JS-计算某年某月某日星期几

var year = Number(prompt("请输入年："));
		var month = Number(prompt("请输入月："));
		var day = Number(prompt("请输入日："));
		var totaldays = 0;

		for(var i = 1900;i < year;i++) {
			if((i % 4 == 0 && i % 100 != 0) || (i % 400 == 0)) {
				totaldays += 366;
			}else{
				totaldays += 365;
			}
		}

		var isrun = ((year % 4 == 0 && year % 100 != 0) || (year % 400 == 0)) ? 1 : 0;

		switch(month - 1) {
			case 11:
				totaldays += 30;
			case 10:
				totaldays += 31;
			case 9:
				totaldays += 30;
			case 8:
				totaldays += 31;
			case 7:
				totaldays += 31;
			case 6:
				totaldays += 30;
			case 5:
				totaldays += 31;
			case 4:
				totaldays += 30;
			case 3:
				totaldays += 31;
			case 2:
				isrun ? (totaldays += 29) : (totaldays +=28);
			case 1:
				totaldays += 31;
		}

		totaldays += day;

		switch(totaldays % 7) {
			case 1:
				console.log("周一");
				break;
			case 2:
				console.log("周二");
				break;
			case 3:
				console.log("周三");
				break;
			case 4:
				console.log("周四");
				break;
			case 5:
				console.log("周五");
				break;
			case 6:
				console.log("周六");
				break;
			case 0:
				console.log("周日");
				break;