createWithEqualityFnImpl in Zustand’s source code explained.

In this article, we will analyse how the createWithEqualityFnImpl is implemented by logging some of its value to get a better understanding.

As you can tell from the above image, createWithEqualityFn calls the function createWithEqualityFnImpl. This pattern is used in vanilla.ts as well as demonstrated below:

export const createStore = ((createState) =>
  createState ? createStoreImpl(createState) : createStoreImpl) as CreateStore

createStore calls createStoreImpl and createWithEqualityFn calls createWithhEqualityFnImpl.

Before we jump into the execution of createWithEqualityFn, let’s first understand the purpose of using createWithEqualityFn.

The above screenshot is from https://github.com/pmndrs/zustand/tree/main

You can use createWithEqualityFn to have more control over re-rendering.

createWithEqualityFn

const createWithEqualityFnImpl = <T>(
  createState: StateCreator<T, [], []>,
  defaultEqualityFn?: <U>(a: U, b: U) => boolean,
) => {
  const api = createStore(createState)

  const useBoundStoreWithEqualityFn: any = (
    selector?: any,
    equalityFn = defaultEqualityFn,
  ) => useStoreWithEqualityFn(api, selector, equalityFn)

  Object.assign(useBoundStoreWithEqualityFn, api)

  return useBoundStoreWithEqualityFn
}

useStoreWithEqualityFn

export function useStoreWithEqualityFn<S extends ReadonlyStoreApi<unknown>>(
  api: S,
): ExtractState<S>

export function useStoreWithEqualityFn<S extends ReadonlyStoreApi<unknown>, U>(
  api: S,
  selector: (state: ExtractState<S>) => U,
  equalityFn?: (a: U, b: U) => boolean,
): U

export function useStoreWithEqualityFn<TState, StateSlice>(
  api: ReadonlyStoreApi<TState>,
  selector: (state: TState) => StateSlice = identity as any,
  equalityFn?: (a: StateSlice, b: StateSlice) => boolean,
) {
  const slice = useSyncExternalStoreWithSelector(
    api.subscribe,
    api.getState,
    api.getInitialState,
    selector,
    equalityFn,
  )
  useDebugValue(slice)
  return slice
}

This is an overloading function. useStoreWithEqualityFn has 3 definitions:

export function useStoreWithEqualityFn<S extends ReadonlyStoreApi<unknown>>(
  api: S,
): ExtractState<S>

export function useStoreWithEqualityFn<S extends ReadonlyStoreApi<unknown>, U>(
  api: S,
  selector: (state: ExtractState<S>) => U,
  equalityFn?: (a: U, b: U) => boolean,
): U

export function useStoreWithEqualityFn<TState, StateSlice>(
  api: ReadonlyStoreApi<TState>,
  selector: (state: TState) => StateSlice = identity as any,
  equalityFn?: (a: StateSlice, b: StateSlice) => boolean,
) {

Let’s see how the slice value looks like:

value: 1 is skipped because we set the code to avoid re-render when the value is 1 as shown below:

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References:

1. https://github.com/pmndrs/zustand/blob/main/src/traditional.ts#L74

2. https://github.com/pmndrs/zustand/blob/main/src/traditional.ts#L39