Reverse a string - why isn't this correct

JavaScript
1 
function reverseString(str) {
  var array = ['hello','Howdy','Greetings from Earth'];
  array = str.split('');
  array = array.reverse();
  array = array.join('');
  str = array.join('');
  return str;
}

reverseString("hello");
reverseString("Howdy");
reverseString("Greetings from Earth");