Destructure and spread on generics not typing #33579

Activity

1. Nathan-Fenner commented on Sep 24, 2019

   

   Nathan Fenner (Nathan-Fenner)

   on Sep 24, 2019

   Last edited by Nathan-Fenner

   More actions

   This is behaving as expected, since you don't know that 10 is a valid value of type T["key"]. In particular, consider:

   const example: {key: 1 | 2, val: string} = {key: 1, val: "abc"};
   
   // No type error! But it would be wrong:
   const sameType: {key: 1|2, val: string} = test(example);

   So you'd need to instead give a type like:

   function test<T extends {key: number}>({key, ...rest}: T): Omit<T, "key"> & {key: number} {
     return {key: 10, ...rest};
   }

   or like (but this one TypeScript also rejects, which is a design limitation since its flow-sensitive typing is not smart enough):

   function test<T extends {key: number}>({key, ...rest}: T): number extends T["key"] ? T : Omit<T, "key"> & {key: number} {
     return {key: 10, ...rest}
   }

   👍React with 👍2Reacted by Andrii Dieiev and JmQu
2. Ryan Cavanaugh (RyanCavanaugh)

   added

   Working as IntendedThe behavior described is the intended behavior; this is not a bugThe behavior described is the intended behavior; this is not a bug

   on Sep 25, 2019
3. typescript-bot commented on Sep 27, 2019

   

   TypeScript Bot (typescript-bot)

   on Sep 27, 2019

   Contributor

   More actions

   This issue has been marked 'Working as Intended' and has seen no recent activity. It has been automatically closed for house-keeping purposes.
4. TypeScript Bot (typescript-bot)

   closed this as completedon Sep 27, 2019
5. microsoft

   locked as resolved and limited conversation to collaborators on Oct 21, 2025