title: Why does NaN appear for simple subtraction in Javascript?
body: <p>This problem occurred to me when i did this:</p>
  
  <pre><code>var y;
  var x;
  
  function move(e) {
  var x=e.x;
  var y=e.y;
  }
  
  var a=y-600;
  alert(a);
  </code></pre>
  
  <p>When i did this the output was:</p>
  
  <pre><code>a=NaN
  </code></pre>