How to retrieve static attributes while declaring an interface

Question

class A {
    public static readonly TYPE = "A";
}

interface forA {
    for: A.TYPE
}

I am facing an issue while trying to access A.TYPE from the forA interface in order to perform type guarding.

The error I encounter is:

TS2702: 'A' only refers to a type, but is being used as a namespace here

.

Anyone know of a workaround for this situation?

Answer 1

Answer №1

To find the desired type, you can use either (typeof A)['TYPE'] or typeof (A.TYPE).

When using the (typeof A)['TYPE'] notation:

Static properties belong to the class constructor, so it's necessary to use typeof A instead of just A. (The constructor is a value called A, which differs from the type A, despite sharing the same name. Therefore, typeof A provides the type of the constructor)
In order to access a property with the key "TYPE", bracket notation needs to be used. TypeScript does not support . as a type operator, perhaps to avoid conflicts with namespace notation. Hence, use (typeof A)['TYPE'] instead of (typeof A).TYPE.

Regarding the typeof (A.TYPE) notation:

As previously mentioned, there exists a value named A representing the constructor for instances of the A class. This value includes a property named TYPE, resulting in A.TYPE. The type of this value can be obtained by querying typeof (A.TYPE), which is likely the simplest method for your requirements.

Hopefully, this explanation proves helpful. Best of luck!

Answer 2

To find the desired type, you can use either (typeof A)['TYPE'] or typeof (A.TYPE).

When using the (typeof A)['TYPE'] notation:

Static properties belong to the class constructor, so it's necessary to use typeof A instead of just A. (The constructor is a value called A, which differs from the type A, despite sharing the same name. Therefore, typeof A provides the type of the constructor)
In order to access a property with the key "TYPE", bracket notation needs to be used. TypeScript does not support . as a type operator, perhaps to avoid conflicts with namespace notation. Hence, use (typeof A)['TYPE'] instead of (typeof A).TYPE.

Regarding the typeof (A.TYPE) notation:

As previously mentioned, there exists a value named A representing the constructor for instances of the A class. This value includes a property named TYPE, resulting in A.TYPE. The type of this value can be obtained by querying typeof (A.TYPE), which is likely the simplest method for your requirements.

Hopefully, this explanation proves helpful. Best of luck!