title: Return numbers which appear only once (JavaScript)
body: <p>Say I have the array <code>[1,2,3,5,2,1,4]</code>. How do I get make JS return <code>[3,4,5]</code>? </p>
  
  <p>I've looked at other questions here but they're all about delete the copies of a number which appears more than once, not both the original and the copies. </p>
  
  <p>Thanks! </p>