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.

Start by locating the DOMStringList declaration and review how its length, contains, item, and indexed access members are typed. Implement the optional string-constrained generic with a string default, then verify that both the existing form and the restricted DOMStringList<'fruits' | 'meats'> usage remain valid.