TS2339 - 'Property doesn't exist on type' error in an apparently valid TS file

Question

I have the following type declarations:

type Root = { r: string };
type A = { a: string };
type B = { b: string };
type Main = Root & (A | B);
         


        

Answer 1

You are right about Main being equivalent to (Root & A) | (Root & B) you are wrong about the interpretation of union types (|). A union type means you can assign either type in the union to main ( so either (Root & A) or (Root & B)), but you can only access common properties of the union members. In this case the common member is only r. Since main ca be either (Root & A) or (Root & B), a might not exist on main, so typescript prevents such an access.

To use the members that only exist on one type in the union you need a type guard. In this case an in type-guard will work best:

const func : (main: Main) => void = main => {
    if ('a' in main) {
        main.a
    } else {
        main.b
    }
}

You can read more about type guards here