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missing properties after applying spread operator on instance of abstract type #32022
Description
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)
Related Issues:
Activity
jcalz commentedon Jun 21, 2019
In classes, getters go on the prototype like methods, and so object spread won't copy them:
Since
a1is only known to beBase, I think thea2behavior is probably working as intended.That the
a4behavior differs is likely a design limitation; the return type ofObject.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.fatcerberus commentedon Jun 22, 2019
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
Basethen it has no way to know it’s really dealing with aDerived.paulluap commentedon Jun 24, 2019
I see. Thanks for the replies.