Confusion. . . 😵‍💫 Why is my daysOverDue property negative?

Question

In my Library class I used filter and reduce to generate a .daysOverDue value from a book using a getter method in the book class. 
Also, a setter method out() is used to set .dueDate to -14 days (in the past): 
```js
chargeFines() {
    const now = new Date();
const latePatrons = this.patrons.filter(patron => 
    (patron.currentBook !== null && patron.currentBook.dueDate < now)
);

for (let patron of latePatrons) {
    const daysLate = Math.floor( (now.getTime() - patron.currentBook.dueDate.getTime()) / (1000 * 60 * 60 * 24));
    patron.balance += this.dailyFine * daysLate;
}
}

My solution:
js
collectFines() {
        let today = new Date();
        console.log(today.getDate())
        return this.patrons
            .filter(patron => (patron.currentBook.daysOverDue > 0))
            .reduce((collector, patron) => { 
                console.log(patron.currentBook.dueDate.getDate());
                patron.balance = -patron.currentBook.daysOverDue * this.dailyFine;
                return collector + -patron.balance;
        }, 0);
    }

Here is my Book class where dueDate is set and daysOverDue is calculated: 
js
set out(value) {
        this._out = value;
        if (value) {
            let today = new Date();
            today.setDate(today.getDate() - 14);
            this.dueDate = today;
        } else {
            this.dueDate = null;
        }
    }
get daysOverDue() {
        let today = new Date();
        let daysOverDue = today.getDate() - this.dueDate.getDate();
        return daysOverDue
    }
```
ASIDE: What's the difference between

this.dueDate = today.setDate(today.getDate() - 14);

and

today.setDate(today.getDate() - 14);
this.dueDate = today;

Travis Alstrand · Accepted Answer

I found this page (https://www.tutorialspoint.com/How-to-subtract-days-from-a-date-in-JavaScript#:~:text=To%20subtract%20days%20from%20a%20JavaScript%20Date%20object%2C%20use%20the,date%20according%20to%20local%20time.) which shows an example of using getDate and setDate about halfway down and subtracting days from them. I'm not exactly sure why you're getting a negative number back, if the date it has is less than 14 would it go negative and not loop through the end of the previous month?
I hope this is helpful, if not please let me know!

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Samuel Kleos

Samuel Kleos

Confusion. . . 😵‍💫 Why is my daysOverDue property negative?

1 Answer

Travis Alstrand

Travis Alstrand

Travis Alstrand

Travis Alstrand