Elements in the date array update automatically

I’m trying to find week days between first date and second date (exclusive first and second). In the array that contains the weekday dates, the values get increment after line 1 in executed, which is unexpected.

private getListOfWeekDaysBetweenTwoDates(
    startDate: Date,
    endDate: Date
  ): Date[] {
    const weekdaysList = [];
    let tempDate = this.addDaystoGivenDate(startDate, 1);
    while (tempDate < endDate) {
      if (this.isWeekDay(tempDate)) {
        weekdaysList.push(tempDate);
      }
//line 1
      tempDate = this.addDaystoGivenDate(tempDate, 1);
    }
    return weekdaysList;
  }

 private addDaystoGivenDate(date: Date, numberOfDays: number): Date {
    return new Date(date.setUTCDate(date.getUTCDate() + numberOfDays));
  }

private isWeekDay(day: Date): boolean {
    if (day.getUTCDay() >= 1 && day.getUTCDay() <= 5) {
      return true;
    }
    return false;
  }

How do I stop array values changing?

>Solution :

Isolating the problem, you’re changing the date after inserting it to the array, but what you’re inserting is a reference to the date:

let d = new Date()
d.setUTCDate(1)
const arr = []
arr.push(d)
console.log(arr)
d.setUTCDate(10)
console.log(arr) //date in arr changes from 1 to 10

That’s because you’re calling date.setUTCDate() over tempDate.

There is more than one way to solve it, but for example you could store not the reference to tempDate but a completely new Date object:

weekdaysList.push(tempDate); => weekdaysList.push(new Date(tempDate));

In the simplified example:

let d = new Date()
const arr = []
d.setUTCDate(1)
arr.push(new Date(d))
console.log(arr)
d.setUTCDate(10)
console.log(arr) //date in arr keeps 1