My chargeFines solution took a more long term view of fines. I used a setInterval to automatically charge Patrons

class Library {
  constructor(name) {
    this.name = name
    this.books = []
    this.patrons = []

// binding 'this' to my 'this.chargeFines' was very important, because the asynchronous 
// 'setInterval' method would cause 'this' to reference 'window' when the scope changes
    this._autoCharge = setInterval(this.chargeFines.bind(this), 8.64e7)

    this._dailyFine = 0.1
  }
  addBook(Book) {
    this.books.push(Book)
  }

  addPatron(Patron) {
    this.patrons.push(Patron)
  }

  chargeFines() {
    const today = new Date()
    for (let i = 0; i < this.books.length; i++) {
      if (this.books[i].patron && this.books[i].dueDate < today) {
        this.books[i].patron.balance += this._dailyFine
      }
    }
    return "Successful auto-charge cycle complete."
  }
}