title: Do a concatenation on javascript array value/key
body: <p>Why concatenation on values or keys of a javascript array is not allowed?</p>
  
  <pre><code>var arr = {'date': year + ", " + month};
  </code></pre>
  
  <p>Or</p>
  
  <pre><code>var arr = {'to' + 'day': day};
  </code></pre>