Combining Type Aliases

You now have all the pieces: aliases, unions, intersections, shapes, literals. The real power appears when they reference each other. Aliases can be built from other aliases, layer by layer:

type Username = string
type UserAge = number

-- a union of two aliases
type ContactMethod = Username | UserAge

-- a shape whose fields use the aliases above
type UserProfile = {
    id: number,
    displayName: Username,
    preferredContact: ContactMethod,
}

-- an intersection extending the shape
type AdminProfile = UserProfile & { permissions: string }

Every layer stays readable: displayName: Username says far more than displayName: string, and AdminProfile is visibly "a UserProfile plus permissions". Update a base alias once, and every composed type inherits the change automatically.

Literal unions compose the same way:

type Theme = "light" | "dark"
type Mode = Theme | "auto"   -- "light" | "dark" | "auto"

Remember the alias is transparent: ContactMethod is still string | number underneath, so working with one means the same typeof narrowing you already know.