title: when to use reduce and reduceRight?
body: <p>Can you describe this for me?</p>
  
  <pre><code>var arr, total;
  arr = [1, 2, 3, 4, 5];
  total = arr.reduce(function(previous, current) {
  return previous + current;
  });
  // total is 15
  </code></pre>