[Web API type definition issue] incorrect type definition for `getElementById()`

Document と DocumentFragment の生成された lib.dom 定義から始め、それらの getElementById シグネチャがどのように生成されるかを追跡します。Issue に示されている Web API の動作を確認し、両方の戻り値の型を Element | null に更新して、生成された定義または関連するチェックに修正後のシグネチャが反映されていることを確認します。

Summary

incorrect type definition for document.getElementById() and shadowRoot.getElementById()

Expected vs. Actual Behavior

The type definition in lib.dom shows the signature as:

interface Document extends Node, DocumentOrShadowRoot, FontFaceSource, GlobalEventHandlers, NonElementParentNode, ParentNode, XPathEvaluatorBase {
    // ... omitted ...
    getElementById(elementId: string): HTMLElement | null;
    // ... omitted ...
}

The signature should be

interface Document extends Node, DocumentOrShadowRoot, FontFaceSource, GlobalEventHandlers, NonElementParentNode, ParentNode, XPathEvaluatorBase {
    // ... omitted ...
    getElementById(elementId: string): Element | null;
    // ... omitted ...
}

Playground Link

https://www.typescriptlang.org/play/?#code/CYewxgrgtgpgdgFwHQCMTAJ5IJZzjAJwAkAVAWQBkACAXioHIAeAZwDcBzAPkfau2BoAiAGYgQg7gHoujSWy70AUIrAg4zBFV51QkWIlToMAQiTDsBDQFEANjH0IAwgAtsN4KfOWEt+-Ceu7lQAhsxUAMoAagDi0b4OyqrqIHZINiDsABS8uBrBcGAwIMIRMXF2DgCUVJKSVAgEEDBUiWoaWgBMtFS60P5I7DA+Ff4AQhgAksCZ9KIg9NWhpbHx-q3JqelZ7F25CPmFxcvlfojVtVQabjb1jTBAA

Browser Support

• This API is supported in at least two major browser engines (not two Chromium-based browsers).

Have Tried The Latest Releases

• This issue applies to the latest release of TypeScript.
• This issue applies to the latest release of @types/web.

Additional Context

Proof:

// Run this in your console:

document.body.innerHTML = '<svg><g id="foo"></g></svg>'

const g = document.body!.firstElementChild!.firstElementChild

console.log(g instanceof SVGGElement) // true

const g2 = document.getElementById('foo')

console.log(g2 instanceof SVGGElement) // still true (getElementById did not return HTMLElement | null)

The type is also wrong for other types of documents:

interface DocumentFragment extends Node, NonElementParentNode, ParentNode {
    readonly ownerDocument: Document;
    getElementById(elementId: string): HTMLElement | null;
}