[javascript] JavaScript validation for empty input field

<script type="text/javascript">_x000D_
  function validateForm() {_x000D_
    var a = document.forms["Form"]["answer_a"].value;_x000D_
    var b = document.forms["Form"]["answer_b"].value;_x000D_
    var c = document.forms["Form"]["answer_c"].value;_x000D_
    var d = document.forms["Form"]["answer_d"].value;_x000D_
    if (a == null || a == "", b == null || b == "", c == null || c == "", d == null || d == "") {_x000D_
      alert("Please Fill All Required Field");_x000D_
      return false;_x000D_
    }_x000D_
  }_x000D_
</script>_x000D_
_x000D_
<form method="post" name="Form" onsubmit="return validateForm()" action="">_x000D_
  <textarea cols="30" rows="2" name="answer_a" id="a"></textarea>_x000D_
  <textarea cols="30" rows="2" name="answer_b" id="b"></textarea>_x000D_
  <textarea cols="30" rows="2" name="answer_c" id="c"></textarea>_x000D_
  <textarea cols="30" rows="2" name="answer_d" id="d"></textarea>_x000D_
</form>