Can't assign existing property to the rest of generic #28952

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1. weswigham commented on Dec 11, 2018

   

   Wesley Wigham (weswigham)

   on Dec 11, 2018

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   Pavel Klimashkin (@klimashkin) - Consider:

   interface AB {
     a: string;
     b: string;
   }
   
   function excludeA<T extends AB>(obj: T) {
     const {a, ...rest} = obj;
   
     rest.b = '3';
   
     return rest;
   }
   
   interface ExactlyAB extends AB {
     a: "a";
     b: "b"
   }
   
   declare var exactAB: ExactlyAB;
   
   excludeA(exactAB).b; // claims to be of type "b", but will be the value "3" with your implementation 
2. weswigham commented on Dec 11, 2018

   

   Wesley Wigham (weswigham)

   on Dec 11, 2018

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   (But in general yes, #28821 is the reason why the assignment doesn't work, even when you correctly assign a T["b"] - I just wanted to let you know that that code is unsafe from a typesystem perspective, even once that bug is fixed)
3. Wesley Wigham (weswigham)

   added

   DuplicateAn existing issue was already createdAn existing issue was already created

   on Dec 11, 2018
4. que-etc commented on Dec 11, 2018

   

   Denis Rul (que-etc)

   on Dec 11, 2018

   Last edited by que-etc

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   Wesley Wigham (@weswigham)
   Does it mean that any reassignment of an object's property, which has a generic type, should be considered invalid?

   const test = <T extends {foo: string}>(obj: T): T => {
       // No errors as for v3.2.2
       obj.foo = 'bar';
   
       return obj;
   };

   Playground.
5. weswigham commented on Dec 12, 2018

   

   Wesley Wigham (weswigham)

   on Dec 12, 2018

   Last edited by weswigham

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   Yea - it's a known hole that gets closed when you use indexed access types (which get automatically made when you fetch properties off mapped types). It's quite troublesome, really, since the only reason we haven't fixed it, AFAIK, is for backwards compatibility (and to make mutating patterns "easier" to type).
6. TypeScript Bot (typescript-bot)

   closed this as completedon Dec 13, 2018
7. typescript-bot commented on Dec 13, 2018

   

   TypeScript Bot (typescript-bot)

   on Dec 13, 2018

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   This issue has been marked as a duplicate and has seen no activity in the last day. It has been closed for automatic house-keeping purposes.
8. klimashkin commented on Sep 5, 2019

   

   Pavel Klimashkin (klimashkin)

   on Sep 5, 2019

   Last edited by klimashkin

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   The issue has been fixed:

   http://www.typescriptlang.org/play/?allowJs=true&target=6#code/JYOwLgpgTgZghgYwgAgJIHkBGArZBvAKGWOTgC5kQBXAW02gG4iTMLq7HniE3b6omAXwIEYVEAjDAA9iGSQAzmAA8AFWQQAHpBAATBWizYANMgDSG7RD0H1AMnzIAXrw4DkggHwAKaTgqqAJQU6DTAKmamAORwUcgAPshRmFGexIQkyAiySvhwppimAHQlUBBKgsgAvMh+uHAGZkxcyGVKRU7VyADMAEzNmWVgVFBybWBCIopg3njkyACMBRS9pjw9proUACyCgUW6DEA

   🎉React with 🎉1Reacted by Ivan
9. microsoft

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