ShadowRoot should be a class, not an interface

[justinfagnani]

TypeScript Version: 3.2.0-dev.201xxxxx

Search Terms: ShadowRoot

Code

const f = (o) => o instanceof ShadowRoot;

Expected behavior:
No errors

Actual behavior:
Error: 'ShadowRoot' only refers to a type, but is being used as a value here.

Playground Link: http://www.typescriptlang.org/play/#src=const%20f%20%3D%20(o)%20%3D%3E%20o%20instanceof%20ShadowRoot%3B

Related Issues: #10401

[weswigham]

PRs welcomed. You can find more information about contributing lib.d.ts fixes at https://github.com/Microsoft/TypeScript/blob/master/CONTRIBUTING.md#contributing-libdts-fixes.

[weswigham]

Note that the constructor needs to be private, since even though ShadowRoot can be instanceof'd and exists in value space, it cannot be constructed.

[justinfagnani]

Note that the constructor needs to be private, since even though ShadowRoot can be instanceof'd and exists in value space, it cannot be constructed.

Isn't this the same as Element, HTMLElement, Attr, etc.? Those don't seem to have private constructors:

declare var HTMLElement: {
    prototype: HTMLElement;
    new(): HTMLElement;
};

Also, I don't understand why these are interfaces and not classes.

[RyanCavanaugh]

Historically it was not possible to augment an existing class declaration with a subsequent interface declaration, so we wrote them as decomposed var/interface pairs so they could be subject to declaration merging so that people could write .d.ts compat files for experimental DOM features.

It was also historically not possible to have private constructors. You can only instanceof something that has a construct signature, so the new()s are there to allow for instanceof checks. Now that inheriting from elements is possible, making the DOM element constructors protected might make some sense.