密码强弱的判断

window.onload = function () {

            document.getElementById('txt').onkeyup = function () {

                var tds = document.getElementById('tb').getElementsByTagName('td');
                for (var i = 0; i < tds.length; i++) {
                    tds[i].style.backgroundColor = '#E6E6E6';
                }
                //获取密码
                var pwdTxt = this.value;
                //调方法
                if (pwdTxt.length > 0) {

                    var num = checkPwd(pwdTxt);
                    if (num <= 1) {
                       tds[0].style.backgroundColor = 'green';
                    } else if (num == 2) {
                       tds[0].style.backgroundColor = 'red';
                       tds[1].style.backgroundColor = 'red';
                    } else if (num == 3) {
                       tds[0].style.backgroundColor = 'blue';
                       tds[1].style.backgroundColor = 'blue';
                       tds[2].style.backgroundColor = 'blue';
                    }
                }
                //判断等级
            };
        };

        //判断密码的等级
        function checkPwd(pwd) {
            var lvl = 0; //存等级的
            if (pwd.match(/\d/)) {//判断是否包含数字
                lvl++;
            }
            if (pwd.match(/[a-zA-Z]/)) {
                lvl++;
            }
            if (pwd.match(/[^0-9a-zA-Z]/)) {
                lvl++;
            }
            if (pwd.length <= 6) {
                lvl--;
            }
            return lvl;
        }
<input id="txt" type="text" name="name" value="" style="width: 300px; border: 1px solid gray;" />
    <table id="tb" style="width: 300px; text-align: center; background-color: #E6E6E6;">
        <tr>
            <td>
                弱
            </td>
            <td>
                中
            </td>
            <td>
                强
            </td>
        </tr>
</table>