Get the type of a property of an Union Type in typescript

March 3, 2022

Suppose we have some interfaces with a name property (and possibly others):

interface Circle {
   name: 'circle',
   radius: number
}

interface Sphere {
   name: 'sphere',
   radius: number
}

interface Square {
   name: 'square',
   side: number
}

type GeometricalObject = Circle | Sphere | Square;

Now, suppose we have a function:

   doSomething(name: SomeType) {
     ...     
   }

Is there a way to assure that the name argument’s type of doSomething() function is the union of the name property possible types in GeometricalObject (i.e. ‘circle’ | ‘sphere’ | ‘square’, in this specific case) programmatically?

I was imagining something like typeof GeometricalObject.name, if that was possible.

>Solution :

interface Circle {
   name: 'circle',
   radius: number
}

interface Sphere {
   name: 'sphere',
   radius: number
}

interface Square {
   name: 'square',
   side: number
}

type GeometricalObject = Circle | Sphere | Square;

type Name = GeometricalObject["name"]; // "circle" | "sphere" | "square"

It is possible; it’s using bracket notation instead of dot notation.

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