TS2532 Compound null check, object cannot be undefined #35260

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1. AnyhowStep commented on Nov 21, 2019

   

   AnyhowStep

   on Nov 21, 2019

   Last edited by AnyhowStep

   Contributor

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   TS, in general, doesn't do multi-variable narrowing where the narrowing of variable x also narrows variable y.

   It can get very expensive and complicated to do so.

   ---

   Off topic, seeing logical operators used on non-boolean operands triggers me =x

   ---

   You should write something like,

   //Using non-boolean expressions as boolean expressions also triggers me
   if (createdAt) {
     return createdAt;
   }
   return version ? version.createdAt : null;

   👍React with 👍1Reacted by Martin Johns
2. Ryan Cavanaugh (RyanCavanaugh)

   added

   Design LimitationConstraints of the existing architecture prevent this from being fixedConstraints of the existing architecture prevent this from being fixed

   on Nov 22, 2019
3. RyanCavanaugh commented on Nov 22, 2019

   

   Ryan Cavanaugh (RyanCavanaugh)

   on Nov 22, 2019

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   Narrowing can't produce conditional forms like what would be needed to handle this
4. fatcerberus commented on Nov 25, 2019

   

   Bruce Pascoe (fatcerberus)

   on Nov 25, 2019

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   For the record, the expression form x || y doesn't prove anything about either x or y except that "at least one of them is truthy--but we don't know which." This is not enough information to narrow either one.

   Duplicate (pretty much) of #31603
5. fpintos commented on Mar 21, 2021

   

   Fabio Pintos (fpintos)

   on Mar 21, 2021

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   I have a similar condition that has been plaguing my ability to refactor long conditions into local variables.
   This actually breaks when we apply a suggested refactoring.

   For example, see this code in playground:

   export default function actOnDataOrUndefined({ data }: { data?: string[] }) {
     // This is fine
     const ok = data && data.length != 0 ? actOnData(data) : null;
   
     // Refactor the condition to a local variable and typescript 
     // reports that the call to actOnData is invalid because data can be undefined
     // when it clearly(?) cannot be, since hasData is true only when data is not undefined.
     const hasData = data && data.length != 0;
     const not_ok = hasData ? actOnData(data) : null;
   }
   
   function actOnData(data: string[]) {
     return data.length;
   }
6. sdandois commented on Jan 27, 2023

   

   Santiago José Dandois (sdandois)

   on Jan 27, 2023

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   Hi, I may be a noob, but I was wondering if it would be possible to detect that createdAt || version.createdAt is the same expression in both cases. Of course, it would only work if the compounds are primitive types like string or number.

   For example, you could pre-process the code into:

   const stuff = (version?: string, createdAt?: string) => {
     const localVariable : string | undefined = version || createdAt
     if (!(localVariable)) {
       return null;
     }
   
     return localVariable;
   };
7. Ryan Cavanaugh (RyanCavanaugh)

   closed this as completedon Feb 24, 2024