Is it possible to make an "inner join" on interfaces

Question

Let's say we have two interfaces First and Second:

interface First {
  a: string;
  b: number;
}

interface Second {
  b: number;
  c: Date;
}

With intersections it is possible to merge two interfaces:

type FirstSecond = First & Second // {a: string, b: number, c: Date}

But is it possible to make an inner join, so the resultant interface only consists of properties that are declared in both interfaces:

type FirstSecond = First /*inner join here*/ Second // {b: number}

This could especially be helpful for generic types.

https://stackoverflow.com/questions/47375916/typescript-how-to-create-type-with-common-properties-of-two-types — Roberto Zvjerković, Jul 30 '19 at 09:03
@jonrsharpe idk what you telling me to do? If it would be that simple for me I would not ask a question here. If you mean that I should declare a third interface like `{ b: number }` then it's not answering my question because I wanted to have a generic solution (see last sentence). — ysfaran, Jul 30 '19 at 10:17
Yes, that's exactly what I mean, extract a third interface with the commonalities. — jonrsharpe, Jul 30 '19 at 10:18

score 6 · Accepted Answer · answered Jul 30 '19 at 09:04

You can use Pick to take the common properties from the intersection. To get the common properties, you can use the Extract conditional type to filter the keys of one type by the keys of the other:


interface First {
  a: string;
  b: number;
}

interface Second {
  b: number;
  c: Date;
}


type IntersectByProperties<T, U> = Pick<T & U, Extract<keyof T, keyof U>>

type R = IntersectByProperties<First, Second>

Is it possible to make an "inner join" on interfaces

1 Answers1