Narrow "string" to "keyof T" with "in" guard? #12892

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1. Ryan Cavanaugh (RyanCavanaugh)

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   SuggestionAn idea for TypeScriptAn idea for TypeScript

   on Dec 13, 2016
2. zpdDG4gta8XKpMCd commented on Dec 13, 2016

   

   ZpdDG4gta (zpdDG4gta8XKpMCd)

   on Dec 13, 2016

   Last edited by zpdDG4gta8XKpMCd

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   yes it should as it was proven at runtime to be true

   (maybe not keyof T itself, but something that extends keyof T)
3. DanielRosenwasser commented on Dec 14, 2016

   

   Daniel Rosenwasser (DanielRosenwasser)

   on Dec 14, 2016

   Last edited by DanielRosenwasser

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   Aleksey-Bykov It is actually not something that extends keyof T; it's the opposite, and keyof T is the lower bound. Adding more elements to the union will make it a supertype (i.e. a less-specific type).
4. zpdDG4gta8XKpMCd commented on Dec 14, 2016

   

   ZpdDG4gta (zpdDG4gta8XKpMCd)

   on Dec 14, 2016

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   true, i always forget that unions are the other way around
5. mhegazy commented on Jan 23, 2017

   

   Mohamed Hegazy (mhegazy)

   on Jan 23, 2017

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   You can capture the type of specs as a generic type, and constraint k to be keyof that. if you still want to drill in and get a property type you can use lookup types, e.g.:

   export interface Map<T> {
     [x: string]: T
   }
   
   interface Spec<T> {
     description: string;
     default: T;
   }
   
   function getDescription<S extends Map<Spec<any>>, K extends keyof S>(specs: S, k: K) {
     if (k in specs) {
       return specs[k].description;
       // error TS2536: Type 'string' cannot be used to index type '{ [k in keyof T]: Spec<T[k]>; }'.
     }
     return null;
   }

   For getting the type of default, for example, you can write it as such:

   function getDefault<S extends Map<Spec<any>>, K extends keyof S>(specs: S, k: K): S[K]["default"] {
     if (k in specs) {
       return specs[k]["default"];
       // error TS2536: Type 'string' cannot be used to index type '{ [k in keyof T]: Spec<T[k]>; }'.
     }
     return null;
   }
   
   getDefault({ x: { description: "s", default: 1 } ,y: { description: "s", default: "ss" } }, "x"); // number
   getDefault({ x: { description: "s", default: 1 } ,y: { description: "s", default: "ss" } }, "y"); // string
6. Mohamed Hegazy (mhegazy)

   removed

   In DiscussionNot yet reached consensusNot yet reached consensus

   SuggestionAn idea for TypeScriptAn idea for TypeScript

   on Jan 23, 2017
7. mhegazy commented on Jan 23, 2017

   

   Mohamed Hegazy (mhegazy)

   on Jan 23, 2017

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   The reason why narrowing is not really correct is S can have more properties at runtime than we know about at design time. so if we narrow k to keyof specs it would be more restrictive than users intend.
8. Mohamed Hegazy (mhegazy)

   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 Jan 23, 2017
9. danvk commented on Jan 24, 2017

   

   Dan Vanderkam (danvk)

   on Jan 24, 2017

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   Thanks for the explanation, Mohamed Hegazy (@mhegazy)
10. Dan Vanderkam (danvk)

    closed this as completedon Jan 24, 2017
11. microsoft

    locked and limited conversation to collaborators on Jun 19, 2018