Validation in java script

function isNumberKey()
 {
    var key = window.event.keycode;
    if (key < 48 || key > 57) {
        return false;
    }
    else {
        return true;
    }
 }
 function IsAlpha() {
     var key = window.event.keycode;
     if ((key >= 65 && key <= 90) || (key >= 97 && key <= 122)) {
         return true;
     }
     else {
         return false;
     }
 }

<tr>
                    <td align="right" style="width:20%">MOBILE NO.</td>
                    <td class="style3"><asp:TextBox ID="TXT_M_NO" class="txt" runat="server" onkeypress="return isNumberKey()" BackColor="lightyellow"></asp:TextBox>
                    </td>
                    <td colspan="2"></td>
               </tr>

<tr>
                     <td align="right" class="style2" style="width:20%">
                     <span style="color:Red;">*</span> EMPLOYEE NAME </td>
                     <td class="style1"><asp:TextBox ID="TXTEMP_NAME" class="txt" runat="server" onkeypress="return IsAlpha()" BackColor="lightyellow"></asp:TextBox>
                     </td>
                    <td align="right" style="width:10%">LAST NAME</td>
                    <td><asp:TextBox ID="TxtLnm" class="txt" runat="server" onkeypress="return IsAlpha()" BackColor="lightyellow"></asp:TextBox>
                    </td>
                </tr>