is undfined javascript error

I am doing a job. Find IE popup a javascript error: "some variable" is undefined.......

I noticed, in javascript if you write like this :

hello
<script type="text/javascript">

if (abc == undefined)
  document.write("undefied") ;
</script>

will popup this kind of message. because 'abc' vairible is undefined.  should be write like this:

hello
<script type="text/javascript">
var abc;
if (abc == undefined)
  document.write("undefied") ;
</script>

be notied: some browser ignored the javascript error. so in some browser can execute correctly, someone not. In fact, error always exists.