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Type inference changed for callbacks with optional parametersΒ #55394
Description
Activity
Changed in
5.1.0-dev.20230317, this is what has been added in this version: diff. So likely the difference comes from #52609 .I think this is a bug. Perhaps it was always there in
inferFromAnnotatedParametersor maybe there is a specific reason why optional parameters behave differently there. I see that the inference source that is used is justgetEffectiveTypeAnnotationNode(declaration)there and that the optional marker is ignored. I'll investigate this later more thoroughly.Itβs kind of a gray area, but the current behavior feels more intuitive fwiw - the optional marker suggests the intent is that the callback either receives a number or else nothing at all.
- locked as resolved and limited conversation to collaborators
on Oct 22, 2025
π Search Terms
optional, inference, callback
π Version & Regression Information
β― Playground Link
https://www.typescriptlang.org/play?jsx=0&module=1&ts=5.1.3#code/KYDwDg9gTgLgBAMwK4DsDGMCWEWMwGxmCgB4AVAPgAowpgATTNAQyIC44qA3Z-JYDmQA0cTCnqgOKJAFsARsQCUcALwU4ciBHzBmKRYLgBvAL4AoM2hwBneM1V5CxKjQjXgAfimyFUZWrgYKH5FAG4LKxRbDQcEAiIoF0h3LzhpeWIRAH1U9N9-dSCQ0KA
π» Code
π Actual behavior
agets inferred asnumberπ Expected behavior
agets inferred asnumber|undefined(this is the case in 5.0.4)I'm not certain if this change is caused by a bug or intentional, but I couldn't find anything from the announcement at least that made this change, so rather asking for clarification on whether this is an intended behavior
Additional information about the issue
The original definition of
filtercomes from rxjs (simplified in the example for reporting purpose) : https://github.com/ReactiveX/rxjs/blob/2947583bb33e97f3db9e6d9f6cea70c62a173060/src/internal/operators/filter.ts#L6