How you choose to define them depends largely on what you want to be able to do in your application. There are several ways to define appropriate constructor functions to implement the Employee hierarchy. It adds the machine property (whose value defaults to the empty string) and also overrides the dept property with the value 'engineering'. It also overrides the dept property with the value 'sales', indicating that all salespersons are in the same department. It adds the quota property (whose value defaults to 100). It adds the projects property (whose value defaults to an empty array, intended to have an array of strings as its value). It adds the reports property (whose value defaults to an empty array, intended to have an array of Employee objects as its value). Employee has the properties name (whose value defaults to the empty string) and dept (whose value defaults to 'general').This shows an object hierarchy with the following objects: The remainder of this chapter uses the employee hierarchy shown in the following figure. Can add or remove properties dynamically to individual Cannot add properties dynamically at run time.Д¬onstructor function or prototype specifies an initial set of Inherit properties by following the prototype chain.Д¬lass definition specifies all properties of all instances of aД¬lass. Inherit properties by following the class chain. The rest of this chapter describes the details of using JavaScript constructors and prototypes to create an object hierarchy and compares this to how you would do it in Java.Д¬omparison of class-based (Java) and prototype-based (JavaScript) objectД¬lass and instance are distinct entities.ДЄll objects can inherit from another object.Дefine a class with a class definition instantiate a class withДefine and create a set of objects with constructor functions.Д¬reate a single object with the new operator.Д¬onstruct an object hierarchy by using class definitions to defineД¬onstruct an object hierarchy by assigning an object as the prototype The following table gives a short summary of some of these differences. Then, when you create a new Manager, it inherits the name and dept properties from the Employee object. Finally, you assign a new object derived from Employee.prototype as the prototype for the Manager constructor function. Next, you define the Manager constructor function, calling the Employee constructor and specifying the reports property. First you define the Employee constructor function, specifying the name and dept properties. So, you can create exactly the Employee - Manager example, but you use slightly different terminology. JavaScript implements inheritance by allowing you to associate a prototypical object with any constructor function. In this case, an instance of the Manager class would have all three properties: name, dept, and reports. For example, assume the Employee class includes only the name and dept properties, and Manager is a subclass of Employee that adds the reports property. The subclass inherits all the properties of the superclass and additionally can add new properties or modify the inherited ones. In a class definition, you can specify that the new class is a subclass of an already existing class. In a class-based language, you create a hierarchy of classes through the class definitions. In addition, any object can be associated as the prototype for another object, allowing the second object to share the first object's properties. Any object can specify its own properties, either when you create it or at run time. A prototype-based language has the notion of a prototypical object, an object used as a template from which to get the initial properties for a new object. An instance has exactly the same properties of its parent class (no more, no less).ДЄ prototype-based language, such as JavaScript, does not make this distinction: it has objects. For example, Victoria could be an instance of the Employee class, representing a particular individual as an employee. An instance, on the other hand, is the instantiation of a class that is.For example, the Employee class could represent the set of all employees. A class is abstract rather than any particular member in aВ set of objects it describes. A class defines all of the properties that characterize a certain set of objects (considering methods and fields in Java, or members in C++, to be properties).Class-based object-oriented languages, such as Java and C++, are founded on the concept of two distinct entities: classes and instances.

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