Quick test wrapping a js function that can have undefined args

undefined.nim

when not defined(js):
  {.error: "only works with the js backend".}

proc undefined*(T: typedesc): T =
  asm "`result` = undefined;"

# defines a javascript function that could exist in a js lib
asm """
function canTakeUndefineds (a, b, c) {
  const result = ``received a: ${a}, b: ${b}, c: ${c}``
  console.log(result)
  return result
}
"""

proc canTakeUndefineds(a: int, b=undefined(cstring), c=undefined(int)): cstring {.importc.}

assert canTakeUndefineds(3)                == "received a: 3, b: undefined, c: undefined"
assert canTakeUndefineds(3, "hello world") == "received a: 3, b: hello world, c: undefined"
assert canTakeUndefineds(3, c=45)          == "received a: 3, b: undefined, c: 45"