Function truthy check doesn't apply if the function is in a boolean expression #37598

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1. Ryan Cavanaugh (RyanCavanaugh)

   added

   Experience EnhancementNoncontroversial enhancementsNoncontroversial enhancements

   SuggestionAn idea for TypeScriptAn idea for TypeScript

   on Mar 25, 2020
2. Ryan Cavanaugh (RyanCavanaugh)

   added this to the Backlog milestone on Mar 25, 2020
3. Andarist commented on Sep 13, 2022

   

   Mateusz Burzyński (Andarist)

   on Sep 13, 2022

   Last edited by Andarist

   Contributor

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   The situation depicted by this particular playground has been actually fixed~ in 4.2.x: TS playground. It was most likely improved by #41599

   However, some cases still don't work, like this one: TS playground

   const thing = {
     isBeautiful() {
       return true;
     },
     isBiggerThanBreadBox() {
       return false;
     },
     isYellow() {
       return false;
     },
   };
   
   if (thing.isBeautiful || thing.isYellow) {
     console.log("Hi");
   }
   
   // Should error like above
   if (thing.isBeautiful() && (thing.isBiggerThanBreadBox || thing.isYellow)) {
     console.log("Hi");
   }
4. amunger commented on Dec 9, 2022

   

   Aaron Munger (amunger)

   on Dec 9, 2022

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   Is the fact that simply negating the check causes the warning to disappear covered by this issue?

   https://www.typescriptlang.org/play?#code/MYewdgzgLgBFAWBLMBzGBeGBvAsAKBhkQgCEBTAQwFcpEAzKgGwAoBKbfQwgJzKiu5g43KmQDcnGAF98UiXnz0YzBMhQA6YuWq0GjdrgIxQkEIzLrGIFMwBEACUS3W8mQrwB6DzADK8EEwAJjBk3Nwg3DCMiADWZDAUAEYgAG5kinTKAISqqJqklDT0TAaSJhBmFlY2Dk4uskA

   const thing = {
     isBeautiful() {
       return true;
     }
   };
   
   if (thing.isBeautiful) {
     console.log("Hi");
   }
   
   // Should error like above
   if (!thing.isBeautiful) {
     console.log("Hi");
   }
   
5. Andarist commented on Dec 9, 2022

   

   Mateusz Burzyński (Andarist)

   on Dec 9, 2022

   Contributor

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   Aaron Munger (@amunger) this is definitely related but I'd say that perhaps it's worth creating a separate issue about this (if there is none about it already). I have a PR open that aims to fix this but it only deals with binary(like) expressions - I didn't touch unary expressions anyhow there.
6. RyanCavanaugh commented on Dec 9, 2022

   

   Ryan Cavanaugh (RyanCavanaugh)

   on Dec 9, 2022

   Member

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   The uncalled function check is intentionally very limited, since there are a lot of cases where you might be checking a function truthiness on purpose. For the case of if (!x) it's reasonable to not see an x in the body of the function, and we didn't want you to have to write !!!x to get around the checking error, so not including that case was intentional.
7. Jake Bailey (jakebailey)

   closed this as completedin #50756on Jan 17, 2023
8. Jake Bailey (jakebailey)

   added a commit that references this issue on Jan 17, 2023

   Improve uncalled function check related to parenthesized and binary e…

   ...

   16c695c