Merge Two Interfaces

Question

Seeking confirmation or clarification

If I have two interfaces. What is the "proper" way to create a merge of those two interfaces?

IFoo {
  // some stuff
}


IBar {
  // some stuff
}


IFooBar extends IFoo, IBar {
 // Empty
}

It works but it feels weird, like I am doing it wrong with the empty IFooBar.

Am I doing this correctly?

I also noticed that this also works:

type IFooBar = IFoo & IBar;

I have an illogical aversion to using type yet, it is much cleaner.

Answer 1

If you're wanting to merge 2 interfaces which contain members more than 1 level deep:

export interface TypeOne  {
  one: {
    two: {
      hello: string;
    }[]
  }
}

export type TypeTwo = {
  one: {
    two: {
      world: string;
    }[]
  }
} & TypeOne;

const x: TypeTwo;
x.one.two[0]. // autocomplete options are 'hello' / 'world'

Answer 2

I think there it is ok, or not ok relating to what meaning of the merged interface. If IFooBar is a new entity from perspective of object-oriented design, then empty interface is all right. But if there is no such entity, but you want just merge some unrelated interfaces (for some hacky code) - then just use IFoo & IBar in variable type definition, or type for shortening this.

It's just my opinion as programmer, that came from object oriented languages like C++ and C#.