Spec inconsistency for type aliases

[impworks]

Specification, chapter 3.10:

An interface can be named in an extends or implements clause, but a type alias for an object type literal cannot.

This contradicts the actual behaviour seen in current Playground:

• Interface cannot be used in the extends clause
• Type alias can be used in the implements clause as well as an interface

[olegdunkan]

Interface cannot be used in the extends clause

It can

interface A { x:number;}
interface B  extends A { y:number;}

Type alias can be used in the implements clause as well as an interface

Now it is true.

Spec now is in stale state.

[impworks]

You're right. I meant the extends clause for a class definition.
The following line is also misleading:

since interface types offer more capabilities they are generally preferred to type aliases

• The type syntax supports mapped types, while interfaces do not
• There's no way to emulate A | B using interfaces (or is there?)

Spec now is in stale state.

So is the spec going to be updated, or it is abandoned for the sake of faster development?

[ikatyang]

So is the spec going to be updated, or it is abandoned for the sake of faster development?

See #17215 (comment)

[olegdunkan]

since interface types offer more capabilities they are generally preferred to type aliases

Type alias is like named references to any kind of type (primitive types, object types, union types, intersection types, mapped types), interface is like named reference to object type. You can't extend (in the sense of adding extensions to the existed class) class with an alias but with an interface can. Declaration merging is working for interfaces (class and interface, interface and interface) and enums, namespaces.

import {A} from "./a"

declare module './i' {
  interface A {
    f2():void;
  }
}

A.prototype.fn2 = function() {

};

By the way, spec doesn't contain that.

[DanielRosenwasser]

I believe this was a change we made where if a type reference resolves to an object type, then you can use it in an implements clause.

Thanks for reporting!

[RyanCavanaugh]

The spec is now an "archived" artifact and we won't be tracking defects against it anymore.