If somebody forgets to call a constructor with 'new', then 'this' refers to the global scope. When you assign properties within the constructor, you are instead augmenting the global scope. The following pattern helps stop that from happening.

ScopeSafeConstructor

function Book(isbn, title, author) {
  if(this instanceof Book) {
    this.isbn = isbn;
    this.title = title;
    this.author = author;
  } else {
    return new Book(isbn, title, author);
  }
}