/**
Extract all optional keys from the given type.

This is useful when you want to create a new type that contains different type values for the optional keys only.

@example
```
import type {OptionalKeysOf, Except} from 'type-fest';

interface User {
	name: string;
	surname: string;

	luckyNumber?: number;
}

const REMOVE_FIELD = Symbol('remove field symbol');
type UpdateOperation<Entity extends object> = Except<Partial<Entity>, OptionalKeysOf<Entity>> & {
	[Key in OptionalKeysOf<Entity>]?: Entity[Key] | typeof REMOVE_FIELD;
};

const update1: UpdateOperation<User> = {
	name: 'Alice'
};

const update2: UpdateOperation<User> = {
	name: 'Bob',
	luckyNumber: REMOVE_FIELD
};
```

@category Utilities
*/
export type OptionalKeysOf<BaseType extends object> = Exclude<{
	[Key in keyof BaseType]: BaseType extends Record<Key, BaseType[Key]>
		? never
		: Key
}[keyof BaseType], undefined>;