Question

i’ve tried to do a very simple IIFE below,

<script type="text/javascript">

    var obj = new Object;

    (function(_obj) {_obj.prop = 'defined';})(obj);

    if(typeof obj.prop === undefined)
     alert('undefined');
    else
     alert(obj.prop);

</script>

Why does the script alert “undefined” than “defined” as a result?

@EDIT

The script shoud have worked as expected except:

• The unintended typeof obj.prop === undefined is wrong, but obj.prop === undefined should be used instead.

• When omitting parenthesis below, the script doesn’t work as expected but none of syntax error is raised from rhino.

  function(_obj) {_obj.prop = 'defined';}(obj);

Answer 1

Your code alerts 'defined' but for the wrong reason.

This…

typeof obj.prop === undefined

should be this…

obj.prop === undefined

…because typeof returns a string representing the type of object.

---

Don’t use the typeof hack when testing for undefined. It’s confusing, and can be the source of bugs such as the one you encountered.

If you’re that worried about undefined being redefined, then do this…

obj.prop === void 0