[scala.js] Dotty does not produce null/undefined property on anonymous class

Compiler version

3.1.0

Minimized code

• With Scala.js 1.8.0

trait Foo extends js.Object {
  var extra: js.UndefOr[String]
}

val anon1 = new js.Object() {
  var extra = "a"
}
val anon2 = new js.Object() {
  var extra = js.undefined
}
val anon3 = new js.Object() {
  var extra = null
}
val foo1 = new Foo {
   var extra = "a"
}
val foo2 = new Foo {
  var extra: js.UndefOr[String] = js.undefined
}
val foo3 = new Foo {
  var extra: js.UndefOr[String] = null
}
println(js.Object.hasProperty(anon1, "extra"))
println(js.Object.hasProperty(anon2, "extra"))
println(js.Object.hasProperty(anon3, "extra"))
println(js.Object.hasProperty(foo1, "extra"))
println(js.Object.hasProperty(foo2, "extra"))
println(js.Object.hasProperty(foo3, "extra"))

Output

On Scala 2.13.7 and 2.12.15,

true
true
true
true
true
true

However on Scala 3.1.0,

true
false
false
true
true
true

which means null or undefined property on anonymous class is missing.

val foo2 = new Foo {
  var extra: js.UndefOr[String] = js.undefined
}
val foo3 = new Foo {
  var extra: js.UndefOr[String] = null
}

Expectation

All properties should exist even if its value is null or undefined.

[sjrd]

This appears to happen only when the type of the var is Unit or Null (whether inferred, or explicitly written). It does not seem to be related to having a supertrait, since I can also reproduce the issue when the trait declares the member but the class implements it with a more specific type that is explicitly Null or Unit.

Also, I can reproduce this with a named class, local or top-level. So it’s also not related to anonymous classes.

The situations where this issue pops up are therefore summarized as:

• A field of type Unit or Null in a non-native JS class.

[xerial]

@sjrd I also hit this issue when trying to implement an object comparison method like deepEquals(jsObjectA, jsObjectB) in Scala 3 + Scala.js.