Append to object

In this medium-level challenge, you'll implement a Generic Type that can append a new field to any object type. For that you take a key and a value and append it to the object type.

type Test = { id: '1' }
type Result = AppendToObject<Test, 'value', 4> // expected to be { id: '1', value: 4 }

Detailed Explanation

type AppendToObject<T, U extends PropertyKey, V> = {
  [Key in keyof T | U]: Key extends keyof T ? T[Key] : V
}

How it works:

• First we constrain the type U to be a PropertyKey so it can be used as a key in the object.
• Then we create a new object with a mapped type of all keys in T and U ([Key in keyof T | U])
• If the key is in T, we return the value of T[Key], otherwise it must be U so we return V

This challenge helps you understand TypeScript's advanced type system and how to apply this concept in real-world scenarios.