mustUse / noignore return values

[wojpawlik]

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:

1. Passing it to a function or method,
2. Calling its method,
3. Using one of its properties,
4. awaiting it, if it's a Promise,
5. yielding it,
6. Returning it.
   Inferred return type should be mustUse / noignore.

Use Cases

1. Detecting unhandled Promise rejections (related: Suggestion: throws clause and typed catch clause #13219),
2. 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) // error

Checklist

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.

[ikokostya]

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:

returnsPromise(); // must be error or warning, because return value is ignored
_ = returnsPromise(); // OK

See for example https://docs.microsoft.com/en-us/dotnet/csharp/discards

[ikokostya]

Also related to #13376

[RyanCavanaugh]

I don't think this covers any ground not already covered by existing issues.

[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.