35252 · IsAlphabet

Challenge Link

Problem

Implement IsAlphabet<S> that returns true if S is a single alphabetic character (a–z or A–Z), and false otherwise.

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>>,
]

Solution

type LowerAlphabet = '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 UpperAlphabet = Uppercase<LowerAlphabet>
type Alphabet = LowerAlphabet | UpperAlphabet

type IsAlphabet<S extends string> =
  S extends Alphabet ? true : false

Explanation

The solution enumerates all 26 lowercase letters as a union type, then derives the uppercase equivalents using the built-in Uppercase<T> utility, and finally checks membership.

Step by step:

1. LowerAlphabet — a union of all 26 lowercase letter string literals: 'a' | 'b' | ... | 'z'

2. UpperAlphabet = Uppercase<LowerAlphabet> — TypeScript's built-in Uppercase distributes over unions, so this produces 'A' | 'B' | ... | 'Z' automatically without listing all 26.

3. Alphabet = LowerAlphabet | UpperAlphabet — the full alphabet, 52 members.

4. S extends Alphabet ? true : false — a simple membership check. If S is one of the 52 literal types, return true; otherwise false.

Why this works:

• TypeScript string literal types support exact equality: 'a' extends 'a' | 'b' | 'c' resolves to true.
• The empty string '' is not in the union → false.
• Multi-char strings like 'ab' are not in the union → false.
• Non-letter chars like '9', '!', '😂' are not in the union → false.

Alternative approach: One could use template literal magic like S extends \${infer C}`` combined with character ranges, but TypeScript doesn't support character ranges natively. The explicit enumeration is clean and efficient.

Key Concepts

• String literal union types — enumerating exact values as a type
• Uppercase<T> utility — built-in mapped string manipulation that distributes over unions
• Conditional type membership check — S extends Union ? true : false