EDIT:
To get the types of a function return you can use the ReturnType type constructor. Combine that with type indexing using bracket notation and you can find the return type of an interface member like so:
type Server = ReturnType<Express['listen']>;
The Node standard module http exposes a type called Server. Since Express uses this type for its server listeners, you can use it to type the listener on your member like so:
import type { Express } from "express";
import type { Server } from "http";
class App {
constructor(app: Express) {
this.listener = app.listen();
}
private listener: Server;
}
If you want to use a custom listen() function with an uninitialized server variable as you're doing in your above code snippet, you just need to make the property optional:
class App {
private server?: Server;
listen(expressApp: Express) {
this.server = expressApp.listen()
}
}