types.ts

// union
type StringOrNumber = string | number;

// intersection
type User = { id: number };
type Admin = { isAdmin: boolean };
type AdminUser = User & Admin; // { id: number; isAdmin: boolean; }

// return type
type GetUserType = () => { id: number; name: string };
type UserType = ReturnType<GetUserType>; // { id: number; name: string }

// return type pres interface
interface StringFormat {
    (str: string, isUpper: boolean): string;
}

let format: StringFormat;

format = function (str: string, isUpper: boolean) {
    return isUpper ? str.toLocaleUpperCase() : str.toLocaleLowerCase();
};

console.log(format('hi', true));