let Month = 11 // according to Date's 0-11 month count
const d = new Date();
// can't make this inline bcos of .setMonth 's return value
d.setMonth(Month + 1)
Month = d.getMonth();
var d = new Date('2018-03-31');
d.setMonth(d.getMonth() + 1, 1);
dt = new Date(d);
document.getElementById("demo").innerHTML = dt;