Retrieve the last day of the previous month

lastDayMonthBefore.js

var date = new Date();
var firstDay = new Date(date.getFullYear(), date.getMonth(), 1);
firstDay.setDate(firstDay.getDate()-1);
console.log(firstDay);
console.log(firstDay.getFullYear() + '-' + (firstDay.getMonth()+1) + '-' + firstDay.getDate());
//fix two char month
var month = firstDay.getMonth()+1;
if(month < 10){
  month = '0' + month;
}
//fix two char date
var day = firstDay.getDate();
if(day < 10){
  day = '0' + day;
}
var lastDayMonthBefore = firstDay.getFullYear() + '-' + month + '-' + day;
console.log(lastDayMonthBefore);

https://jsfiddle.net/opfxwn9p/