Undefined Variables

Looking for clarification of undefined variables vs. error in
JavaScript code.

e.g.
<script>
alert( z ); // this will be an error, i.e. an exception
</script>

<script>
alert( this.z );
alert( window.z ); // both equivalent, no error, but window.z is
'undefined'
</script>

why is it that 'z' alone without the global object generates an error,
shouldn't it also be undefined? for example if i did

z = 0;

the javascript interpreter knows to create a global variable z