books.js

var books = [
  {
    id: 1,
    name: 'Intro to Bash',
    description: 'A great book about bash',
    info: {
      stock: 3,
      copies: 3
    }
  },
  {
    id: 2,
    name: 'Fundamentals of Web',
    description: 'Get to grips with the fundamentals of web tech',
    info: {
      stock: 2,
      copies: 2
    }
  }
]

var ledger = [
  {
    user: 'Darren',
    action: 'borrow',
    book: 1
  },
  {
    user: 'Kiki',
    action: 'return',
    book: 2
  },
  {
    user: 'James',
    action: 'borrow',
    book: 1
  }
]

function updateStock() {
  // This function should go through the ledger, determining whether action was
  // to borrow or return a copy of the book, and then increment or decrement the
  // 'stock' parameter of the relevant book
}

updateStock();