IsAlphabet

In this medium-level challenge, you'll implement an IsAlphabet<S> type that determines whether a given single-character string type is an alphabetic letter (a-z or A-Z).

type cases = [
  Expect<Equal<IsAlphabet<'A'>, true>>,
  Expect<Equal<IsAlphabet<'z'>, true>>,
  Expect<Equal<IsAlphabet<'9'>, false>>,
  Expect<Equal<IsAlphabet<'!'>, false>>,
  Expect<Equal<IsAlphabet<'😂'>, false>>,
  Expect<Equal<IsAlphabet<''>, false>>,
]

Detailed Explanation

type IsAlphabet<S extends string> =
  Uppercase<S> extends Lowercase<S>
    ? false
    : Lowercase<S> extends Uppercase<S>
      ? false
      : true;

How it works:

• For alphabetic characters, Uppercase and Lowercase produce different results (e.g., Uppercase<'a'> is 'A' and Lowercase<'a'> is 'a'). This means Uppercase<S> extends Lowercase<S> will be false for letters.
• For non-alphabetic characters (digits, symbols, emoji), Uppercase and Lowercase return the same string, so Uppercase<S> extends Lowercase<S> will be true, and we return false.
• The empty string '' also passes through Uppercase and Lowercase unchanged, so it correctly returns false.
• The double check ensures robustness -- we verify both directions of the case transformation differ.

An alternative approach explicitly lists all 26 letters:

type Alphabet = 'a' | 'b' | 'c' | 'd' | 'e' | 'f' | 'g' | 'h' | 'i' | 'j' | 'k' | 'l' | 'm' | 'n' | 'o' | 'p' | 'q' | 'r' | 's' | 't' | 'u' | 'v' | 'w' | 'x' | 'y' | 'z';
 
type IsAlphabet<S extends string> = Lowercase<S> extends Alphabet ? true : false;

This challenge helps you understand TypeScript's built-in string manipulation types (Uppercase and Lowercase) and how to apply them for character classification in real-world scenarios.