Iterator Helpers: `return` method typing is weaker than ideal

⚙ Compilation target

ESNext

⚙ Library

esnext.iterator.d.ts

Missing / Incorrect Definition

The return method always exists on iterators returned by Iterator.from (iter in below code) and iterators returned by Iterator Helpers methods (iter2 in below code).

However, it is defined as optional in the current definition. This could be improved.

Please note that you can't just add a non-optional return method definition to the IteratorObject type -- iterators returned by builtin methods other than Iterator Helpers don't have that method (iter3).

P.S. if you are interested in a motivating example, I don't have one. Sorry 🤪

Sample Code

const iter = Iterator.from({ next() { return { done: false, value: 0 }} });

iter.return(); // should not error

const iter2 = iter.map(x => x);

iter2.return(); // should not error

const iter3 = [].values();

iter3.return(); // Note: this error IS correct

Playground

Documentation Link

https://tc39.es/proposal-iterator-helpers/

Spec states that below objects always have the return method:

• %WrapForValidIteratorPrototype%
• %IteratorHelperPrototype%

esnext.iterator.d.ts から始め、サンプルコードと照らし合わせて IteratorObject、Iterator.from、Iterator Helpers の定義を調べます。リンク先の Iterator Helpers 仕様に基づいて期待される動作を確認します。Iterator.from とヘルパーの結果では return() を許可し、[].values() では引き続き拒否する必要があります。iter3 に対するエラーを導入せず、宣言がこれらのケースを区別できれば完了です。