方法一：
读取当前日期，再将day-7，判断day<=0针对不同月份的显示情况：
let today = date.toISOString().split('T')[0];
let oneWeekBefore = '';
let year = date.getFullYear();
let month = date.getMonth() + 1;
let day = date.getDate() - 7;
if(day <= 0 &&(month == 1||3||5||7||8||10||12)){
day = 31+day;
month --;
}
else if(day <= 0 &&(month == 2)){
day = 28+day;
month --;
}
else{
day = 30+day;
month --;
};
day = day > 9 ? day: '0' + day;
if(month == 0){
oneWeekBefore = (year-1) + '-12-' + day;
}
else{
month = month > 9 ? month: '0' + month;
oneWeekBefore = year + '-' + month + '-' + day;
};
方法二：
先读取当前日期，再读取七天前日期。
let date = new Date();
let today = date.toISOString().split('T')[0];
let oneWeekBeforeDate = new Date(date.getTime() - (7 * 24 * 60 * 60 * 1000));
let oneWeekBefore = oneWeekBeforeDate.toISOString().split('T')[0];
显然，第二种方法简便了很多。
