// Type User can have an ID which is either a number or a string type User<CustomType extends (number | string)> = { id: CustomType, name?: string, age?: number }
// In this case, we define CustomType as a string let myUser:User = { id: "1234-1234-1234", name: "John Doe", age: 24 }
// In this case, we define CustomType as a number let myOtherUser:User = { id: 1234, name: "Jane Seymore", age: 48 }