ElementListTagNameMap should be a mapped type derived from ElementTagNameMap

[justinfagnani]

TypeScript Version: 2.2.1

Code

Right now there's some extra boilerplate when defining custom elements and augmenting dom.d.ts's tag name maps that could go away if mapped types were used more:

export class FooElement extends HTMLElement {}
customElements.define('foo', FooElement);

declare global {
  interface ElementTagNameMap {
    "foo": FooElement,
  }
  interface HTMLElementTagNameMap {
    "foo": FooElement,
  }
  interface ElementListTagNameMap {
    "foo": NodeListOf<FooElement>;
  }
}

It would be nice to only have to augment one interface, like so:

declare global {
  interface HTMLElementTagNameMap {
    "foo": FooElement,
  }
}

We could get closer if ElementListTagNameMap was defined like this:

type ElementListTagNameMap = {
  [P in keyof ElementTagNameMap]: NodeListOf<ElementTagNameMap[P]>;
}

[mhegazy]

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.

[justinfagnani]

Looks like it was fixed in microsoft/TypeScript-DOM-lib-generator@14029e6