Problem

In its current state, it's not possible to use the instanceof operator to check if the value of a property is a Realm.List, Realm.Result or Realm.Collection:

This simple playground implements the current types and show the issue:

// API
declare namespace Realm {
    interface Collection {}
    const Collection: {};
}
declare class Realm {}

// Usage
const collection = Realm.Collection;
if (collection instanceof Realm.Collection) { //   will produce a TS error

}

Solution

One alternative is to declare classes on the Realm namespace:

// API
declare namespace Realm {
    interface Collection {}
    class Collection implements Collection {}
}
declare class Realm {}

// Usage
const collection = Realm.Collection;
if (collection instanceof Realm.Collection) {//   works

}

Another alternative is to simplify our type by completely abandoning the use of the TypeScript namespace and expose these classes as static properties on the Realm class:

// API
declare class Collection {}
declare class Realm {
    static Collection: typeof Collection;
}

// Usage
const collection = Realm.Collection;
if (collection instanceof Realm.Collection) {//   works

}

This would be a breaking change, since developers relying on the global "Realm" type (declared by the namespace) would have to start explicitly importing types from "realm".

How important is this improvement for you?

I'd like to see it, but have a workaround