Object Hierarchy and Inheritance in JavaScript

JavaScript is an object-oriented language based on prototypes, rather than, as is common, being class-based. Because of this different basis, it can be less apparent how JavaScript allows you to create hierarchies of objects and to have inheritance of properties and their values. This paper attempts to clarify the situation. If you're interested in precise details of how this all works, you can read the ECMA-262 JavaScript language specification (as a PDF file or a Microsoft Word self-extracting binary).

This paper assumes that you're already somewhat familiar with JavaScript and that you have used JavaScript functions to create simple objects. For information on this subject, see Chapter 10, "Object Model," in the JavaScript Guide.

The sections in this paper are:

Class-based object-oriented languages, such as Java and C++, are founded on the concept of two distinct entities: classes and instances. A class defines all of the properties (considering methods and fields in Java, or members in C++, to be properties) that characterize a certain set of objects. A class is an abstract thing, rather than any particular member of the set of objects it describes. For example, the Employee class could represent the set of all employees. An instance, on the other hand, is the instantiation of a class; that is, one of its members. For example, Victoria could be an instance of the Employee class, representing a particular individual as an employee. An instance has exactly the properties of its parent class (no more, no less).

A prototype-based language, such as JavaScript, does not make this distinction: it simply has objects. 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. Any object can specify its own properties, either when you create it or even at runtime. In addition, any object can be associated as the prototype for another object, allowing the second object to share the first object's properties.

In class-based languages, you define a class in a separate class definition. In that definition you can specify special methods, called constructors, to use to create instances of the class. A constructor method can specify initial values for the instance's properties and perform other processing appropriate at creation time. You use the new operator in association with the constructor method to create class instances.

JavaScript follows a similar model, but does not have a class definition separate from the constructor. Instead, you define a constructor function to create objects with a particular initial set of properties and values. Any JavaScript function can be used as a constructor. You use the new operator with a constructor function to create a new object.

In a class-based language, you create a hierarchy of classes through the class definitions. In a class definition, you can specify that the new class is a subclass of an already existing class. The subclass inherits all the properties of the superclass and additionally can add new properties or modify the inherited ones. For example, assume the Employee class includes only name and dept properties and Manager is a subclass of Employee that adds the reports property. In this case, an instance of the Manager class would have all three properties: name, dept, and reports.

JavaScript implements inheritance by allowing you to associate a prototypical object with any constructor function. So, you can create exactly the Employee-Manager example, but you use slightly different terminology. First you define the Employee constructor function, specifying the name and dept properties. Next, you define the Manager constructor function, specifying the reports property. Finally, you assign a new Employee object as the prototype for the Manager constructor function. Then, when you create a new Manager, it inherits the name and dept properties from the Employee object.

In class-based languages, you typically create a class at compile time and then you instantiate instances of the class either at compile time or at runtime. You cannot change the number or the type of properties of a class after you define the class. In JavaScript, however, at runtime you can add or remove properties from any object. If you add a property to an object that is used as the prototype for a set of objects, the objects for which it is the prototype also get the new property.

Table 1 gives a short summary of some of these differences. The rest of this paper 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.

Table 1 Comparison of class-based (Java) and prototype-based (JavaScript) object systems
Class-based (Java) Prototype-based (JavaScript)
Class and instance are distinct entities.

All objects are instances.

Define a class with a class definition; instantiate a class with constructor methods.

Define and create a set of objects with constructor functions.

Create a single object with the new operator.

Same.

Construct an object hierarchy by using class definitions to define subclasses of existing classes.

Construct an object hierarchy by assigning an object as the prototype associated with a constructor function.

Inherit properties by following the class chain.

Inherit properties by following the prototype chain.

Class definition specifies all properties of all instances of a class. No way to add properties dynamically at runtime.

Constructor function or prototype specifies an initial set of properties. Can add or remove properties dynamically to individual objects or to the entire set of objects.

The Employee Example

The rest of this paper works with the simple employee hierarchy shown in Figure 1.

