function update() {
                //document.form表单的name值.input输入框的name值
		var username = document.form1.username;
		var mobile = document.form1.mobile;
		var qq = document.form1.qq;
        }



<form name="form1" method="post" action="">
  <table width="500" border="1" cellpadding="5" cellspacing="0">
    <tr>
      <td colspan="2" align="center" bgcolor="#FFFF00">通讯录</td>
    </tr>
    <tr>
      <td width="80" align="center">姓名</td>
      <td width="204"><input type="text" name="username" id="username"></td>
    </tr>
    <tr>
      <td align="center">手机号码</td>
      <td><input type="tel" name="mobile" id="mobile"></td>
    </tr>
    <tr>
      <td align="center">QQ号码</td>
      <td><input type="number" name="qq" id="qq"></td>
    </tr>
</table>
</form>