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JavaScript Practice Object Interaction Checking Out and Returning a Book Demo: Charging Fines to Patrons

Mohamad Faiz Khairi Mohd Nazri
Mohamad Faiz Khairi Mohd Nazri
5,668 Points
Posted by Mohamad Faiz Khairi Mohd Nazri
Mohamad Faiz Khairi Mohd Nazri
5,668 Points

My chargeFines method using for loops

chargeFines() {
    const dt = new Date;

    for (let i = 0; i < this.patrons.length; i++) {
      if (this.patrons[i].currentBook !== null && this.patrons[i].currentBook.dueDate < dt) {
        const dateDiff = new Date(dt - this.patrons[i].currentBook.dueDate);
        const daysLate = dateDiff.getDate();
        this.patrons[i].balance += (daysLate - 1) * this.dailyFine;
      }
    }
  }

Above is my way of writing the chargeFines method with for loops.

However, when i log the daysLate to the console, it give me 1 extra days and I don't know why. So my solution is I deduct the extra days through this code

this.patrons[i].balance += (daysLate - 1) * this.dailyFine;

Can somebody help me check my code. Thanks.

1 Answer

Steven Parker
Steven Parker
231,275 Points
Steven Parker
Steven Parker
231,275 Points

Converting a time difference directly into a date isn't entirely legitimate. Using "getDate" on it is even less so, it will become extremely inaccurate for spans longer than a month.

To avoid inaccuracies in day difference calculations, you might consider these concepts:

  • always store or convert dates to UTC to avoid zone difference issues
  • if only whole day differences are desired, set hours/minutes/seconds/fractions to 0 on all dates
  • after subtracting dates, don't attempt to convert the result into a date
  • instead, just divide the result by the number of milliseconds in a day