Figure 1    A simple object hierarchy

  • Employee has the properties name (whose value defaults to the empty string) and dept (whose value defaults to "general").
  • Manager is based on Employee. It adds the reports property (whose value defaults to an empty array, intended to have an array of Employee objects as its value).
  • WorkerBee is also based on Employee. It adds the projects property (whose value defaults to an empty array, intended to have an array of strings as its value).
  • SalesPerson is based on WorkerBee. It adds the quota property (whose value defaults to 100). It also overrides the dept property with the value "sales", indicating that all salespersons are in the same department.
  • Engineer is based on WorkerBee. It adds the machine property (whose value defaults to the empty string) and also overrides the dept property with the value "engineering".
  • Creating the Hierarchy

    There are several ways you can define appropriate constructor functions to implement the Employee hierarchy. How you choose to define them depends largely on what you want to be able to do in your application. We'll get into all that later.

    For now, let's use very simple (and comparatively inflexible) definitions just to see how we get the inheritance to work. In these definitions, you can't specify any property values when you create an object. The newly-created object simply gets the default values, which you can change at a later time. Figure 2 illustrates the hierarchy with these simple definitions.

    In a real application, you would probably define constructors that allow you to provide property values at object creation time. Options for doing so are described later in "More Flexible Constructors". For now, these simple definitions let us look at how the inheritance occurs.

    Figure 2    What the definitions look like

    The simple Java and JavaScript Employee definitions below are similar. The only difference is that you need to specify the type for each property in Java but not in JavaScript and you need to create an explicit constructor method for the Java class.

    JavaScript Java
    function Employee () {
    this.name = "";
    this.dept = "general";
    }
    public class Employee {
       public String name;
       public String dept;
       public Employee () {
          this.name = "";
          this.dept = "general";
       }
    }

    The Manager and WorkerBee definitions show the difference in how you specify the next object higher in the inheritance chain. In JavaScript, you add a prototypical instance as the value of the prototype property of the constructor function. You can do so at any time after you define the constructor. In Java, you specify the superclass within the class definition. You cannot change the superclass outside the class definition.

    JavaScript Java
    function Manager () {
    this.reports = [];
    }
    Manager.prototype = new Employee;
    function WorkerBee () {
    this.projects = [];
    }
    WorkerBee.prototype = new Employee;
    public class Manager extends Employee {
       public Employee[] reports;
       public Manager () {
          this.reports = new Employee[0];
       }
    }
    public class WorkerBee extends Employee {
       public String[] projects;
       public WorkerBee () {
          this.projects = new String[0];
       }
    }

    The Engineer and SalesPerson definitions create objects that descend from WorkerBee and hence from Employee. An object of these types has properties of all the objects above it in the chain. In addition, these definitions override the inherited value of the dept property with new values specific to these objects.

    JavaScript Java
    function SalesPerson () {
       this.dept = "sales";
       this.quota = 100;
    }
    SalesPerson.prototype = new WorkerBee;
    function Engineer () {
       this.dept = "engineering";
       this.machine = "";
    }
    Engineer.prototype = new WorkerBee;
    public class SalesPerson extends WorkerBee {
       public double quota;
       public SalesPerson () {
          this.dept = "sales";
          this.quota = 100.0;
       }
    }
    public class Engineer extends WorkerBee {
       public String machine;
       public Engineer () {
          this.dept = "engineering";
          this.machine = "";
       }
    }

    Using these definitions, you can create instances of these objects that get the default values for their properties. Figure 3 illustrates using these JavaScript definitions to create new objects and shows the property values for the new objects.

    NOTE: As described earlier, the term instance has a specific technical meaning in class-based languages. In these languages, an instance is an individual member of a class and is fundamentally different from a class. In JavaScript, "instance" does not have this technical meaning because JavaScript does not have this difference between classes and instances. However, in talking about JavaScript, "instance" can be used informally to mean an object created using a particular constructor function. So, in this example, you could informally say that jane is an instance of Engineer. Similarly, although the terms parent, child, ancestor, and descendant do not have formal meanings in JavaScript, we can use them informally to refer to objects higher or lower in the prototype chain.
    Figure 3    Creating objects with the simple definitions

    Object Properties

    This section discusses how objects inherit properties from other objects in the prototype chain and what happens when you add a property at runtime.

