Getting the date of next Monday

to fix if today is Monday then sun 7 This will do:

var d = new Date();
d.setDate(d.getDate() + (((1 + 7 - d.getDay()) % 7) || 7));
console.log(d);

If the actual day of the week you want is variable (Sunday, Thursday, ...) and you want a choice whether today could be a possible match or not, and it might be that you want to start with another date (instead of today), then this function may be useful:

function getNextDayOfTheWeek(dayName, excludeToday = true, refDate = new Date()) {
    const dayOfWeek = ["sun","mon","tue","wed","thu","fri","sat"]
                      .indexOf(dayName.slice(0,3).toLowerCase());
    if (dayOfWeek < 0) return;
    refDate.setHours(0,0,0,0);
    refDate.setDate(refDate.getDate() + +!!excludeToday + 
                    (dayOfWeek + 7 - refDate.getDay() - +!!excludeToday) % 7);
    return refDate;
}

console.log("Next is: " + getNextDayOfTheWeek("Wednesday", false));