获取多少天前的日期

function getBeforeDate(n) {
    let d = new Date();
    let calculation = 1000 * 60 * 60 * 24 * n;
    let getTimeStr = new Date().getTime();
    let date = new Date(getTimeStr + calculation);
    let year = date.getFullYear();
    let mon = date.getMonth() + 1;
    let day = date.getDate();
    let s = year + '-' + (mon < 10 ? ('0' + mon) : mon) + '-' + (day < 10 ? ('0' + day) : day);
    return s;
}