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mustUse / noignore return values #38303
Description
Search Terms
no ignore noignore must use mustUse
Suggestion
declare function returnsPromise(): noignore Promise<void>Not using the returned value would be an error. Value can be used by:
- Passing it to a function or method,
- Calling its method,
- Using one of its properties,
awaiting it, if it's a Promise,yielding it,- Returning it.
Inferred return type should bemustUse/noignore.
Use Cases
- Detecting unhandled Promise rejections (related: Suggestion:
throwsclause and typed catch clause #13219), - Detecting unnecessary calls to pure functions (related: Add pure and immutable keywords to ensure code has no unintended side-effects #17181).
Examples
declare function returnsPromise(): noignore Promise<void>
foo(returnsPromise()) // ok (1)
returnsPromise().catch(...) // ok (2)
await returnsPromise() // ok (4)
return returnsPromise() // ok (6)
returnsPromise() // error
array.splice(0, 1) // ok
bar(array.slice(1)) // ok (1)
array.slice(1).forEach(...) // ok (2)
array.slice(1).length // error, `.length` unused
baz(array.slice(1).length) // weird but ok (3, 1)
return array.slice(1).length // weird but ok (3, 6)
yield array.slice(1) // ok (5)
yield* array.slice(1) // ok (5)
return array.slice(1) // ok (6)
array.slice(1) // errorChecklist
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.
Activity
ikokostya commentedon May 3, 2020
In most cases unused return value should be considered as error or warning. For this reason make sense in opposite approach when we can specify that return value should be ignored:
See for example https://docs.microsoft.com/en-us/dotnet/csharp/discards
ikokostya commentedon May 3, 2020
Also related to #13376
RyanCavanaugh commentedon May 5, 2020
I don't think this covers any ground not already covered by existing issues.
typescript-bot commentedon May 7, 2020
This issue has been marked as a 'Duplicate' and has seen no recent activity. It has been automatically closed for house-keeping purposes.