AI solution for TypeScript puzzle on BFE.dev
51. implement Capitalize<T>

One possible solution could be:

type MyCapitalize<S extends string> = S extends `${infer F}${infer R}`
  ? `${Uppercase<F>}${R}`
  : S;

In this solution, we are using a conditional type with template literal types to split the input string into its first character (infer F) and the rest of the string (infer R). We then use the Uppercase utility type to transform F to uppercase and concatenate it with R to get the capitalized string. If S is empty or not a string, we just return it as is.

We can test the solution:

type A = MyCapitalize<'bfe'> // 'Bfe'
type B = MyCapitalize<'Hello, world!'> // 'Hello, world!'
type C = MyCapitalize<1234> // 1234