js 获取多少天前

getBeforeDate: function(day, str) {
var now = new Date().getTime(); //获取毫秒数
var before = new Date(now - ((day > 0 && day ? day : 0) * 86400 * 1000));
var year = before.getFullYear();
var month = before.getMonth()+1;
var date = before.getDate();
return year + (str ? str : '-') + (month < 9 ? '0' + month : month) + (str ? str : '-') + (date < 9 ? '0' + date : date)
}