JS obtains the time period of this week and last week (one time period is from Monday to Sunday)

Due to business needs, get this week 、 Last week period , Originally lazy to write , Search the Internet , Many seats are not in a time cycle from Monday to Sunday , Then I wrote down .

/*************************************
* Get this Monday
* It is a time period from Monday to Sunday
* var d = new Date();
* d.getFirstDayOfWeek(lastWeekNum);
* @param lastWeekNum tinyint The number of days subtracted from this Sunday , Is empty or 0, Get this Monday 00:00:00
*************************************/
Date.prototype.getFirstDayOfWeek = function(lastWeekNum) {
    var d = (new Date(this.setDate(this.getDate() - this.getDay()+ (this.getDay() == 0 ? -6:1) )));
    if (lastWeekNum) {
        if (isNaN(lastWeekNum)) lastWeekNum = 7;
        d = d.setDate(d.getDate()-lastWeekNum);
        d = new Date(d);
    }
    return d.getFullYear()+'-'+ (d.getMonth()+1) +'-'+ d.getDate()+' 00:00:00';
}
/*************************************
* Get this Sunday
* It is a time period from Monday to Sunday
* var d = new Date();
* d.getLastDayOfWeek(lastWeekNum);
* @param lastWeekNum tinyint The number of days subtracted from this Sunday , Is empty or 0, Get this Sunday 23:59:59
*************************************/
Date.prototype.getLastDayOfWeek = function(lastWeekNum) {
    var d = (new Date(this.setDate(this.getDate() - this.getDay() +7)));
    if (lastWeekNum) {
        if (isNaN(lastWeekNum)) lastWeekNum = 7;
        d = d.setDate(d.getDate()-lastWeekNum);
        d = new Date(d);
    }
    return d.getFullYear() +'-'+ (d.getMonth()+1) +'-'+ d.getDate()+' 23:59:59';
}