计算月初和月末的js

function getLast(cur){ //cur 当前时间
var firstDay = new Date(cur.getFullYear() + '/' + (cur.getMonth()+1) + '/' + cur.getDate() + ' 00:00:00');
firstDay.setDate(1);
//月末
var lastDay = new Date(+firstDay);
lastDay.setMonth(lastDay.getMonth() + 1);
lastDay.setDate(0);

return [firstDay,lastDay];
}