title: Format a number to two decimal places
body: <p>Give me a native (no jQuery, Prototype, etc. please) JavaScript function that converts numbers as follows: </p>
  
  <pre><code>input:  0.39, 2.5,  4.25, 5.5,  6.75, 7.75, 8.5
  output: 0.39, 2.50, 4.25, 5.50, 6.75, 7.75, 8.50
  </code></pre>
  
  <p>E.g., in Ruby, I'd do something like this:</p>
  
  <pre><code>&gt;&gt; sprintf("%.2f", 2.5)
  =&gt; "2.50"
  </code></pre>
  
  <p>The output may be a number or a string. I don't really care because I'm just using it to set <code>innerHTML</code>.</p>
  
  <p>Thank you.</p>