[Types] Consider exposing PropertyNames types

🚀 Feature Proposal

Please consider exposing NonFunctionPropertyNames<T>, FunctionPropertyNames<T> and ConstructorPropertyNames<T>

Motivation

Sometimes types are not infered as desired. Exposing these types could help developers to provide Typescript hints to infer types in some corner cases

Example

Consider your actual type spyOn:

    function spyOn<T extends {}, M extends NonFunctionPropertyNames<Required<T>>>(
        object: T,
        method: M,
        accessType: 'get'
    ): SpyInstance<Required<T>[M], []>;
    function spyOn<T extends {}, M extends NonFunctionPropertyNames<Required<T>>>(
        object: T,
        method: M,
        accessType: 'set'
    ): SpyInstance<void, [Required<T>[M]]>;
    function spyOn<T extends {}, M extends FunctionPropertyNames<Required<T>>>(
        object: T,
        method: M
    ): Required<T>[M] extends (...args: any[]) => any
        ? SpyInstance<ReturnType<Required<T>[M]>, ArgsType<Required<T>[M]>>
        : never;
    function spyOn<T extends {}, M extends ConstructorPropertyNames<Required<T>>>(
        object: T,
        method: M
    ): Required<T>[M] extends new (...args: any[]) => any
        ? SpyInstance<InstanceType<Required<T>[M]>, ConstructorArgsType<Required<T>[M]>>
        : never;

Consider a mock on global.Date:

   const dateSpy: jest.SpyInstance<Date> = jest.spyOn<
          NodeJS.Global,
          'Date'
    >(global, 'Date');

The actual code throws the following syntax error:

Type 'SpyInstance<string, []>' is not assignable to type 'SpyInstance<Date, any>'.
  Types of property 'mock' are incompatible.
    Type 'MockContext<string, []>' is not assignable to type 'MockContext<Date, any>'.
      Type 'string' is not assignable to type 'Date'.t

Why is this happening?

Well, if we look NodeJS.Global Types:

interface Global {
        Array: typeof Array;
        ArrayBuffer: typeof ArrayBuffer;
        Boolean: typeof Boolean;
        Buffer: typeof Buffer;
        DataView: typeof DataView;
        Date: typeof Date;
...
}

Then, if we look for Date

interface DateConstructor {
    new(): Date;
    new(value: number | string): Date;
    new(year: number, month: number, date?: number, hours?: number, minutes?: number, seconds?: number, ms?: number): Date;
    (): string;
    readonly prototype: Date;
    ...
}

declare var Date: DateConstructor;

As you can realize, Typescript tries to match spyOn in the order provided in the type file. As long as DateConstructor is a valid FunctionPropertyNames<T>, Typescript asumes I’m using the following call:

    function spyOn<T extends {}, M extends FunctionPropertyNames<Required<T>>>(
        object: T,
        method: M
    ): Required<T>[M] extends (...args: any[]) => any
        ? SpyInstance<ReturnType<Required<T>[M]>, ArgsType<Required<T>[M]>>
        : never;

But as long as Date includes (): string, Typescript infers its a SpyInstance<string, []>

Solution

Exposing these types (NonFunctionPropertyNames<T>, FunctionPropertyNames<T> and ConstructorPropertyNames<T>) would help us provide hints to Typescript:

    const dateSpy: jest.SpyInstance<Date> = jest.spyOn<
          NodeJS.Global,
          ConstructorPropertyNames<NodeJS.Global>
        >(global, 'Date');

Do you think it could be possible? Thank you so much for your time and effort

[mrazauskas]

Interestingly since #12089 jest-mock is already exporting MethodKeysOf, PropertyKeysOf, ConstructorArgumentsOf types which are equivalent to FunctionPropertyNames, NonFunctionPropertyNames, ConstructorPropertyNames (edit: apparently ConstructorPropertyNames wasn’t present in jest-mock types, the issue is referencing @types/jest):

https://github.com/facebook/jest/blob/7d4595eb935556b8214e1dda45b0b68dfc67068f/packages/jest-mock/src/index.ts#L36-L41

[SimenB]

PR welcome 🙂