Possible to extend types in Typescript?

Question

Say I have the following type:

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

I now want to extend this type, i.e.

type UserEvent extends Event = {
   UserId: string; 
}

This doesn't work. How can I do this?

Answer 1

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.

Answer 2

you can intersect types:

type TypeA = {
    nameA: string;
};
type TypeB = {
    nameB: string;
};
export type TypeC = TypeA & TypeB;

somewhere in you code you can now do:

const some: TypeC = {
    nameB: 'B',
    nameA: 'A',
};

Answer 3

What you are trying to achieve is equivalent to

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

interface UserEvent extends Event {
   UserId: string; 
}

The way you defined the types does not allow for specifying inheritance, however you can achieve something similar using intersection types, as artem pointed out.

Answer 4

You can also do:

export type UserEvent = Event & { UserId: string; };