I added a "safe(int): int" proc to the above example (for convenience):
# nim js -d:release -d:nodejs --verbosity:0
type Foo = ref object
a: cstring
b: int
proc isUndefined*[T](x: T): bool {.importcpp: "((#)==undefined)".}
proc safe*(x: int): int {.importcpp: "((#)||0)".}
proc main() =
var o {.noinit.}: Foo
{.emit: "var `o` = {};".}
o.a = "test"
#[ this is Nim not javascript
if o.b == nil:
echo "o.b is not is nil"
]#
if o.b.isUndefined:
echo "but then we are Nim and Nim is King"
echo o.b.safe # 0
o.b = 3
echo o.b.safe # 3
main()
