ExtractToObject

Problem Link

Problem

Implement a type that extracts the value of a property from a union of objects and merges it with the rest of the object.

type T0 = ExtractToObject<{ id: 1; city: 'New York' } | { id: 2; city: 'Paris' }, 'city'>
// { id: 1; city: 'New York' } | { id: 2; city: 'Paris' }   (same here, city is scalar)

// More illustrative:
type T1 = ExtractToObject<
  { id: 1; address: { city: 'New York'; zip: '10001' } } |
  { id: 2; address: { city: 'Paris'; zip: '75001' } },
  'address'
>
// { id: 1; city: 'New York'; zip: '10001' } | { id: 2; city: 'Paris'; zip: '75001' }

Solution

type ExtractToObject<T, U extends keyof T> =
  T extends T
    ? Omit<T, U> & T[U]
    : never

With flattening:

type Flatten<T> = { [K in keyof T]: T[K] }

type ExtractToObject<T, U extends keyof T> =
  T extends T
    ? Flatten<Omit<T, U> & T[U]>
    : never

How it works:

1. T extends T distributes the operation over each member of the union.
2. For each union member, Omit<T, U> removes the key U from the object.
3. T[U] is the type of the extracted property (an object type in the interesting case).
4. Intersecting with T[U] merges its properties into the result.
5. Flatten collapses & into a clean object type.

Key Takeaways

• Distribution with T extends T applies the transformation per union member, preserving the union structure.
• Omit<T, U> & T[U] is the standard “spread/inline nested object” pattern.
• Flatten<T> ({ [K in keyof T]: T[K] }) resolves intersection types into a single readable object type.