why the result is NaN?
<pre><code>var a = 10;
sayHi();
function sayHi()
{
var a = a + 10;
alert(a);
return a;
}
alert(a);
alert(sayHi()+10);
</code></pre>

<p>why the above result is not 20 and 30? i feel the first is 20 , then 30. </p>
