AI solution for TypeScript puzzle on BFE.dev
16. implement FirstChar<T>

type FirstChar<T extends string> = T extends `${infer U}${string}` ? U : never;

Explanation:

• We define a type FirstChar that takes a generic type parameter T that extends the string type.
• We use conditional types to check if T matches the pattern of having a first character followed by any number of characters. We use template literal types to achieve this, where ${infer U} captures the first character and ${string} captures the rest of the string.
• If it matches the pattern, we return the captured first character U. Otherwise, we return never to indicate that the input is invalid.