I'm a little bit confused with what you really want to get referencing your variable names, so I include them all:
const currentYear = 2021;
const currentMonth = 10;
const prevMonthFirstDay = new Date(currentYear, currentMonth - 2, 1);
const prevMonthLastDay = new Date(currentYear, currentMonth - 1, 0);
const currentMonthFirstDay = new Date(currentYear, currentMonth - 1, 1);
const currentMonthLastDay = new Date(currentYear, currentMonth, 0);
const nextMonthFirstDay = new Date(currentYear, currentMonth, 1);
const nextMonthLastDay = new Date(currentYear, currentMonth + 1, 0);
console.log('prev month first day:', prevMonthFirstDay);
console.log('prev month last day:', prevMonthLastDay);
console.log('current month first day:', currentMonthFirstDay);
console.log('current month last day:', currentMonthLastDay);
console.log('next month first day:', nextMonthFirstDay);
console.log('next month last day:', nextMonthLastDay);
on my system this will gives:
prev month first day: Wed Sep 01 2021 00:00:00 GMT+0700 (Western Indonesia Time)
prev month last day: Thu Sep 30 2021 00:00:00 GMT+0700 (Western Indonesia Time)
current month first day: Fri Oct 01 2021 00:00:00 GMT+0700 (Western Indonesia Time)
current month last day: Sun Oct 31 2021 00:00:00 GMT+0700 (Western Indonesia Time)
next month first day: Mon Nov 01 2021 00:00:00 GMT+0700 (Western Indonesia Time)
next month last day: Tue Nov 30 2021 00:00:00 GMT+0700 (Western Indonesia Time)
so now it will only a matter of formatting:
console.log(currentMonthFirstDay.toLocaleDateString('en-GB').split('/').reverse().join('-'));
which will give you:
2021-10-01