箭头函数正在更改日期常量

Question

可以编写此箭头函数以保留firstOfMonth值吗？

const today = new Date();
const firstOfMonth = new Date(today.getFullYear(), today.getMonth(), 1);
console.log("First Day Of Month: " + firstOfMonth.getDay());
//"First Day Of Month: 3"

const firstSunday = fDate => new Date(fDate.setDate(fDate.getDate() - fDate.getDay()));
console.log("First Sunday: " + firstSunday(firstOfMonth));
//"First Sunday: Sun Apr 28 2019 00:00:00 GMT-0400 (Eastern Daylight Time)"

console.log("First Day Of Month: " + firstOfMonth.getDay());
//"First Day Of Month: 0"