Number index signature doesn't work with Readonly generic #17857

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1. olegdunkan commented on Aug 17, 2017

   

   olegdunkan

   on Aug 17, 2017

   Last edited by olegdunkan

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   Readonly is a mapped type and requires named properties in T, Readonly from number index signature is the empty type {}.
   (edited)

   👍React with 👍1Reacted by Aluan Haddad

   👎React with 👎1Reacted by Junyoung/"Clare" Jang
2. aluanhaddad commented on Aug 17, 2017

   

   Aluan Haddad (aluanhaddad)

   on Aug 17, 2017

   Last edited by aluanhaddad

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   In addition to what olegdunkan stated, Readonly<T> and other mapped types should be applied at the point of use for correct inference, not instantiated ahead of time and used as annotations.

   Consider rewriting the example function as

   function foobar(): void {
     const x = Object.freeze({
       0: 'asdf',
       1: 'fdsa',
       [-1]: 'dddd',
     });
     const y = Object.freeze({
       0: 'qwer',
       1: 'rewq',
       [-1]: 'eeee',
     });
     const z: string = x[0] + y[0];
   }
3. Ryan Cavanaugh (RyanCavanaugh)

   added

   QuestionAn issue which isn't directly actionable in codeAn issue which isn't directly actionable in code

   on Aug 17, 2017
4. remagpie commented on Aug 18, 2017

   

   Joonmo Yang (remagpie)

   on Aug 18, 2017

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   olegdunkan
   But when I use string index instead, I don't get any error.
   Like this:

   type Foo = Readonly<{
     [s: string]: string;
   }>;
   
   type Bar = {
     readonly [s: string]: string;
   };
   
   function foobar(): void {
     const x: Foo = {
       ['asdf']: 'asdf',
       ['fdsa']: 'fdsa',
       ['dddd']: 'dddd',
     };
     const y: Bar = {
       ['qwer']: 'qwer',
       ['rewq']: 'rewq',
       ['eeee']: 'eeee',
     };
     const z: string = x['asdf'] + y['qwer'];
   }
   

   P.S. I think it should be separate issue, but I don't get any error when I do const z: string = x[0] + y[0]; too.
5. olegdunkan commented on Aug 18, 2017

   

   olegdunkan

   on Aug 18, 2017

   Last edited by olegdunkan

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   Declaration of Readonly is

   type Readonly<T> = {
       readonly [P in keyof T]: T[P];
   };

   The type of x in var x:keyof {[k:number]:string} is never
   The type of x in var x:keyof {[k:string]:string} is string

   Then we can assume that in the case of a number index signature tsc produces empty type.
   For string index signature next statement is true:

   A mapped type takes one of the forms
   { [ P in K ] : T }
   { [ P in K ] ? : T }
   { readonly [ P in K ] : T }
   { readonly [ P in K ] ? : T }
   where P is an identifier, K is a type that must be assignable to string

   When P is a string literal type, a property with that name is introduced. Otherwise, when P is type string, an index signature is introduced.

   #12114 (comment)
6. remagpie commented on Aug 18, 2017

   

   Joonmo Yang (remagpie)

   on Aug 18, 2017

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   Oh, it was related to mapped type's operation.
   Thank you for reference.
7. Joonmo Yang (remagpie)

   closed this as completedon Aug 18, 2017
8. microsoft

   locked and limited conversation to collaborators on Jun 14, 2018