Well, it depends on if you are trying to create an array of the constructors or of objects?

• It sounds like you want to create an array of constructors, so type Handler as {new (): Base}. This is type of a constructor.
• If you want to create an array of objects, instead of writing Derived in the object of type OneNode, write new Derived().

Working code:

// Now I want to collect the derived classes in an array...
interface OneNode {
  handler: { new (): Base };
}
const availableNodes: OneNode[] = [{ handler: Derived }];

// ...and instantiate some of them only when needed
const y = new (availableNodes[0]?.handler)();
y.fun();

Source for a list of constructors solution: https://stackoverflow.com/a/13408029/2834938