    Inheriting Properties

    Assume you create the mark object as a WorkerBee as shown in Figure 3 with this statement:

    mark = new WorkerBee;
    When JavaScript sees the new operator, it creates a new generic object and passes this new object as the value of the this keyword to the WorkerBee constructor function. The constructor function explicitly sets the value of the projects property. It also sets the value of the internal __proto__ property to the value of WorkerBee.prototype. (That property name has 2 underscore characters at the front and 2 at the end.) The __proto__ property determines the prototype chain used to return property values. Once these properties are set, JavaScript returns the new object and the assignment statement sets the variable mark to that object.

    This process doesn't explicitly put values in the mark object (local values) for the properties mark inherits from the prototype chain. When you ask for the value of a property, JavaScript first checks to see if the value exists in that object. If it does, that value is returned. If the value isn't there locally, JavaScript checks the prototype chain (using the __proto__ property). If an object in the prototype chain has a value for the property, that value is returned. If no such property is found, JavaScript says the object doesn't have the property. In this way, the mark object has the following properties and values:

    mark.name = "";
    mark.dept = "general";
    mark.projects = [];
    The mark object inherits values for the name and dept properties from the prototypical object in mark.__proto__. It is assigned a local value for the projects property by the WorkerBee constructor. Simply put, this gives you inheritance of properties and their values in JavaScript. Some subtleties of this process are discussed in "Property Inheritance Revisited".

    Because these constructors don't let you supply instance-specific values, this information is generic. The property values are the default ones shared by all new objects created from WorkerBee. You can, of course, change the values of any of these properties. So, you could give specific information for mark as shown here:

    mark.name = "Doe, Mark";
    mark.dept = "admin";
    mark.projects = ["navigator"];

    Adding Properties

    In JavaScript at runtime you can add properties to any object. You are not constrained to use only the properties provided by the constructor function. To add a property that is specific to a single object, you simply assign a value to the object, as in:

    mark.bonus = 3000;
    Now, the mark object has a bonus property, but no other WorkerBee has this property.

    If you add a new property to an object that is being used as the prototype for a constructor function, you add that property to all objects that inherit properties from the prototype. For example, you can add a specialty property to all employees with the following statement:

    Employee.prototype.specialty = "none";
    As soon as JavaScript executes this statement, the mark object also has the specialty property with the value of "none". Figure 4 shows the effect of adding this property to the Employee prototype and then overriding it for the Engineer prototype.

    Figure 4    Adding properties

    More Flexible Constructors

    The constructor functions used so far do not let you specify property values when you create an instance. As with Java, you can provide arguments to constructors to initialize property values for instances. Figure 5 shows one way to do this.

    Figure 5    Specifying properties in a constructor, take 1

    Here are the Java and JavaScript definitions for these objects.

    JavaScript Java
    function Employee (name, dept) {
    this.name = name || "";
    this.dept = dept || "general";
    }
    public class Employee {
       public String name;
       public String dept;
       public Employee () {
          this("", "general");
       }
       public Employee (name) {
          this(name, "general");
       }
       public Employee (name, dept) {
          this.name = name;
          this.dept = dept;
       }
    }
    function WorkerBee (projs) {
    this.projects = projs || [];
    }
    WorkerBee.prototype = new Employee;
    public class WorkerBee extends Employee {
       public String[] projects;
       public WorkerBee () {
          this(new String[0]);
       }
       public WorkerBee (String[] projs) {
          this.projects = projs;
       }
    }
    function Engineer (mach) {
       this.dept = "engineering";
       this.machine = mach || "";
    }
    Engineer.prototype = new WorkerBee;
    public class Engineer extends WorkerBee {
       public String machine;
       public WorkerBee () {
          this.dept = "engineering";
          this.machine = "";
       }
       public WorkerBee (mach) {
          this.dept = "engineering";
          this.machine = mach;
       }
    }

    These JavaScript definitions use a special idiom for setting default values:

    this.name = name || "";
    The JavaScript logical OR operator (||) evaluates its first argument. If that argument is converts to true, the operator returns it. Otherwise, the operator returns the value of the second argument. Therefore, this line of code tests to see if name has a useful value for the name property. If it does, it sets this.name to that value. Otherwise, it sets this.name to the empty string. This paper uses this idiom for brevity; however, it can be puzzling at first glance.

