Possible to extend types in Typescript?

The keyword extends can be used for interfaces and classes only.

If you just want to declare a type that has additional properties, you can use intersection type:

type UserEvent = Event & {UserId: string}

UPDATE for TypeScript 2.2, it’s now possible to have an interface that extends object-like type, if the type satisfies some restrictions:

type Event = {
   name: string;
   dateCreated: string;
   type: string;
}

interface UserEvent extends Event {
   UserId: string; 
}

It does not work the other way round – UserEvent must be declared as interface, not a type if you want to use extends syntax.

And it’s still impossible to use extend with arbitrary types – for example, it does not work if Event is a type parameter without any constraints.