Build the library class diagram.

A library lends copies of books to members. The code in the editor does it with loose dictionaries and string names: it works, but nothing says what holds what, and nothing enforces the numbers. Rewrite it as the classes of this diagram, so each line on it is a line you can point to in your code:

%% title: The library loan
classDiagram
  class Library {
    +add_book(title, barcodes)
    +join(member)
    +lend(title, member, day) Loan
    +give_back(loan)
  }
  class Book {
    +title str
  }
  class BookCopy {
    +barcode str
  }
  class Member {
    +name str
  }
  class Loan {
    +borrowed_day int
    +fine(policy, returned_day) int
  }
  class FinePolicy {
    <<interface>>
    +fine(days_late) int
  }
  class PerDayFine
  class LoanError
  class NoCopyAvailable
  Library "1" *-- "*" Book : catalogue
  Book "1" *-- "1..*" BookCopy : copies
  Library "1" *-- "*" Loan : loans
  Library "*" o-- "*" Member : members
  Loan "*" --> "1" BookCopy : of
  Loan "*" --> "1" Member : by
  Loan ..> FinePolicy : prices with
  FinePolicy <|.. PerDayFine
  LoanError <|-- NoCopyAvailable

Build this API (names are Python's; JavaScript and TypeScript use camelCase where they differ):

• LoanError, and NoCopyAvailable which is a LoanError.
• FinePolicy (the interface: fine(days_late)) and PerDayFine(rupees_per_day) which implements it: rupees_per_day * days_late.
• BookCopy(barcode) with .barcode.
• Book(title, barcodes) creates a BookCopy for each barcode, in .copies. A book needs at least one copy (1..*): an empty list raises ValueError (JavaScript: an Error whose message contains at least one copy).
• Member(name) with .name.
• Loan(copy, member, borrowed_day) refers to the very objects it is given, in .copy and .member. Loan.fine(policy, returned_day) is policy.fine(days_late), where a loan lasts 14 days and days_late = max(0, returned_day - (borrowed_day + 14)). The policy is only a parameter: it works with any object that has fine(days_late), including ones the tests write.
• Library() with books (a dict from title to Book), members, and loans. add_book(title, barcodes) creates the Book. join(member) keeps the same Member object it is given (it never creates members). lend(title, member, day) returns a new Loan on the first copy of the title that has no active loan, and raises NoCopyAvailable("no free copy of <title>") when every copy is out. A member may have at most 2 active loans (0..2): a third raises LoanError("<name> already has 2 loans (limit 2)"), and this check comes before the search for a copy. give_back(loan) ends a loan, so its copy can be lent again; for a loan that is not active it raises LoanError.

JavaScript and TypeScript: giveBack, borrowedDay, addBook, daysLate, PerDayFine(rupeesPerDay); the interface is the class FinePolicy (TypeScript: an abstract class), because the tests check instanceof.

The tests check the story of the lesson (2 copies, the third borrower refused, a fine of 10 for a loan returned on day 20), that each relationship is really there (copy is the object you passed, PerDayFine is a FinePolicy, NoCopyAvailable is a LoanError), both numbers, a fine policy the test writes, and that no loan is left as a loose dictionary in your code.