Creates types that allow you to create a map of properties of certain type of an object. This map is then recognised by compiler to be compatible with object's keys.

PropertyTypeMapper.ts

/**
 * Creates a union type with names of all properties of T where S extends T[Key]
 *
 * @example
 * ```ts
 * type T = { a: string; b?: string; c: string | undefined; d: string | null; e: number };
 * type StringPropertiesFromT = PropertiesOfType<T, string>; // 'a' | 'b' | 'c' | 'd'
 * ```
 */
export type PropertiesOfType<T, S> = {
  [Key in keyof T]-?: S extends T[Key] ? (T[Key] extends S ? Key : never) : never;
}[keyof T];

/**
 * Creates a new union type with names of all properties T where S does not extend T[Key].
 * @example
 * ```ts
 * type T = { a: string; b?: string; c: string | undefined; d: string | null; e: number };
 * type StringPropertiesFromT = PropertiesOfType<T, string>; // 'e'
 * ```
 */
export type PropertiesNotOfType<T, S> = Exclude<keyof T, PropertiesOfType<T, S>>;

/**
 * Creates a new interface type from T with properties that extend S.
 *
 * @example
 * ```ts
 * type T = { a: string; b?: string; c: string | undefined; d: string | null; e: number };
 * type StringPropertiesFromT = KeysOfType<T, string>; // {a: string, b?: string, c: string | undefined, d: string | null}
 * ```
 */
// export type KeysOfType<T, S> = Pick<T, PropertiesOfType<T, S>>;
export type KeysOfType<T, S> = Omit<T, PropertiesNotOfType<T, S>>;

type T = { a: string; b?: string; c: string | undefined; d: string | null; e: number };
type StringPropertiesFromT = PropertiesOfType<T, string | null>; // 'a' | 'b' | 'c' | 'd'
type NotStringPropertiesFromT = PropertiesNotOfType<T, string | null>;
type StringPropertiesFromT = KeysOfType<T, string | undefined>;