await-promise is incompatible with the Promise/A+ spec

[alexandru]

Bug Report

• TSLint version: 5.8.0
• TypeScript version: 2.6.1

TypeScript code being linted

  const tweetsIterator = twitterClient.scanUserTimeline(account.twitter)
  while (true) {
    const tweets = await tweetsIterator.next()
    if (tweets.value && tweets.value.length) {
      await db.bulkInsertTweets(twitterArchiveIndex, tweets.value)
    }
    if (tweets.done) {
      break
    }
  }

Actual behavior

Invalid 'await' of a non-Promise value. (await-promise)
  68 |       const tweetsIterator = twitterClient.scanUserTimeline(account.twitter)
  69 |       while (true) {
> 70 |         const tweets = await tweetsIterator.next()
     |                       ^
  71 |         if (tweets.value && tweets.value.length) {
  72 |           await db.bulkInsertTweets(twitterArchiveIndex, tweets.value)
  73 |         }

Expected behavior

The error is triggered because the object awaited is a Funfix Future which does not inherit from Promise.

According to the Promises/A+ specification the Promise class is NOT required for await to work. You only need a "thenable" object, meaning a type that implements then with the described behavior.

In TypeScript this is exposed as PromiseLike:

interface PromiseLike<T> {
    /**
     * Attaches callbacks for the resolution and/or rejection of the Promise.
     * @param onfulfilled The callback to execute when the Promise is resolved.
     * @param onrejected The callback to execute when the Promise is rejected.
     * @returns A Promise for the completion of which ever callback is executed.
     */
    then<TResult1 = T, TResult2 = never>(onfulfilled?: ((value: T) => TResult1 | PromiseLike<TResult1>) | undefined | null, onrejected?: ((reason: any) => TResult2 | PromiseLike<TResult2>) | undefined | null): PromiseLike<TResult1 | TResult2>;
}

Possibly related: #3381

Please make this check look for PromiseLike and not for Promise.

Thanks,

[ajafff]

You can configure what the rule should treat as promise: https://palantir.github.io/tslint/rules/await-promise/

[alexandru]

What's the motivation for Thenable to not be the default?

[ajafff]

The rule doesn't check for assignability / compatibility. It just checks the name. Thenable is a builtin type and even if it was, you are not required to implement that interface.

[alexandru]

You mean it literally checks that the type's name is that string?

[ajafff]

You mean it literally checks that the type's name is that string?

Yep, it checks the name of every base type and recurses into union and intersection types.

[adidahiya]

How's this for a docs update to clarify the issue? #3519

[alexandru]

Thanks for the clarifications @ajafff, @adidahiya.

Cheers,