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1. fatcerberus commented on Jun 16, 2023

   

   Bruce Pascoe (fatcerberus)

   on Jun 16, 2023

   Last edited by fatcerberus

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   In general, { foo: A | B } is not assignable to { foo: A } | { foo: B } (there are some exceptions, but that's the general rule). This happens to be safe with one property, but not when there are multiple. Since the uncertainty is specifically in the value of prop and not the entire object, the structure of the type should reflect that.

   👍React with 👍1Reacted by Martin Johns
2. FelixZY commented on Jun 16, 2023

   

   Felix Zedén Yverås (FelixZY)

   on Jun 16, 2023

   Last edited by FelixZY

   Author

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   I see. For this particular case I wanted very specifically to have a single property and to do

   type ABase = {
     // ...
   }
   
   export type AString = ABase & {
     prop: {
       type: "string";
       num?: undefined;
       str: string;
     };
   }
   
   export type ANumber = ABase & {
     prop: {
       type: "number";
       num: number;
       str?: undefined;
     };
   }
   
   export type A = AString | ANumber;

   I guess this could be done with even more type variables or Extract instead but I find the lack of intellisense (vscode) on the second type parameter makes Extract a bit unfriendly/unsafe to use.

   ---

   EDIT:

   Solved this by doing:

   type AStringProp = {
     prop: {
       type: "string";
       num?: undefined;
       str: string;
     };
   }
   
   type ANumberProp = {
     prop: {
       type: "number";
       num: number;
       str?: undefined;
     };
   }
   
   type AProp = AStringProp | ANumberProp;
   
   export type A = {
     // ...
     prop: AProp,
   }
   export type AString = A & { prop: AStringProp };
   export type ANumber = A & { prop: ANumberProp };
3. fatcerberus commented on Jun 16, 2023

   

   Bruce Pascoe (fatcerberus)

   on Jun 16, 2023

   More actions

   You could write it like this:

   type ABase = {
     // ...
   }
   
   export type AStringProp = {
       type: "string";
       num?: undefined;
       str: string;
   };
   
   export type ANumberProp = {
       type: "number";
       num: number;
       str?: undefined;
   };
   
   export type A = ABase & {
     prop: 
       | AStringProp
       | ANumberProp
   };
4. Daniel Rosenwasser (DanielRosenwasser)

   added

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

   on Jun 16, 2023
5. Ryan Cavanaugh (RyanCavanaugh)

   closed this as completedon Feb 24, 2024