typescript.md

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TypeScript Guide	8

Basic usage

The difference when using TypeScript is that instead of writing create(...), you have to write create<T>()(...) (notice the extra parentheses () too along with the type parameter) where T is the type of the state to annotate it. For example:

import { create } from 'zustand'

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

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

Why can't we simply infer the type from the 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, i.e. (get: () => T) => T, it "gives" T via return (making it covariant), but it also "takes" T via get (making it contravariant). "So where does T come from?" TypeScript wonders. It's 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 (i.e. both covariant and contravariant), TypeScript will be unable to infer it. Another simple example would be this:

const createFoo = {} as <T>(f: (t: T) => T) => T
const x = createFoo((_) => 'hello')

Here again, x is unknown instead of string.

More about the inference (just for the people curious and interested in TypeScript)

In some sense this inference failure is not a problem because a value of type <T>(f: (t: T) => T) => T cannot be written. That is to say you can't write the real runtime implementation of createFoo. Let's try it:

const createFoo = (f) => f(/* ? */)

createFoo needs to return the return value of f. And to do that we first have to call f. And to call it we have to pass a value of type T. And to pass a value of type T we first have to produce it. But how can we produce a value of type T when we don't even know what T is? The only way to produce a value of type T is to call f, but then to call f itself we need a value of type T. So you see it's impossible to actually write createFoo.

So what we're saying is, the inference failure in case of createFoo is not really a problem because it's impossible to implement createFoo. But what about the inference failure in case of create? That also is not really a problem because it's impossible to implement create too. Wait a minute, if it's impossible to implement create then how does Zustand implement it? The answer is, it doesn't.

Zustand lies that it implemented create's type, it implemented only the most part of it. Here's a simple proof by showing unsoundness. Consider the following code:

import { create } from 'zustand'

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

This code compiles. But if we run it, we'll get an exception: "Uncaught TypeError: Cannot read properties of undefined (reading 'foo')". This is because get would return undefined before the initial state is created (hence you shouldn't call get when creating the initial state). The types promise that get will never return undefined but it does initially, which means Zustand failed to implement it.

And of course Zustand failed because it's impossible to implement create the way types promise (in the same way it's impossible to implement createFoo). In other words we don't have a type to express the actual create we have implemented. We can't type get as () => T | undefined because it would cause inconvenience and it still won't be correct as get is indeed () => T eventually, just if called synchronously it would be () => undefined. What we need is some kind of TypeScript feature that allows us to type get as (() => T) & WhenSync<() => undefined>, which of course is extremely far-fetched.

So we have two problems: lack of inference and unsoundness. Lack of inference can be solved if TypeScript can improve its inference for invariants. And unsoundness can be solved if TypeScript introduces something like WhenSync. To work around lack of inference we manually annotate the state type. And we can't work around unsoundness, but it's not a big deal because it's not much, calling get synchronously anyway doesn't make sense.

Why the currying `()(...)`?

TLDR: It is 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 to be a string and E is inferred to be unknown. You might want to annotate E as Foo, because you are certain of the shape of error doSomething() would throw. However, you can't do that. You can either pass all generics or none. Along with annotating E as Foo, you will also have to annotate T as string even though it gets inferred anyway. The solution is to make a curried version of withError that does nothing at runtime. Its 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())
}

This way, T gets inferred and you get to annotate E. Zustand has the same use case when we want to annotate the state (the first type parameter) but allow other parameters to get inferred.

Alternatively, you can also use combine, which infers the state so that you do not need to type it.

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

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

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 as if the state is the first parameter, when in fact the state is the shallow-merge ({ ...a, ...b }) of both first parameter and the second parameter's return. For example, get from the second parameter has type () => { bears: number } and that is a lie as it should be () => { bears: number, increase: (by: number) => void }. And useBearStore still has the correct type; for example, useBearStore.getState is typed as () => { bears: number, increase: (by: number) => void }.

It isn't really a lie because { bears: number } is still a subtype of { bears: number, increase: (by: number) => void }. Therefore, there will be no problem in most cases. You should just be careful while using replace. For example, set({ bears: 0 }, true) would compile but will be unsound as it will delete the increase function. Another instance where you should be careful is if you use Object.keys. Object.keys(get()) will return ["bears", "increase"] and not ["bears"]. The return type of get can make you fall for these mistakes.

combine trades off a little type-safety for the convenience of not having to write a type for state. Hence, you should use combine accordingly. It is fine in most cases and you can use it conveniently.

Note that we don't use the curried version when using combine because combine "creates" the state. When using a middleware that creates the state, it isn't necessary to use the curried version because the state now can be inferred. Another middleware that creates state is redux. So when using combine, redux, or any other custom middleware that creates the state, we don't recommend using the curried version.

