Why is output of variable NaN : <p>I wrote simple code like this</p>

<pre><code>var value = function(a){return a*3;}
var number = value(a);
</code></pre>

<p>when i type <code>value(5);</code> i will get number 15 thats OK, but after typing <code>number;</code> ill get <code>NaN</code>.
I just want to write in variable the result of function.</p>
