Possible to extend types in Typescript?

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?

1

4 Answers

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.

3

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',
};

1

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.

2

A generic extension type can be written as follows:

type Extension<T> = T & { someExtensionProperty: string }

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