Is there a way to “extract” the type of TypeScript interface property?

Question

Let's suppose there's a typing file for library X which includes some interfaces.

interface I1 {
    x: any;
}
    
interface I2 {
    y: {
        a: I1,
        b: I1,
        c: I1
    }
    z: any
}

In order to work with this library I need pass around an object that is of exactly the same type as I2.y. I can of course create identical interface in my source files:

interface MyInterface {
    a: I1,
    b: I1,
    c: I1
}

let myVar: MyInterface;

but then I get the burden of keeping it up to date with the one from library, moreover it can be very large and result in lot of code duplication.

Therefore, is there any way to "extract" the type of this specific property of the interface? Something similar to let myVar: typeof I2.y (which doesn't work and results in "Cannot find name I2" error).

---

Edit: after playing a bit in TS Playground I noticed that following code achieves exactly what I want to:

declare var x: I2;
let y: typeof x.y;

However it requires a redundant variable x to be declared. I am looking for a way to achieve this without that declaration.

Answer 1

It wasn't possible before but luckily it is now, since TypeScript version 2.1. It has been released on the 7th of December 2016 and it introduces indexed access types also called lookup types.

The syntax looks exactly like element access but written in place of types. So in your case:

interface I1 {
    x: any;
}

interface I2 {
    y: {
        a: I1,
        b: I1,
        c: I1
    }
    z: any
}

let myVar: I2['y'];  // indexed access type

Now myVar has type of I2.y.

Check it out in TypeScript Playground.

Answer 2

To expand on the accepted answer, you can also assign the type using the type keyword and use it in other places.

// Some obscure library
interface A {
  prop: {
    name: string;
    age: number;
  }
}

// Your helper type
type A_Prop = A['prop']

// Usage
const myThing: A_prop = { name: 'June', age: 29 };

Answer 3

An interface is like the definition of an object. Then y is a property of your I2 object, that is of a certain type, in that case "anonymous".

You could use another interface to define y and then use it as your y type like this

interface ytype {
   a: I1;
   b: I1;
   c: I1;
}

interface I2 {
    y: ytype;
    z: any;
}

You can put your interface in a file and use extract so you can import it in other files of your projects

export interface ytype {
   a: I1;
   b: I1;
   c: I1;
}



 export interface I2 {
        y: ytype;
        z: any;
    }

You can import it that way :

   import {I1, I2, ytype} from 'your_file'