当获得一个月的总周数时的奇怪行为

Question

I have strange problem. I have to calculate total weeks of the given date month.

My logic is working perfectly it will return me correct week number but the problem is my original date was also getting changed during calculating weeks.

I have stored my original date in other variable and passed other variable to the function but still the original date is also getting changed. How can i restrict change my original date?

 var currentDate;
 $(document).ready(function () {
    currentDate = new Date();
 });
function goNextMonth() {
   var dt = currentDate;
   console.log("Before Date: " + currentDate);
   var weeks = getWeeksOfMonth(dt);
   console.log("After Date: " + currentDate + ", Weeks: " + weeks);
}
function getWeeksOfMonth(date) {
    var firstDay = new Date(date.setDate(1)).getDay();
    var totalDays = new Date(date.getFullYear(), date.getMonth() + 1, 0).getDate();
    return Math.ceil((firstDay + totalDays) / 7);
}

Cosole Log:

HTML code

 <input type="button" value="Next Month" onclick="goNextMonth()" />

Answer 1

Turn this line...

var firstDay = new Date(date.setDate(1)).getDay();

... into this:

var dateClone = new Date(date);
dateClone.setDate(1);
var firstDay = dateClone.getDay();

The problem is that with date.setDate(1) expression you change the original date object (passed into this function as a param). Obviously, that's not what you want.