readonly mapped type modifiers for Map / ReadonlyMap #30633

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1. Ryan Cavanaugh (RyanCavanaugh)

   added

   SuggestionAn idea for TypeScriptAn idea for TypeScript

   In DiscussionNot yet reached consensusNot yet reached consensus

   on Mar 28, 2019
2. JakeTunaley commented on Mar 28, 2019

   

   JakeTunaley

   on Mar 28, 2019

   Last edited by JakeTunaley

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   This would actually be a breaking change if someone is using a type like Readonly<Map<Foo, Bar>>, because right now methods like Map.prototype.put exist on that type, but after this change they would not.

   Also, instead of just adding a special case for Map, why not do this in general?

   class SomeClass {
       public readonly readonlyField: any;
       public mutableField: any;
   
       public get property (): any {}
       public set property (value: any) {}
   
       public set propertyOnlySet (value: any) {}
   
       public readonly readonlyMethod () {}
       public mutationMethod () {}
   }
   
   // Readonly<SomeClass> will be the same as:
   class SomeClass {
       public readonly readonlyField: any;
       public readonly mutableField: any;
   
       public get property (): any {}
   
       public readonly readonlyMethod () {}
   }

   That would be incredibly useful, though it would benefit from a new error checking that readonly methods don't mutate class state.

   class SomeClass {
       public mutableField: any;
   
       public set property (value: any) {}
   
       public mutationMethod () {}
   
       public readonly readonlyMethod () {
           this.mutableField = 0; // Error: Cannot mutate a property inside a readonly method
           this.property = 0; // Error: Cannot mutate a property inside a readonly method
           this.mutationMethod(); // Error: Cannot call a non-readonly instance method from inside a readonly method
       }
   }

   👍React with 👍2Reacted by Daniel Contreras and Peter Flynn
3. bjchambers commented on Mar 28, 2019

   

   Ben Chambers (bjchambers)

   on Mar 28, 2019

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   Hmm... you are correct that it has the potential to break existing code. However, it seems like the same argument would apply to the following change, which made similar changes to ReadonlyArray:

   #29435

   👍React with 👍1Reacted by Peter Flynn