    With these definitions, when you create an instance of an object, you can specify values for the locally defined properties. As shown in Figure 5, you can use this statement to create a new Engineer:

    jane = new Engineer("belau");
    Jane's properties are now:

    jane.name == "";
    jane.dept == "general";
    jane.projects == [];
    jane.machine == "belau"
    Notice that with these definitions, you cannot specify an initial value for an inherited property such as name. If you want to specify an initial value for inherited properties in JavaScript, you need to add more code to the constructor function.

    So far, the constructor function has created a generic object and then specified local properties and values for the new object. You can have the constructor add more properties by directly calling the constructor function for an object higher in the prototype chain. Figure 6 shows these new definitions.

    Figure 6    Specifying properties in a constructor, take 2

    Let's look at one of these definitions in detail. Here's the new definition for the Engineer constructor:

    function Engineer (name, projs, mach) {
    this.base = WorkerBee;
    this.base(name, "engineering", projs);
    this.projects = mach || "";
    }
    Assume we create a new Engineer object as follows:

    jane = new Engineer("Doe, Jane", ["navigator", "javascript"], "belau");
    JavaScript follows these steps:

    1.   First, the new operator creates a generic object and sets its __proto__ property to Engineer.prototype.
    2.   The new operator then passes the new object to the Engineer constructor as the value of the this keyword.
    3.   Next, the constructor creates a new property called base for that object and assigns the value of the WorkerBee constructor to the base property. This makes the WorkerBee constructor a method of the Engineer object.
    NOTE: The name of the base property is not special. You can use any legal property name; base is simply evocative of its purpose.
    4.   Next, the constructor calls the base method, passing as its arguments two of the arguments passed to the constructor ("Doe, Jane" and ["navigator", "javascript"]) and also the string "engineering". Explicitly using "engineering" in the constructor indicates that all Engineer objects have the same value for the inherited dept property and this value overrides the value inherited from Employee.
    5.   Because base is a method of Engineer, within the call to base, JavaScript binds the this keyword to the object created in step 1. Thus, the WorkerBee function in turn passes the "Doe, Jane" and ["navigator", "javascript"] arguments to the Employee constructor function. Upon return from the Employee constructor function, the WorkerBee function uses the remaining argument to set the projects property.
    6.   Upon return from the base method, the Engineer constructor initializes the object's machine property to "belau".
    7.   Upon return from the constructor, JavaScript assigns the new object to the jane variable.
    You might think that, having called the WorkerBee constructor from inside the Engineer constructor, you've set up inheritance appropriately for Engineer objects. This is not the case. Calling the WorkerBee constructor ensures that an Engineer object starts out with the properties specified in all constructor functions that are called. However, if you later add properties to the Employee or WorkerBee prototypes, those properties are not inherited by the Engineer object. For example, assume you have these statements:

    function Engineer (name, projs, mach) {
    this.base = WorkerBee;
    this.base(name, "engineering", projs);
    this.projects = mach || "";
    }
    jane = new Engineer("Doe, Jane", ["navigator", "javascript"], "belau");
    Employee.prototype.specialty = "none";
    The jane object does not inherit the specialty property. You still need to explicitly set up the prototype to ensure dynamic inheritance. Assume instead you have these statements:

    function Engineer (name, projs, mach) {
    this.base = WorkerBee;
    this.base(name, "engineering", projs);
    this.projects = mach || "";
    }
    Engineer.prototype = new WorkerBee;
    jane = new Engineer("Doe, Jane", ["navigator", "javascript"], "belau");
    Employee.prototype.specialty = "none";
    Now the value of the jane object's specialty property is "none".

