When coding a class, the ES2015 feature of function parameter destructuring allows using a single constructor parameter that is a record with a simplified syntax for defining its fields. We make use of this new feature for obtaining a simplified class definition syntax illustrated by the following example:

class Book {
  constructor ({isbn, title, year, ...}) {
    this.isbn = isbn;
    this.title = title;
    this.year = year;
    ...
  }
  ...
}

A single-valued reference property, such as the property publisher of the object type Book, allows storing internal references to objects of another type, such as Publisher. When creating a new object, the constructor function needs to have a parameter for allowing to assign a suitable value to the reference property. In a typed programming language, such as Java, we would have to take a decision if this value is expected to be an (internal) object reference or an (external) ID reference. In JavaScript, however, we can take a more flexible approach and allow using either of them, as shown in the following example:

class Book {
  constructor ({isbn, title, year,
      publisher, publisher_id}) {
    ...
    // assign object reference or ID reference
    if (publisher || publisher_id)  {
      this.publisher = publisher || publisher_id;
    }
    ...
  }
  ...
}

Notice that the record parameter's publisher field represents a JS object reference while its publisher_id field represents an ID reference. In JavaScript, we can use a disjunctive expression like expr1 || expr2 for getting the value of the first expression, if it is defined (and not 0), or else the value of the second expression. We handle the resulting ambiguity in the property setter by checking the type of the argument as shown in the following code fragment:

set publisher(p) {
  var publisher_id = "";
  // p can be an ID reference or an object reference 
  publisher_id = (typeof p !==  "object") ? p : p.name;
  ...
  this._publisher = Publisher.instances[ publisher_id];
  ...
}

Notice that the name of a publisher is used as an ID reference, since it is the standard ID of the Publisher class.