo: get().foo,
}))

This code compiles, but guess what happens when you run it? You'll get an exception "Uncaught TypeError: Cannot read properties of undefined (reading 'foo') because after all get would return undefined before the initial state is created (hence kids don't call get when creating the initial state). But the types tell that get is () => { foo: number } which is exactly the lie I was taking about, get is that eventually but first it's () => undefined.

Okay we're quite deep in the rabbit hole haha, long story short zustand has a bit crazy runtime behavior that can't be typed in a sound way and inferrable way. We could make it inferrable with the right TypeScript features that don't exist today. And hey that tiny bit of unsoundness is not a problem.

Why that currying `()(...)`?

TLDR: It's a workaround for microsoft/TypeScript#10571.

Imagine you have a scenario like this...

declare const withError: <T, E>(p: Promise<T>) =>
  Promise<[error: undefined, value: T] | [error: E, value: undefined]>
declare const doSomething: () => Promise<string>

const main = async () => {
  let [error, value] = await withError(doSomething())
}

Here T is inferred as string and E is inferred as unknown. Now for some reason you want to annotate E as Foo because you're certain what shape of error doSomething() would throw. But too bad you can't do that, you can either pass all generics or none. So now along with annotating E as Foo you'll also have to annotate T as string which gets inferred anyway. So what to do? What you do is make a curried version of withError that does nothing in runtime, it's purpose is to just allow you annotate E...

declare const withError: {
  <E>(): <T>(p: Promise<T>) =>
    Promise<[error: undefined, value: T] | [error: E, value: undefined]>
  <T, E>(p: Promise<T>):
    Promise<[error: undefined, value: T] | [error: E, value: undefined]>
}
declare const doSomething: () => Promise<string>
interface Foo { bar: string }

const main = async () => {
  let [error, value] = await withError<Foo>()(doSomething())
}

And now T gets inferred and you get to annotate E too. Zustand has the same use case we want to annotate the state (the first type parameter) but allow the rest type parameters to get inferred.

Alternatively you can also use combine which infers the state instead of you having to type it...

import create from "zustand"
import { combine } from "zustand/middleware"

const useStore = create(combine({ bears: 0 }, (set) => ({
  increase: (by: number) => set((state) => ({ bears: state.bears + by })),
}))

But be a little careful...

We achieve the inference by lying a little in the types of set, get and store that you receive as parameters. The lie is that they're typed in a way as if the state is the first parameter only when in fact the state is the shallow-merge ({ ...a, ...b }) of both first parameter and the second parameter's return. So for example get from the second parameter has type () => { bears: number } and that's a lie as it should be () => { bears: number, increase: (by: number) => void }. And useStore still has the correct type, ie for example useStore.getState is typed as () => { bears: number, increase: (by: number) => void }.

It's not a lie lie because { bears: number } is still a subtype { bears: number, increase: (by: number) => void }, so in most cases there won't be a problem. Just you have to be careful while using replace. For eg set({ bears: 0 }, true) would compile but will be unsound as it'll delete the increase function. (If you set from "outside" ie useStore.setState({ bears: 0 }, true) then it won't compile because the "outside" store knows that increase is missing.) Another instance where you should be careful you're doing Object.keys, Object.keys(get()) will return ["bears", "increase"] and not ["bears"] (the return type of get can make you fall for this).

So combine trades-off a little type-safety for the convience of not having to write a type for state. Hence you should use combine accordingly, usually it's not a big deal and it's okay to use it.

Using middlewares

You don't have to do anything special to use middlewares in TypeScript.

import create from "zustand"
import { devtools, persist } from "zustand/middleware"

interface BearState {
  bears: number
  increase: (by: number) => void
}

const useStore = create<BearState>()(devtools(persist((set) => ({
  bears: 0,
  increase: (by) => set((state) => ({ bears: state.bears + by })),
}))))

Just make sure you're using them immediately inside create so as to make the contextual inference work. Doing something even remotely fancy like the following myMiddlewares would require more advanced types.

import create from "zustand"
import { devtools, persist } from "zustand/middleware"

const myMiddlewares = f => devtools(persist(f))

interface BearState {
  bears: number
  increase: (by: number) => void
}

const useStore = create<BearState>()(myMiddlewares((set) => ({
  bears: 0,
  increase: (by) => set((state) => ({ bears: state.bears + by })),
})))

Authoring middlewares and advanced usage

Imagine you had to write this hypothetical middleware...

import create from "zustand"

const foo = (f, bar) => (set, get, store) => {
  store.foo = bar
  return f(set, get, store);
}

const useStore = create(foo(() => ({ bears: 0 }), "hello"))
console.log(store.foo.toUpperCase())

Yes, if you didn't know Zustand middlewares do and are allowed to mutate the store. But how could we possibly encode the mutation on the type-level? That is to say how could do we type foo so that this code compiles?

For an usual statically typed language this is impossible, but thanks to TypeScript, Zustand has something called an "higher kinded mutator" that makes this possible. If you're dealing with complex type problems like typing a middleware or using the StateCreator type, then you'll have to understand this implementation detail, for that check out #710.

If you're eager to know what the answer is to this particular problem then it's here.

