JavaScript validation for empty input field

<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>