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1. MartinJohns commented on Mar 17, 2023

   

   Martin Johns (MartinJohns)

   on Mar 17, 2023

   Contributor

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   Duplicate of #39885 / #1805.
2. Ryan Cavanaugh (RyanCavanaugh)

   added

   DuplicateAn existing issue was already createdAn existing issue was already created

   on Mar 17, 2023
3. mckravchyk commented on Mar 17, 2023

   

   Maciej Kravchyk (mckravchyk)

   on Mar 17, 2023

   Last edited by mckravchyk

   Author

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   Ok, thanks. I never came across this in 3 years of using TS and I have been using plenty overloads. It's good to know that, better late then never but I wished the docs mentioned it. This is an issue that will not come up immediately, so there may be plenty of code using overloads that will need to be refactored upon realization that there is a major bug that has been marked as wontfix. I only found: "Always prefer parameters with union types instead of overloads when possible" but I don't think it sufficiently warns against it. I would interpret it as "use union types if you don't need to narrow the return type based on the input arg".

   And when I try to implement the same with a conditional type in the return, I find myself between the rock and a hard place. I run into a bug I encountered earlier #52051 . As a matter of fact, my workaround to that bug was using overloads :)

   Edit: It seems to be possible to override it with casting after all.

   function get<Arg extends string | string[] | undefined>(
     idOrIds: Arg
   ) : Arg extends string ? string | null
     : Arg extends string[] ? Record<string, string | null>
     : Record<string, string> {
     if (typeof idOrIds === 'string') {
       return ''; // Not assignable
     }
       
   }
   
   function get2<Arg extends string | string[] | undefined>(
     idOrIds: Arg
   ) : Arg extends string ? string | null
     : Arg extends string[] ? Record<string, string | null>
     : Record<string, string> {
     if (typeof idOrIds === 'string') {
       return '' as ReturnType<typeof get2<Arg>>; // Ok
     }
       
   }

   Playground

   Edit2: Hmm, actually I cannot get the above function to work properly when the argument is not supplied. I will investigate it further and update this if anything applies to the current bug.

   Edit3: It's possible to use a combo of overload to deal with the missing parameter and conditional types to deal with the union bug.

   function get3(): Record<string, string>;
   function get3<Arg extends string | string[] | undefined>(
     idOrIds?: Arg
   ) : Arg extends string ? string | null
     : Arg extends string[] ? Record<string, string | null>
     : Arg extends undefined ? Record<string, string> : never;
   function get3(idsOrId?: string | string[]): string | null | Record<string, string | null> | Record<string, string>
      {
     if (typeof idsOrId === 'string') {
       return '';
     }
       
   }
   
   
   let a = get3('');
   let b = get3([]);
   let c = get3(undefined);
   let d = get3();
   
   function get4(): Record<string, string>;
   function get4<Arg extends string | string[] | undefined>(
     idOrIds?: Arg
   ) : Arg extends string ? string | null
     : Arg extends string[] ? Record<string, string | null>
     : Arg extends undefined ? Record<string, string> : never;
   function get4(idsOrId?: string | string[]): string | null | Record<string, string | null> | Record<string, string> {
     return get3(idsOrId);
   }

   Playground
4. Maciej Kravchyk (mckravchyk)

   closed this as completedon Mar 17, 2023
5. microsoft

   locked as resolved and limited conversation to collaborators on Oct 22, 2025