AI solution for TypeScript puzzle on BFE.dev
60. implement UndefinedToNull<T>

Solution:

type UndefinedToNull<T> = T extends undefined
  ? null
  : T extends object
  ? {
      [K in keyof T]: UndefinedToNull<T[K]>;
    }
  : T;

Explanation: We use conditional types in TypeScript to create a new type based on a condition. In this case, we check if the input type T is undefined or not. If it is undefined, we assign the type null to it. If it's not undefined, we check whether it is an object or not. If it's not an object, it means it's a non-object type like string or number. In this case, we just return the same type T. If it's an object, we create a mapped type that iterates over all the keys of the object and apply UndefinedToNull recursively to each key's value.