Simple Javascript doesn't return anything -

i'm trying learn javascript. i've written code should return a, b, c, d , if letter in var letters, should alert message "yes true".
when run code doesn't return me anything.

can see why?

<script type="text/javascript">      var letters = ["a", "b", "c", "d"]     var numbers = ["one", "two", "three", "four"]      for(x=0; x < letters.length; x++) {         document.write(letters);         if(a in letters)  {             document.write("yes true");         }     }  </script> 

a variable have not defined, dont confused "a" (string)

and letters array. x index in array iterating through. letters[x] specific element index x.

so, "a" == letters[x] compare fix code.

as side note, can suggest better way of solving problem.

you trying solve iterating through whole list , comparing elements. ok, if want learn concepts iteration, thiscould done lot easier inbuild methods in string object, indexof in case.

<script type="text/javascript">      var letters = ["a", "b", "c", "d"]     var numbers = ["one", "two", "three", "four"]      if(letters.indexof("a") >= 0)  {         document.write("yes true");     }  </script> 

indexof returns index in array of element pass argument, -1 if not in array. if number bigger or equal 0 means element exist in array.