you can use the following code:
const monthNames = ["January", "February", "March", "April", "May", "June",
"July", "August", "September", "October", "November", "December"
];
formatDate = function(d) {
return d.getFullYear()+"-"+monthNames[d.getMonth()]
}
$("#mymonth").change(function(){
var mymonth = $(this).val();
$("#mymonth2").val(formatDate(mymonth));
});