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.

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',};

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.