    Property Inheritance Revisited

    The preceding sections have described how constructors and prototypes provide hierarchies and inheritance in JavaScript. As with all languages, there are some subtleties that were not necessarily apparent in these earlier discussions. This section discusses some of those subtleties.

    Local versus Inherited Values

    Let's revisit property inheritance briefly. As discussed earlier, when you access an object property, JavaScript performs these steps:

    1. Check to see if the value exists locally. If it does, return that value.

    2. If there isn't a local value, check the prototype chain (using the __proto__ property).

    3. If an object in the prototype chain has a value for the specified property, return that value.

    4. If no such property is found, the object does not have the property.
    The outcome of this simple set of steps depends on how you've defined things along the way. In our original example, we had these definitions:

    function Employee () {
    this.name = "";
    this.dept = "general";
    }
    function WorkerBee () {
    this.projects = [];
    }
    WorkerBee.prototype = new Employee;
    With these definitions, assume you create amy as an instance of WorkerBee with this statement:

    amy = new WorkerBee;
    The amy object has one local property, projects. The values for the name and dept properties are not local to amy and so are gotten from the amy object's __proto__ property. Thus, amy has these property values:

    amy.name == "";
    amy.dept = "general";
    amy.projects == [];
    Now assume you change the value of the name property in the prototype associated with Employee:

    Employee.prototype.name = "Unknown"
    At first glance, you might expect that new value to propagate down to all the instances of Employee. However, it does not.

    When you create any instance of the Employee object, that instance gets a local value for the name property (the empty string). This means that when you set the WorkerBee prototype by creating a new Employee object, WorkerBee.prototype has a local value for the name property. Therefore, when JavaScript looks up the name property of the amy object (an instance of WorkerBee), JavaScript finds the local value for that property in WorkerBee.prototype. It therefore does not look farther up the chain to Employee.prototype.

    If you want to change the value of an object property at runtime and have the new value be inherited by all descendants of the object, you cannot define the property in the object's constructor function. Instead, you add it to the constructor's associated prototype. For example, assume you change the code above to the following:

    function Employee () {
       this.dept = "general";
    }
    Employee.prototype.name = "";
    function WorkerBee () {
    this.projects = [];
    }
    WorkerBee.prototype = new Employee;
    amy = new WorkerBee;
    Employee.prototype.name = "Unknown";
    In this case, the name property of amy becomes "Unknown".

    As these examples show, if you want to have default values for object properties and you want to be able to change the default values at runtime, you should set the properties in the constructor's prototype, not in the constructor function itself.

    Determining Instance Relationships

    You may want to know what objects are in the prototype chain for an object, so that you can tell from what objects this object inherits properties. In a class-based language, you might have an instanceof operator for this purpose. JavaScript does not provide instanceof, but you can write such a function yourself.

    As discussed in "Inheriting Properties", when you use the new operator with a constructor function to create a new object, JavaScript sets the __proto__ property of the new object to the value of the prototype property of the constructor function. You can use this to test the prototype chain.

    For example, assume you have the same set of definitions we've been using, with the prototypes set appropriately. Create an __proto__ object as follows:

    chris = new Engineer("Pigman, Chris", ["jsd"], "fiji");
    With this object, the following statements are all true:

    chris.__proto__ == Engineer.prototype;
    chris.__proto__.__proto__ == WorkerBee.prototype;
    chris.__proto__.__proto__.__proto__ == Employee.prototype;
    chris.__proto__.__proto__.__proto__.__proto__ == Object.prototype;
    chris.__proto__.__proto__.__proto__.__proto__.__proto__ == null;
    Given this, you could write an instanceOf function as follows:

    function instanceOf(object, constructor) { 
       while (object != null) {
          if (object == constructor.prototype)
             return true;
          object = object.__proto__;
       }
       return false;
    }
    With this definition, the following expressions are all true:

    instanceOf (chris, Engineer)
    instanceOf (chris, WorkerBee)
    instanceOf (chris, Employee)
    instanceOf (chris, Object)
    But this expression is false:

    instanceOf (chris, SalesPerson)

