Typescript get optional keys from interface

optionalKeys.d.ts

type Undefined<T> = { [P in keyof T]: P extends undefined ? T[P] : never }

type FilterFlags<Base, Condition> = {
  [Key in keyof Base]:
  Base[Key] extends Condition ? Key : never
};

type AllowedNames<Base, Condition> =
  FilterFlags<Base, Condition>[keyof Base];

type SubType<Base, Condition> =
  Pick<Base, AllowedNames<Base, Condition>>;

type OptionalKeys<T> = Exclude<keyof T, NonNullable<keyof SubType<Undefined<T>, never>>>


interface Something {
  firstName: string
  lastName: string
  phone?: string
  age: number
}
type AllOptionalKeys = OptionalKeys<Something> // "phone"

Could you check if {} extends Pick<T, Key>? Something like this, since undefined is different than optional..

type OptionalKeys<T extends object> = { [P in keyof T]: {} extends Pick<T, P> ? P : never }[keyof T]

@gisheri yeah that works. A lot simpler. Can't even remember now why I was using this 😂

Could you check if {} extends Pick<T, Key>? Something like this, since undefined is different than optional..

type OptionalKeys<T extends object> = { [P in keyof T]: {} extends Pick<T, P> ? P : never }[keyof T]

Can you tell me how this works. {} extends Pick<T, P>
Optional and non-optional attribute types do not behave the same way for this