fix #12303 (#15964)

compiler/jsgen.nim

@@ -1074,8 +1074,13 @@ proc genAsgnAux(p: PProc, x, y: PNode, noCopyNeeded: bool) =
       lineF(p, "$1 = $2;$n", [a.rdLoc, b.rdLoc])
     else:
       useMagic(p, "nimCopy")
+      # supports proc getF(): var T
+      if x.kind in {nkHiddenDeref, nkDerefExpr} and x[0].kind in nkCallKinds:
           lineF(p, "nimCopy($1, $2, $3);$n", 
                 [a.res, b.res, genTypeInfo(p, y.typ)])
+      else:
+        lineF(p, "$1 = nimCopy($1, $2, $3);$n",
+              [a.res, b.res, genTypeInfo(p, y.typ)])
   of etyBaseIndex:
     if a.typ != etyBaseIndex or b.typ != etyBaseIndex:
       if y.kind == nkCall:

tests/js/t12303.nim

@@ -0,0 +1,16 @@
+discard """
+  output: "{ b: 2 }"
+"""
+
+import jsconsole, jsffi
+
+type
+  A = ref object
+   b: B
+
+  B = object
+    b: int
+
+var a = cast[A](js{})
+a.b = B(b: 2)
+console.log a.b

tests/varres/tvarres0.nim

@@ -4,6 +4,7 @@ discard """
 123
 1234
 12345
+123456
 '''
 """
 
@@ -28,6 +29,8 @@ getF().a = 1234
 echo getF().a
 getF() = Foo(a: 12345)
 echo getF().a
+(addr getF())[] = Foo(a: 123456)
+echo getF().a
 
 
 block: # #13848