Using middlewares

You do not 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 useBearStore = create<BearState>()(
  devtools(
    persist(
      (set) => ({
        bears: 0,
        increase: (by) => set((state) => ({ bears: state.bears + by })),
      }),
      { name: 'bearStore' },
    ),
  ),
)

Just make sure you are 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, { name: 'bearStore' }))

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

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

Also, we recommend using devtools middleware as last as possible. For example, when you use it with immer as a middleware, it should be devtools(immer(...)) and not immer(devtools(...)). This is becausedevtools mutates the setState and adds a type parameter on it, which could get lost if other middlewares (like immer) also mutate setState before devtools. Hence using devtools at the end makes sure that no middlewares mutate setState before it.

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 useBearStore = create(foo(() => ({ bears: 0 }), 'hello'))
console.log(useBearStore.foo.toUpperCase())

Zustand middlewares can 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 a usual statically typed language, this is impossible. But thanks to TypeScript, Zustand has something called a "higher-kinded mutator" that makes this possible. If you are dealing with complex type problems, like typing a middleware or using the StateCreator type, you will have to understand this implementation detail. For this, you can check out #710.

If you are eager to know what the answer is to this particular problem then you can see it here.

Common recipes

Middleware that doesn't change the store type

import { create, State, StateCreator, StoreMutatorIdentifier } 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: StateCreator<T, [], []>,
  name?: string,
) => 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 ? [`${name}:`] : []), get())
  }
  const setState = store.setState
  store.setState = (...a) => {
    setState(...a)
    console.log(...(name ? [`${name}:`] : []), store.getState())
  }

  return f(loggedSet, get, store)
}

export const logger = loggerImpl as unknown as Logger

// ---

const useBearStore = 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: StateCreator<T, [], []>,
  bar: A,
) => 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 Write<T extends object, U extends object> = Omit<T, keyof U> & U

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

// ---

const useBearStore = create(foo(() => ({ bears: 0 }), 'hello'))
console.log(useBearStore.foo.toUpperCase())

create without curried workaround

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

import { create } from "zustand"

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

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

Slices pattern

import { create, StateCreator } from 'zustand'

interface BearSlice {
  bears: number
  addBear: () => void
  eatFish: () => void
}

interface FishSlice {
  fishes: number
  addFish: () => void
}

interface SharedSlice {
  addBoth: () => void
  getBoth: () => void
}

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

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

const createSharedSlice: StateCreator<
  BearSlice & FishSlice,
  [],
  [],
  SharedSlice
> = (set, get) => ({
  addBoth: () => {
    // you can reuse previous methods
    get().addBear()
    get().addFish()
    // or do them from scratch
    // set((state) => ({ bears: state.bears + 1, fishes: state.fishes + 1 })
  },
  getBoth: () => get().bears + get().fishes,
})

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

A detailed explanation on the slices pattern can be found here.

If you have some middlewares then replace StateCreator<MyState, [], [], MySlice> with StateCreator<MyState, Mutators, [], MySlice>. For example, if you are using devtools then it will be StateCreator<MyState, [["zustand/devtools", never]], [], MySlice>. See the "Middlewares and their mutators reference" section for a list of all mutators.

Bounded useStore hook for vanilla stores

import { useStore } from 'zustand'
import { createStore } from 'zustand/vanilla'

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

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

function useBearStore(): BearState
function useBearStore<T>(selector: (state: BearState) => T): T
function useBearStore<T>(selector?: (state: BearState) => T) {
  return useStore(bearStore, selector!)
}

You can also make an abstract createBoundedUseStore function if you need to create bounded useStore hooks often and want to DRY things up...

import { useStore, StoreApi } from 'zustand'
import { createStore } from 'zustand/vanilla'

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

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

const createBoundedUseStore = ((store) => (selector) => useStore(store)) as <
  S extends StoreApi<unknown>,
>(
  store: S,
) => {
  (): ExtractState<S>
  <T>(selector: (state: ExtractState<S>) => T): T
}

type ExtractState<S> = S extends { getState: () => infer X } ? X : never

const useBearStore = createBoundedUseStore(bearStore)

Middlewares and their mutators reference

• devtools — ["zustand/devtools", never]
• persist — ["zustand/persist", YourPersistedState]
YourPersistedState is the type of state you are going to persist, ie the return type of options.partialize, if you're not passing partialize options the YourPersistedState becomes Partial<YourState>. Also sometimes passing actual PersistedState won't work. In those cases, try passing unknown.
• immer — ["zustand/immer", never]
• subscribeWithSelector — ["zustand/subscribeWithSelector", never]
• redux — ["zustand/redux", YourAction]
• combine — no mutator as combine does not mutate the store