MyUppercase

In this medium-level challenge, you'll implement your own version of the built-in Uppercase<T> intrinsic type, converting all lowercase letters in a string literal type to uppercase without using the built-in.

type cases = [
  Expect<Equal<MyUppercase<'a'>, 'A'>>,
  Expect<Equal<MyUppercase<'Z'>, 'Z'>>,
  Expect<
    Equal<MyUppercase<'A z h yy 😃cda\n\t  a   '>, 'A Z H YY 😃CDA\n\t  A   '>
  >,
]

Detailed Explanation

type UpperMap = {
  a: 'A'; b: 'B'; c: 'C'; d: 'D'; e: 'E'; f: 'F'; g: 'G';
  h: 'H'; i: 'I'; j: 'J'; k: 'K'; l: 'L'; m: 'M'; n: 'N';
  o: 'O'; p: 'P'; q: 'Q'; r: 'R'; s: 'S'; t: 'T'; u: 'U';
  v: 'V'; w: 'W'; x: 'X'; y: 'Y'; z: 'Z';
};
 
type MyUppercase<T extends string> =
  T extends `${infer First}${infer Rest}`
    ? `${First extends keyof UpperMap ? UpperMap[First] : First}${MyUppercase<Rest>}`
    : T;

How it works:

• The type recursively processes the string one character at a time using template literal inference with ${infer First}${infer Rest}
• For each character, it checks whether First is a key in UpperMap (i.e., a lowercase letter from a to z)
• If the character is a lowercase letter, it is replaced with the corresponding uppercase letter from the map; otherwise, the character is kept as-is (handling uppercase letters, digits, spaces, special characters, and emoji)
• The recursion continues with Rest until the string is empty, at which point the base case returns the empty string T
• Template literal types in TypeScript naturally preserve escape sequences like \n and \t, so they pass through without special handling

This challenge helps you understand recursive template literal type transformations and character-level string processing, and how to apply these concepts in real-world scenarios.