Plus equals operator to increase value each time
<p>I am sure this is very simple but I can't get seem to get this to work just right.  Here is my code:</p>

<pre><code>var zero = 10;

function some(elem, num) {
    return elem += num;
}

console.log(some(zero, 10));
console.log(some(zero, 10));
console.log(some(zero, 10));
</code></pre>

<p>From what I understand, I was expecting the variable in <code>zero</code> to increase each time by 10..  So, it would be 10, 20 and 30.  Instead, I keep getting 10.  Obviously, this isn't right.</p>

<p>My question is, how would I increase the variable <code>zero</code> by 10 each time and save that value back to the variable <code>zero</code>?</p>
