I am not able to understand how generics are used in this code in nodejs module http.d.ts

Question

  type RequestListener<
        Request extends **typeof IncomingMessage = typeof IncomingMessage**,
        Response extends **typeof ServerResponse = typeof ServerResponse**,
    > = (req: InstanceType<Request>, res: InstanceType<Response> & { req: InstanceType<Request> }) => void;

what is the meaning of the bold part here

I thought of trying hit and try method to see what it means. But I don't know how and what to check here. I think that request extends the IncomingMessage type, but don't know how is that possible.

Answer 1

If you have a class like IncomingMessage , typeof IncomingMessage is the type of the constructor function of that class (whereas IncomingMessage is the type of instances of the class). So that code is saying that Request is a constructor function that extends the IncomingMessage constructor function. (And the same sort of thing for Response .)

The = ____ part says what the default for the type parameter is if none is provided explicitly. So if you did listener: RequestListener; , the type parameters would take their default values ( typeof IncomingMessage and typeof ServerResponse ), but if you did listener: RequestListener<X, Y> , the type parameters would take the values X and Y instead.