This will retrieve the next Monday, returning the current date if already a Monday:
var d = new Date();
d.setDate(d.getDate() + (1 + 7 - d.getDay()) % 7);
console.log(d);
To return the following Monday even if the current date is a Monday:
var d = new Date();
d.setDate(d.getDate() + (((1 + 7 - d.getDay()) % 7) || 7));
console.log(d);