Description

Paul Lu (paulluap)

opened on Jun 21, 2019

Issue body actions

TypeScript Version: 3.5.2

Search Terms:
spread operator type error, Object.assign,
Code

abstract class Base { 
    abstract get a(): string;
    abstract get b(): string;
}
class A extends Base{ 
    a: string
    b: string
}
const a1 : Base = new A() 
const a2 : Base = {...a1}  //reports error: Type '{}' is missing the following properties from type 'Base': a, bts(2739)

const a3 : Base = a1
const a4 : Base = Object.assign({}, a1)

Expected behavior:
code compiles

Actual behavior:
reports error on a2
Type '{}' is missing the following properties from type 'Base': a, bts(2739)

Playground Link:
https://www.typescriptlang.org/play/#src=abstract%20class%20Base%20%7B%20%0D%0A%20%20%20%20abstract%20get%20a()%3A%20string%3B%0D%0A%20%20%20%20abstract%20get%20b()%3A%20string%3B%0D%0A%7D%0D%0Aclass%20A%20extends%20Base%7B%20%0D%0A%20%20%20%20a%3A%20string%0D%0A%20%20%20%20b%3A%20string%0D%0A%7D%0D%0Aconst%20a1%20%3A%20Base%20%3D%20new%20A()%20%0D%0Aconst%20a2%20%3A%20Base%20%3D%20%7B...a1%7D%0D%0Aconst%20a3%20%3A%20Base%20%3D%20a1%0D%0Aconst%20a4%20%3A%20Base%20%3D%20Object.assign(%7B%7D%2C%20a1)

Related Issues:

Reacted by Robouste

Activity

1. jcalz commented on Jun 21, 2019

   

   Joe Calzaretta (jcalz)

   on Jun 21, 2019

   Contributor

   More actions

   In classes, getters go on the prototype like methods, and so object spread won't copy them:

   class X {
     get x() {
       return "x";
     }
   }
   let y = { ...new X() }; // {}
   console.log(y); // {}

   Since a1 is only known to be Base, I think the a2 behavior is probably working as intended.

   That the a4 behavior differs is likely a design limitation; the return type of Object.assign() is an intersection of its input types, which is incorrect for some edge cases.

   This is related to #10727 and #28234 about how we don't have a real spread type and tend to use intersections in generic cases and Object.assign(), but that spread expressions with known concrete class instance only preserve non-method properties.

   Reacted by Paul Lu and Svyat Kovtun
2. Anders Hejlsberg (ahejlsberg)

   added

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

   on Jun 21, 2019
3. fatcerberus commented on Jun 22, 2019

   

   Bruce Pascoe (fatcerberus)

   on Jun 22, 2019

   More actions

   Yeah, I think this is the same deal as how in C++ if you only have a Base reference and call a method on it, you get the base class implementation unless the method is virtual (which requires a vtable, i.e. runtime dispatch). TS types have no runtime representation so if the compiler sees Base then it has no way to know it’s really dealing with a Derived.
4. paulluap commented on Jun 24, 2019

   

   Paul Lu (paulluap)

   on Jun 24, 2019

   Author

   More actions

   I see. Thanks for the replies.
5. Paul Lu (paulluap)

   closed this as completedon Jun 24, 2019

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