var str = "";
var week = new Date().getDay();
if (week == 0) {
    str = "今天是星期日";
} else if (week == 1) {
    str = "今天是星期一";
} else if (week == 2) {
    str = "今天是星期二";
} else if (week == 3) {
    str = "今天是星期三";
} else if (week == 4) {
    str = "今天是星期四";
} else if (week == 5) {
    str = "今天是星期五";
} else if (week == 6) {
    str = "今天是星期六";
}

var str1 = "今天是星期";
var week = new Date().getDay();
switch (week) {
    case 0 :
        str1 += "日";
        break;
    case 1 :
        str1 += "一";
        break;
    case 2 :
        str1 += "二";
        break;
    case 3 :
        str1 += "三";
        break;
    case 4 :
        str1 += "四";
        break;
    case 5 :
        str1 += "五";
        break;
    case 6 :
        str1 += "六";
        break;
}
alert(str1);

var str = "今天是星期" + "日一二三四五六".charAt(new Date().getDay());
alert(str);