输入一个日期，判断这个日期是这一年的第几天

1.switch实现(利用switch穿透)

<script>
		var date = +prompt("请输入一个日期(例如：20160211):");
        var year = parseInt(date / 10000);//年
        var month = parseInt(date / 100) % 100;//月份
        var day = date % 100;//天
        var days = 0;//一年中第几天
        var array = [31,28,31,30,31,30,31,31,30,31,30,31];
        if(month > 12 || month < 0 || day > 31 || day < 0 || isNaN(date)){
            alert("日期格式输入有误！！！");
        }else{
        	switch(month){
                case 12:
                    days += 30;
                case 11:
                    days += 31;
                case 10:
                    days += 30;
                case 9:
                    days += 31;
                case 8:
                    days += 31;
                case 7:
                    days += 30;
                case 6:
                    days += 31;
                case 5:
                    days += 30;
                case 4:
                    days += 31;
                case 3:
                    if(year % 4 == 0 && year % 100 != 0 || year % 400 == 0)
                        days += 29;
                    else
                        days += 28;
                case 2:
                    days += 31;
                case 1:
                    days += day;
            }
        }
        console.log(date + ":" + year + "年" + month + "月" + day + "日" + "，是这一年中的第：" + days + "天");
</script>

2.for+array实现

<script>
		var date = +prompt("请输入一个日期(例如：20160211):");
        var year = parseInt(date / 10000);
        var month = parseInt(date / 100) % 100;
        var day = date % 100;
        var days = 0;
        var array = [31,28,31,30,31,30,31,31,30,31,30,31];
        if(month > 12 || month < 0 || day > 31 || day < 0 || isNaN(date)){
            alert("日期格式输入有误！！！");
        }else{
            // for(var i = 0; i < month - 1; i++){
            //         days += array[i];
            // }
            // if(month > 2)
            //     if(year % 4 == 0 && year % 100 != 0 || year % 400 == 0)
            //         days += 1;
            // days += day;
        }
        console.log(date + ":" + year + "年" + month + "月" + day + "日" + "，是这一年中的第：" + days + "天");
</script>