TypeScript typings of EqualityFn

[SanderDeWaal1992]

The arguments of EqualityFn are defined as being unknown even though they are known (i.e. the same as ...newArgs of ResultFn).

Our build did fail today due to the recently added TypeScript types. My suggestion would be to type the args of EqualityFn as any (just like the typings on DefinitelyTyped did which does make that users do not have to cast the unknown type) or to strong-type it, e.g.:

 export type EqualityFn<Args = any> = (
  newArgs: Args,
  lastArgs: Args,
) => boolean;

export default function memoizeOne<
  ResultFn extends (this: any, ...newArgs: T) => ReturnType<ResultFn>, 
  T extends any[]
>(resultFn: ResultFn, isEqual: EqualityFn<T> = areInputsEqual): ResultFn {
...

[alexreardon]

I'll get onto this soon

[alexreardon]

I suspect this would be sufficient:

type EqualityFn = (newArgs: ReadonlyArray<any>, lastArgs: ReadonlyArray<any>) => boolean;

Do you think the readonly part will cause issues?

[alexreardon]

having a strong type on the equality function might be a bit lame for consumers

[SanderDeWaal1992]

I suspect this would be sufficient:

type EqualityFn = (newArgs: ReadonlyArray<any>, lastArgs: ReadonlyArray<any>) => boolean;

Do you think the readonly part will cause issues?

I made an example snippet:

const test = memoizeOne((
	arg1: string
) => {
	return "test" + arg1;
}, (
	a: Readonly<[string]>,
	b: Readonly<[string]>,
) => {
	return someCustomComparison(a as [string], b as [string]);
});

This will result in TS error:

Argument of type '(a: readonly [string], b: readonly [string]) => any' is not assignable to parameter of type 'EqualityFn'.
Types of parameters 'a' and 'newArgs' are incompatible.
Property '0' is missing in type 'readonly any[]' but required in type 'readonly [string]'.ts(2345)

I'm not sure how I -as a user- could type it correctly for TS. I think its obvious for users that the arguments should not be adjusted and that just 'any' should be fine.

[alexreardon]

I think this should be fine then:

type EqualityFn = (newArgs: any[], lastArgs: any[]) => boolean;

it is fairly loose, but i don't think the friction is worth typing it stricter for now

[alexreardon]

Fixed in 5.1.1