[nuxi]

You can avoid the SyntaxError by abusing boolean expressions a bit:

  > obj = {}
  {}
  > obj != null && (obj.bar = 42)
  42
  > obj
  { bar: 42 }

Not as pretty as your LHS case, but it works.

[Wingy]

With the length of that you might as well do:

    if (obj) obj.bar = 42