var functionName = function() {} vs function functionName() {}

he difference is that functionOne is defined at run-time, whereas functionTwo is defined at parse-time for a script block. For example:

<script>
  // Error
  functionOne();

  var functionOne = function() {
  }
</script>

<script>
  // No error
  functionTwo();

  function functionTwo() {
  }
</script>