Literal types

A value can itself be a type: the type "md" accepts only the string "md". This is called a literal type. A union of several values, like "sm" | "md" | "lg", describes a fixed set of values and accepts nothing else. It does the job other languages use an enum for, and it's removed when the code is compiled, so it adds no code at all.

The benefit is that a typo becomes a type error. Right now padding("xl") returns 16 without any warning, because the final return 16 catches every value that isn't "sm" or "md". Once you add the type, that call is rejected before the code runs.

Watch out for the widening you saw in the type inference lesson: the type of let preferred = "lg" is string, not "lg", so it isn't accepted where a Size is expected. The fix is to use const, or to write the variable's type explicitly: let preferred: Size = "lg".