async iterator does not call return() for an abrupt completion at for await of loop

[niceandneat]

Bug Report

🔎 Search Terms

async iterator, for await loop

🕗 Version & Regression Information

• This changed after version v4.9.3

⏯ Playground Link

Target: ES2017 (with actual result)
Playground link with relevant code

Target: ES2018 (with expected result)
Playground link with relevant code

💻 Code

tsconfig.json

{
  "compilerOptions": {
    "target": "ES2017",
  }
}

index.ts

const asyncIterable = {
  [Symbol.asyncIterator]() {
    return {
      next() {
        console.log('next()');
        return { value: '!', done: false };
      },
      return(value: string) {
        console.log('return()');
        return { value, done: true };
      },
    };
  }
};

const main = async () => {
  for await (const x of asyncIterable) {
    console.log(x);
    break;
  }
}

main();

Thanks to @kbridge for making MCVE.

🙁 Actual behavior

next()
!

Should call return() for an abrupt completion at for await of loop (e.g. break, throw). This happens when for-await-loop code is transformed to target equal or lower than ES2017.

🙂 Expected behavior

next()
!
return()

If target for compilation is equal or higher than ES2018, It works as expected. This is also aligned with javascript specification.

[fatcerberus]

Looks like a duplicate of #52936

[typescript-bot]

This issue has been marked as a 'Duplicate' and has seen no recent activity. It has been automatically closed for house-keeping purposes.