RequiredByKeys

In this medium-level challenge, you'll implement RequiredByKeys<T, K> which makes only the specified keys of T required while leaving the rest unchanged, defaulting to making all properties required when K is not provided.

interface User {
name?: string
age?: number
address?: string
}
 
type UserRequiredName = RequiredByKeys<User, 'name'> // { name: string; age?: number; address?: string }
 

Detailed Explanation

type Merge<T> = { [K in keyof T]: T[K] }
 
type RequiredByKeys<T, K extends keyof T = keyof T> = Merge<
  Omit<T, K> & Required<Pick<T, K>>
>

How it works:

• K extends keyof T = keyof T constrains K to valid keys of T and defaults to all keys when K is not provided, matching the behavior of the built-in Required<T>.
• Pick<T, K> extracts only the properties specified by K, and Required<...> removes their optional modifiers.
• Omit<T, K> extracts the remaining properties that should stay unchanged (keeping their original optional/required status).
• The intersection Omit<T, K> & Required<Pick<T, K>> combines both parts, but intersection types are not displayed cleanly by TypeScript.
• The Merge<T> helper flattens the intersection into a single object type using a mapped type { [K in keyof T]: T[K] }, which produces a clean, unified result that matches the expected test case types.

This challenge helps you understand selective property modifier manipulation and intersection type flattening and how to apply these concepts in real-world scenarios.