空输入字段的JavaScript验证

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