title: Case-by-case Coffee-script push method
body: <p>When I push the index of itemList in list 'a'.<br>
  Right in for loop, it works fine.<br>
  But when I push the index in if statement, it doesn't work.<br>
  I don't know what's secret exists behind the coffee-script engine. </p>
  
  <pre><code>a=[]
  for item, index of itemList
      a.push(index)
      if item.isExchanged is true
          a.push(index)
  </code></pre>