fix: FunctionKey and NonFunctionKeys

[promer94]

#41 Description
FunctionKeys and NonFunctionKeys cant work with optional keys
example

type FunctionKeys<T extends object> = {
  [K in keyof T]: T[K] extends Function ? K : never
}[keyof T]

export type NonFunctionKeys<T extends object> = {
  [K in keyof T]: T[K] extends Function ? never : K
}[keyof T];

type MixedProps = {
  name: string;
  setName: (name: string) => void;
  someKeys?: string;
  someFn?: (...args: any) => any;
};
// "setName" | undefined
type Keys = FunctionKeys<MixedProps>
// "name" | "someKeys" | "someFn" | undefined
type newKey = NonFunctionKeys<MixedProps>

Checklist

• I have read CONTRIBUTING.md
• I have linked all related issues above
• I have rebased my branch

For bugfixes:

• I have added at least one unit test to confirm the bug have been fixed
• I have checked and updated TOC and API Docs when necessary

[piotrwitek]

Hey @promer94, thanks for the PR. The current behavior is intentional.

To get non-nullable keys you could use type composition like this:

type OmitOptional<T> = Omit<T, OptionalKeys<T>>;
type Keys = FunctionKeys<OmitOptional<MixedProps>>

I like that you extended tests, and I would gladly merge them but without optional keys change highlighted in the comments.

[piotrwitek]

Please remove this change, it's not desired.

[piotrwitek]

• // Expect: "name" | "someKeys"

[piotrwitek]

// Expect: "setName" | "someFn"

[promer94]

Thanks for your feedback 😀.
But i'm confused now.

type FunctionKeys<T extends object> = {
  [K in keyof T]: T[K] extends Function ? K : never
}[keyof T]

type MixedProps = {
  name: string;
  setName: (name: string) => void;
  someKeys?: string;
  someFn?: (...args: any) => any;
};
// Expect: "setName" | undefined  
type Keys = FunctionKeys<MixedProps>

if i don't change the implementation, the type of Keys would be 'setName' | undefined rather than 'setName' | 'someFn'.

I'm new to Typescript, let me know if i am wrong

[piotrwitek]

@promer94 Ok I see now, it seems the type is broken, it definitely shouldn't include undefined.
Could you please try to exclude only undefined value but still preserving optional keys?

This should work

export type FunctionKeys<T extends object> = {
  [K in keyof T]-?: NonUndefined<T[K]> extends Function ? K : never
}[keyof T];

[piotrwitek]

That looks sweet! Great job @promer94, thanks.