ExtractToObject

In this medium-level challenge, you'll implement ExtractToObject<T, U> which takes an object type and a key whose value is an object, then flattens that nested object's properties into the parent object while removing the original key.

type Test = { id: '1', myProp: { foo: '2' }}
type Result = ExtractToObject<Test, 'myProp'> // expected to be { id: '1', foo: '2' }

Detailed Explanation

type ExtractToObject<T, U extends keyof T> =
  T[U] extends object
    ? { [K in keyof T | keyof T[U] as K extends U ? never : K]:
        K extends keyof T[U] ? T[U][K] : K extends keyof T ? T[K] : never }
    : never

A cleaner way to express this using Omit and intersection:

type ExtractToObject<T, U extends keyof T> = {
  [K in keyof (Omit<T, U> & T[U])]: (Omit<T, U> & T[U])[K]
}

How it works:

• U extends keyof T constrains U to be a valid key of T, which causes ExtractToObject<test4, 'prop4'> to produce a ts-expect-error since 'prop4' is not a key of test4
• Omit<T, U> removes the specified key U from the object type, keeping all other properties
• & T[U] intersects the result with the value type at key U, effectively merging the nested object's properties into the parent
• The outer mapped type { [K in keyof (...)]: (...)[K] } flattens the intersection into a clean object type, so instead of Omit<T, U> & T[U] you get a single object with all properties listed
• For example, given { id: '1', myProp: { foo: '2' } } with U = 'myProp', the result is { id: '1', foo: '2' }

This challenge helps you understand object type manipulation using Omit, intersections, and mapped type flattening, and how to apply these concepts in real-world scenarios.