Support with construct in JS

[LPTK]

For instance:

class C[A]: { x: A }
//│ Defined class C

c = C{x = 1}
//│ c: c & {C#A: 'A | 1 .. 'A, x: 1}

d = c with { x = "hi"; y = 2 }
//│ d: c & {C#A: 'A | 1 .. 'A, y: 2, x: "hi"}

d.x
//│ res: "hi"

To do this, I guess we should create a new object, copy over all the fields of the previous object (there is a JS method to do that but I forget its name), and set the new fields in the new object, potentially overriding old ones. We may have to also explicitly set the prototype of the new object to the same thing as the old one, so we retain class instance-ness.

[chengluyu]

The method you mentioned might be Object.assign. For class instances, we can use Object.setPrototypeOf.