How to type a variable to contain a derived class [duplicate]

Question

I don't know how to type a variable to contain a derived class that will be instantiated when needed.

// I have a base class... abstract class Base { constructor() { console.log("Base"); } abstract fun(); } // ...for various derived classes class Derived extends Base { constructor() { super(); console.log("Derived"); } fun() { console.log("Have fun") } } // Now I want to collect the derived classes in an array... interface OneNode { handler: Base; // What should I put here? Obviously if I put "any" don't have complains } const availableNodes: OneNode[] = [ {handler: Derived} // Here ts complains that "Property 'fun' is missing in type 'typeof Derived' but required in type 'Base'. ts(2741)" ] // ...and instantiate some of them only when needed const y = new availableNodes[0].handler(); // Here ts complains that "This expression is not constructable. Type 'Base' has no construct signatures. ts(2351)" y.fun();

If I ignore the TypeScript complains or type the "handler" field as "any", everything works as expected. But I want to properly type the "handler" variable.

Did you mean {handler: new Derived()}? You are assigning the class itself, not an instance. A class is not an instance of itself nor its parent. const node: OneNode = Derived will fail the same way. const node: OneNode = new Derived() will work just fine. — knittl
– knittl, Commented Nov 18, 2024 at 15:52
You're looking for new() => Base as shown in this playground link. Does that fully address the question? If so then I'll write an answer (or, more likely, find a duplicate). If not, then what's missing? — jcalz
– jcalz, Commented Nov 18, 2024 at 15:53

Eli · Accepted Answer · 2024-11-18 17:09:02Z

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

Thanks @eli! Your first solution works like a charm. Cannot apply the 2nd because the various derived class constructors do things and need a parameter known only later.

Collectives™ on Stack Overflow

How to type a variable to contain a derived class [duplicate]

1 Answer 1

1 Comment

Linked

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

1 Comment

Linked

Related