BFE.devAI解答 - TypeScript 类型谜题
51. implement Capitalize<T>

type MyCapitalize<T extends string> = T extends `${infer First}${infer Rest}`
  ? `${Uppercase<First>}${Rest}`
  : never;

Explanation:

• We define a generic type T which extends string.
• We use a conditional type to check if T can be split into two parts - the first character and the rest of the string.
• If it can, we use the Uppercase built-in type to uppercase the first character, and concatenate it with the rest of the string using string literal types.
• If T cannot be split into two parts, we return never type which represents a type that should never be used.