FR: Warn when using await on PromiseLike

[kwasimensah]

TypeScript Version: 3.7.x-dev.201xxxxx

Search Terms:

PromiseLike await

Code

const promiseA = new Promise(resolve => {
  resolve(5);
})

async function main() {
  const a = await 5; // typescript will give warning 80007
  const b = await promiseA; // expected
  const c = await ({} as PromiseLike<number>); // The cast to PromiseLike silences 80007
}

Expected behavior:

There should be warning on the line with const c. I found this while using chai-as-promised. "await expect(promise).to.eventually.be(arg)" was throwing UnhandledPromiseRejectionWarning. Chai.PromisedAssertion is a PromiseLike and not a Promise (https://github.com/DefinitelyTyped/DefinitelyTyped/blob/master/types/chai-as-promised/index.d.ts#L105) and it's not safe to use with await. But the type system hid that from me.

Actual behavior:

No warning.

Playground Link:

https://www.typescriptlang.org/play/index.html?ssl=10&ssc=2&pln=1&pc=1#code/MYewdgzgLgBADgJxAWwJYQKYEEYF4ZgYDuMACkmpgBQIYQgA2AbhngHwwDeAUDDLfWYYqAVgCUAbm4BfMd24BDCAE8wwGADMArmqipwMZAtRgqYrrxihIsBXhgKix2CKl9r0GACN7j5-Ap0bDcrcE91fD9UWCpOaQcIMkDMABlUAGsMAB4wLWQvDAQ2SXlpIA

Related Issues:

#35419

[DanielRosenwasser]

Thoughts @rbuckton?

[rbuckton]

I'm not sure I understand why you would issue a warning on await on PromiseLike. That is expressly a feature of ECMAScript's await keyword and allows the adoption of foreign (i.e., non-native) promises. If you try to await a "thenable" (i.e., something with a callable then that does not conform to the definition of PromiseLike), you already receive an error:

https://www.typescriptlang.org/play/index.html#code/MYewdgzgLgBADgJxAWwJYQKYEEYF4ZgYDuMACkmpgBQIYQgA2AbhngHwwDeAUDDLfWYYqAVgCUAbm4BfMd26owUDAgBmAQ2CsAKgAsMYdQCMGrHnyj6wVSTPnqIATzDAYqgK4uoqcDGTrFGy5eGFBIWHU8GHUiANgRKT4w6BgjKJi4+Ap0bETQ8BTXfAzUWCpOaWiIMmzMABlUAGsMAB4wd2QjFTZbJILYABN02NKYcsqHGD0DY1NbaSA

I'm not familiar with chai's PromisedAssertion or any of its caveats, but if it's not a valid promise it probably shouldn't be typed as one.

[kwasimensah]

1. Chai should use proper promises (in fact the extension that’s causing this seems dead)


2. There’s nothing in the PromiseLike type that seems to actually guarantee that it works with await. In fact looking at the Javascript spec for await (https://tc39.es/ecma262/#sec-async-function-definitions) and its definition of what a Promise is (https://tc39.es/ecma262/#sec-ispromise) I don’t think custom Promise types are actually compatible with await/async and warning 80007 should actually fire for anything that’s not a native promise. Bluebird documents that’s it’s not compatible with async/await at Official recommendation for using Bluebird with async functions? petkaantonov/bluebird#1434 (comment)

[rbuckton]

The PromiseLike type was specifically crafted to work with async/await per the spec. What a non-native promise loses over a native promise is that there is an extra turn of the microtask queue and that async functions only return native promises. The fact that native promises take one less turn is a recent addition as well.

All that await cares about is that the object has a callable then that can accept two callback arguments, and that is specifically what PromiseLike in TypeScript was designed to represent.

[kwasimensah]

Sorry for the noise. Turns out there's some brittle stuff with proxying that muddles the typing here. chaijs/chai-as-promised#266 (comment)

[typescript-bot]

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