Make returned state (data, error, isValidating) readonly

[shuding]

We can use mark the properties of the SWR return state readonly, to avoid modifying them. For example it's dangerous to directly modify the data object, which could cause an infinite loop of rerender.

[swarup4741]

Can i work on this feature ?

[shuding]

Yep feel free to pick it up 👍

[swarup4741]

Could this be a possible fix ?

previously

swr/src/types.ts

Lines 69 to 79 in b819c25

	export type SWRHook = <Data = any, Error = any>(
	...args:
	| readonly [Key]
	| readonly [Key, Fetcher<Data> | null]
	| readonly [Key, SWRConfiguration<Data, Error> | undefined]
	| readonly [
	Key,
	Fetcher<Data> | null,
	SWRConfiguration<Data, Error> | undefined
	]
	) => SWRResponse<Data, Error>

After

export type SWRHook = <Data = Readonly<any>, Error = Readonly<any>>(
  ...args:
    | readonly [Key]
    | readonly [Key, Fetcher<Data> | null]
    | readonly [Key, SWRConfiguration<Data, Error> | undefined]
    | readonly [
        Key,
        Fetcher<Data> | null,
        SWRConfiguration<Data, Error> | undefined
      ]
) => SWRResponse<Data, Error>

[shuding]

I think this only marks the parameters as read only, not the return value.

[swarup4741]

I think this only marks the parameters as read only, not the return value.

Then i think we need to freeze the data and error object while returning them

return {
    mutate: boundMutate,
    get data() {
      stateDependencies.data = true
      return Object.freeze(data)
    },
    get error() {
      stateDependencies.error = true
      return Object.freeze(error)
    },
    get isValidating() {
      stateDependencies.isValidating = true
      return isValidating
    }
  } as SWRResponse<Data, Error>
}

Also i found another type error

swr/src/use-swr.ts

Lines 270 to 276 in b819c25

	cache.set(keyErr, err)
	if (stateRef.current.error !== err) {
	// Keep the stale data but update error.
	setState({
	isValidating: false,
	error: err
	})

here err has type unknown .... i think it shall be of type Error | undefined