Make `DOMStringList` generic

Search Terms

DOMStringList, DOMStringMap, DOMString

Suggestion

I'd be nice if DOMStringList had an optional generic so that a more restricted string type could be used for the list item type. The generic could default to string so that existing code using the type does not break.

The implementation would be along the lines of:

interface DOMStringList<T extends string = string> {
    readonly length: number;
    contains(string: T): boolean;
    item(index: number): T | null;
    [index: number]: T;
}

Use Cases

Libraries that wrap native DOM methods may return the DOMStringLists but want more strict types.

Examples

const dbOpenRequest = indexedDB.open("store", 1);

dbOpenRequest.onupgradeneeded = () => {
  const db = dbOpenRequest.result;
  db.createObjectStore('fruits');
  db.createObjectStore('meats');
}

dbOpenRequest.onsuccess = () => {
  const db = dbOpenRequest.result;
  const storeNames = db.objectStoreNames as DOMStringList<'fruits' | 'meats'>;
};

Checklist

My suggestion meets these guidelines:

• This wouldn't be a breaking change in existing TypeScript/JavaScript code
• This wouldn't change the runtime behavior of existing JavaScript code
• This could be implemented without emitting different JS based on the types of the expressions
• This isn't a runtime feature (e.g. library functionality, non-ECMAScript syntax with JavaScript output, etc.)
• This feature would agree with the rest of TypeScript's Design Goals.

まず DOMStringList の宣言を見つけ、length、contains、item、およびインデックスアクセスのメンバーがどのように型付けされているかを確認します。string をデフォルトとする、string に制約されたオプショナルなジェネリックを実装し、既存の形式と、制限付きの DOMStringList<'fruits' | 'meats'> の使用の両方が引き続き有効であることを確認します。