Practicing Objects video is confusing

Question

I am confused by the code below, I have provided my answer to the chargeFines method which works and the teachers but I can not see where she updates the patrons array.

She never directly updates it but when she logs the patron object to console it has updated. ?

class Library {
  constructor() {
    this.books = [];
    this.patrons = [];
    this.dailyFine = 0.1;

  }
  addBook(book) {
  // addd books to the  books array
   this.books.push(book);

  }
  addPatron(patron) {
    // add patrons to the patrons array
    this.patrons.push(patron);
  }
  chargeFines() {
    //fines are charged for overdue books by 1 cent per day over duedate
    // use filter to return a list of patrons who's books are over due..
    // get todays date
    var daysOverDue = [];
    const daysAllowed = 14;
     const today = new Date();
    const overDue = this.patrons.filter((patron) =>{
      if (patron.currentBook != null) {
        var dueDate = patron.currentBook.dueDate;
        var timeDiff = Math.abs(dueDate.getTime() - today.getTime());
        var diffDays = Math.ceil(timeDiff / (1000 * 3600 * 24));
      }
      if (diffDays > 14) {
        return patron;
      }
    })
    //console.log(overDue ); 
    for(let i = 0; i < overDue.length; i++) {
      let dueDate = patron.currentBook.dueDate;
      let timeBetween = Math.abs(dueDate.getTime() - today.getTime());
      let totalDaysOut = Math.floor(timeBetween / (1000 * 3600 * 24));
      let fine = (totalDaysOut * this.dailyFine);
       if (this.patrons[i].name === overDue[i].name) {
          this.patrons[i].balance = fine;
      }

    }

  }
}

/* Her solution is more elegant but doesnt seem to update the patrons array held in this class (Library) ?

  chargeFines() {
    const now = new Date();

    const latePatrons = this.patrons.filter(patron => 
        (patron.currentBook !== null && patron.currentBook.dueDate < now)
    );

    for (let patron of latePatrons) {
        const dateDiff = new Date(now - patron.currentBook.dueDate);
        const daysLate = dateDiff.getDate();
        patron.balance += this.dailyFine * daysLate;
    }
}



The return is implicit
*/

In her code in the comments above she returns an array into an array called latePatrons via using the filter method on this.patrons.filter which will not update the patrons array but return a new array to latePatrons.

She then iterates over this array via the for loop and updates the balance property.

In index. html

she uses console.log(patron) which is the name of the object an it reflects the change?

<script>
  // our test code here
  const library = new Library();
  const book = new Book('Harry Potter and the Sorcerer\'s Stone', 'J.K. Rowling', '978-0439708180');
  const bookB = new Book('Art of fighting', 'Bruce Leee ', '556677');
  const patron = new Patron("Laurence Kite", "lawwz@hotmail.com");
  const patron2 = new Patron("Bruce Lee", "Lee@kungfu.com");
  library.addPatron(patron);
  library.addPatron(patron2);
  library.addBook(book);
  library.addBook(bookB);
  patron.checkOut(book);
  patron2.checkOut(bookB);
  console.log(patron);
  console.log(patron2);
  library.chargeFines();
  console.log(patron);
  console.log(patron2);
</script>

She has not updated the object directly, but a returned copy. ?

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Laurence kite

Laurence kite

Practicing Objects video is confusing