Common recipes

Middleware that does not change the store type

import create, { State, StateCreator, StoreMutatorIdentifier, Mutate, StoreApi } from "zustand"

type Logger =
  < T extends State
  , Mps extends [StoreMutatorIdentifier, unknown][] = []
  , Mcs extends [StoreMutatorIdentifier, unknown][] = []
  >
    ( f: StateCreator<T, Mps, Mcs>
    , name?: string
    ) =>
      StateCreator<T, Mps, Mcs>

type LoggerImpl =
  <T extends State>
    ( f: PopArgument<StateCreator<T, [], []>>
    , name?: string
    ) =>
      PopArgument<StateCreator<T, [], []>>

const loggerImpl: LoggerImpl = (f, name) => (set, get, store) => {
  type T = ReturnType<typeof f>
  const loggedSet: typeof set = (...a) => {
    set(...a)
    console.log(...(name ? [`${name}:`] : []), get())
  }
  store.setState = loggedState

  return f(loggedSet, get, store)
}

export const logger = loggerImpl as unknown as Foo

type PopArgument<T extends (...a: never[]) => unknown> =
  T extends (...a: [...infer A, infer _]) => infer R
    ? (...a: A) => R
    : never

// ---

const useStore = create<BearState>()(logger((set) => ({
  bears: 0,
  increase: (by) => set((state) => ({ bears: state.bears + by })),
}), "bear-store"))

Middleware that changes the store type

import create, { State, StateCreator, StoreMutatorIdentifier, Mutate, StoreApi } from "zustand"

type Foo =
  < T extends State
  , A
  , Mps extends [StoreMutatorIdentifier, unknown][] = []
  , Mcs extends [StoreMutatorIdentifier, unknown][] = []
  >
    ( f: StateCreator<T, [...Mps, ['foo', A]], Mcs>
    , bar: A
    ) =>
      StateCreator<T, Mps, [['foo', A], ...Mcs]>

declare module 'zustand' {
  interface StoreMutators<S, A> {
    foo: Write<Cast<S, object> { foo: A }>
  }
}

type FooImpl =
  <T extends State, A>
    ( f: PopArgument<StateCreator<T, [], []>>
    , bar: A
    ) => PopArgument<StateCreator<T, [], []>>

const fooImpl: FooImpl = (f, bar) => (set, get, _store) => {
  type T = ReturnType<typeof f>
  type A = typeof bar

  const store = _store as Mutate<StoreApi<T>, [['foo', A]]>
  store.foo = bar
  return f(set, get, _store)
}

export const foo = fooImpl as unknown as Foo

type PopArgument<T extends (...a: never[]) => unknown> =
  T extends (...a: [...infer A, infer _]) => infer R
    ? (...a: A) => R
    : never

type Write<T extends object, U extends object> =
  Omit<T, keyof U> & U

type Cast<T, U> =
  T extends U ? T : U;

// ---

const useStore = create(foo(() => ({ bears: 0 }), "hello"))
console.log(store.foo.toUpperCase())

create without curried workaround

The recommended way to use create is using the curried workaround ie create<T>()(...) because this enabled you to infer the store type. But for some reason if you don't want to use the workaround then you can pass the type parameters like the following. Note that in some cases this acts as an assertion instead of annotation, so it's not recommended.

import create, { State, StateCreator, StoreMutatorIdentifier, Mutate, StoreApi } from "zustand"

interface BearState {
  bears: number
  increase: (by: number) => void
}

const useStore = create<
  BearState,
  [
    ['zustand/persist', BearState],
    ['zustand/devtools', never]
  ]
>(devtools(persist((set) => ({
  bears: 0,
  increase: (by) => set((state) => ({ bears: state.bears + by })),
})))

Independent slices pattern

import create, { State, StateCreator, StoreMutatorIdentifier, Mutate, StoreApi } from "zustand"

interface BearSlice {
  bears: number
  addBear: () => void
}
const createBearSlice: StateCreator<BearSlice, [], []> = (set) => ({
  bears: 0,
  addBear: () => set((state) => ({ bears: state.bears + 1 }))
})

interface FishSlice {
  fishes: number
  addFish: () => void
}
const createFishSlice: StateCreator<FishSlice, [], []> = (set) => ({
  fishes: 0,
  addFish: () => set((state) => ({ fishes: state.fishes + 1 }))
})

const useStore = create<BearSlice & FishSlice>()((...a) => ({
  ...createBearSlice(...a),
  ...createFishSlice(...a)
}))

If you have some middlewares then replace StateCreator<MySlice, [], []> with StateCreator<MySlice, Mutators, []>. Eg if you're using devtools then it'll be StateCreator<MySlice, [["zustand/devtools", never]], []>.

Also you can even write StateCreator<MySlice> instead of StateCreator<MySlice, [], []> as the second and third parameter have [] as their default value.

Interdependent slices pattern

import create, { State, StateCreator, StoreMutatorIdentifier, Mutate, StoreApi } from "zustand"

interface BearSlice {
  bears: number
  addBear: () => void
  eatFish: () => void
}
const createBearSlice: StateCreator<BearSlice & FishSlice, [], [], BearSlice> = (set) => ({
  bears: 0,
  addBear: () => set((state) => ({ bears: state.bears + 1 })),
  eatFish: () => set((state) => ({ fishes: state.fishes - 1 }))
})

interface FishSlice {
  fishes: number
  addFish: () => void
}
const createFishSlice: StateCreator<BearSlice & FishSlice, [], [], FishSlice> = (set) => ({
  fishes: 0,
  addFish: () => set((state) => ({ fishes: state.fishes + 1 }))
})

const useStore = create<BearSlice & FishSlice>()((...a) => ({
  ...createBearSlice(...a),
  ...createFishSlice(...a)
}))

If you have some middlewares then replace StateCreator<MyState, [], [], MySlice> with StateCreator<MyState, Mutators, [], MySlice>. Eg if you're using devtools then it'll be StateCreator<MyState, [["zustand/devtools", never]], [], MySlice>.