    Global Information in Constructors

    When you create constructors, you need to be careful if you set global information in the constructor. For example, assume that you want a unique ID to be automatically assigned to each new employee. You could use this definition for Employee:

    var idCounter = 1;
    function Employee (name, dept) {
       this.name = name || "";
       this.dept = dept || "general";
       this.id = idCounter++;
    }
    With this definition, when you create a new Employee, the constructor assigns it the next ID in sequence and then increments the global ID counter. So, if your next statement were:

    victoria = new Employee("Pigbert, Victoria", "pubs")
    harry = new Employee("Tschopik, Harry", "sales")
    victoria.id is 1 and harry.id is 2. At first glance that seems fine. However, idCounter gets incremented every time an Employee object is created, for whatever purpose. If you create the entire Employee hierarchy we've been working with, the Employee constructor is called every time we set up a prototype. That is, assume you have this code:

    var idCounter = 1;
    function Employee (name, dept) {
       this.name = name || "";
       this.dept = dept || "general";
       this.id = idCounter++;
    }
    function Manager (name, dept, reports) {...}
    Manager.prototype = new Employee;
    function WorkerBee (name, dept, projs) {...}
    WorkerBee.prototype = new Employee;
    function Engineer (name, projs, mach) {...}
    Engineer.prototype = new WorkerBee;
    function SalesPerson (name, projs, quota) {...}
    SalesPerson.prototype = new WorkerBee;
    mac = new Engineer("Wood, Mac");
    Further assume that the definitions we've omitted here have the base property and call the constructor above them in the prototype chain. In this case, by the time the mac object is created, mac.id is 5.

    Depending on the application, it may or may not matter that the counter has been incremented these extra times. If you care about the exact value of this counter, one possible solution involves instead using this constructor:

    function Employee (name, dept) {
       this.name = name || "";
       this.dept = dept || "general";
       if (name)
          this.id = idCounter++;
    }
    When you create an instance of Employee to use as a prototype, you do not supply arguments to the constructor. Using this definition of the constructor, when you do not supply arguments, the constructor does not assign a value to the id and does not update the counter. Therefore, for an Employee to get an assigned id, you must specify a name for the employee. In our example, mac.id would be 1.

    No Multiple Inheritance

    Some object-oriented languages allow multiple inheritance. That is, an object can inherit the properties and values from unrelated parent objects. JavaScript does not support multiple inheritance.

    As we've already said, inheritance of property values occurs at runtime by JavaScript searching the prototype chain of an object to find a value. Because an object has a single associated prototype, JavaScript cannot dynamically inherit from more than one prototype chain.

    In JavaScript you can have a constructor function call more than one other constructor function within it. This gives the illusion of multiple inheritance. For example, consider the following statements:

    function Hobbyist (hobby) {
       this.hobby = hobby || "scuba";
    }
    function Engineer (name, projs, mach, hobby) {
       this.base1 = WorkerBee;
       this.base1(name, "engineering", projs);
       this.base2 = Hobbyist;
       this.base2(hobby);
       this.projects = mach || "";
    }
    Engineer.prototype = new WorkerBee;
    dennis = new Engineer("Doe, Dennis", ["collabra"], "hugo")
    Further assume that the definition of WorkerBee is as we've previously seen it. In this case, the dennis object has these properties:

    dennis.name == "Doe, Dennis"
    dennis.dept == "engineering"
    dennis.projects == ["collabra"]
    dennis.machine == "hugo"
    dennis.hobby == "scuba"
    So dennis does get the hobby property from the Hobbyist constructor. However, assume you then add a property to the Hobbyist constructor's prototype:

    Hobbyist.prototype.equipment = ["mask", "fins", "regulator", "bcd"]
    The dennis object does not inherit this new property.

    Last Updated: 12/18/97 15:19:54


    Copyright 1997 Netscape Communications Corporation