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JavaScriptSource Staff Jul 24, 2006
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Core JavaScript Guide 1.5

Chapter 7   Working with Objects

JavaScript is designed on a simple object-based paradigm. An object is a construct with properties that are JavaScript variables or other objects. An object also has functions associated with it that are known as the object's methods. In addition to objects that are predefined in the Navigator client and the server, you can define your own objects.

This chapter describes how to use objects, properties, functions, and methods, and how to create your own objects.

This chapter contains the following sections:

Objects and Properties

A JavaScript object has properties associated with it. You access the properties of an object with a simple notation:


Both the object name and property name are case sensitive. You define a property by assigning it a value. For example, suppose there is an object named myCar (for now, just assume the object already exists). You can give it properties named make, model, and year as follows:

myCar.make = "Ford";
myCar.model = "Mustang";
myCar.year = 1969;

An array is an ordered set of values associated with a single variable name. Properties and arrays in JavaScript are intimately related; in fact, they are different interfaces to the same data structure. So, for example, you could access the properties of the myCar object as follows:

myCar["make"] = "Ford"
myCar["model"] = "Mustang"
myCar["year"] = 1967

This type of array is known as an associative array, because each index element is also associated with a string value. To illustrate how this works, the following function displays the properties of the object when you pass the object and the object's name as arguments to the function:

function show_props(obj, obj_name) {
   var result = "";
   for (var i in obj)
      result += obj_name + "." + i + " = " + obj[i] + "\n";
   return result

So, the function call show_props(myCar, "myCar") would return the following:

myCar.make = Ford
myCar.model = Mustang
myCar.year = 1967

Creating New Objects

JavaScript has a number of predefined objects. In addition, you can create your own objects. In JavaScript 1.2 and later, you can create an object using an object initializer. Alternatively, you can first create a constructor function and then instantiate an object using that function and the new operator.

Using Object Initializers

In addition to creating objects using a constructor function, you can create objects using an object initializer. Using object initializers is sometimes referred to as creating objects with literal notation. "Object initializer" is consistent with the terminology used by C++.

The syntax for an object using an object initializer is:

objectName = {property1:value1, property2:value2,..., propertyN:valueN}

where objectName is the name of the new object, each propertyI is an identifier (either a name, a number, or a string literal), and each valueI is an expression whose value is assigned to the propertyI. The objectName and assignment is optional. If you do not need to refer to this object elsewhere, you do not need to assign it to a variable.

If an object is created with an object initializer in a top-level script, JavaScript interprets the object each time it evaluates the expression containing the object literal. In addition, an initializer used in a function is created each time the function is called.

The following statement creates an object and assigns it to the variable x if and only if the expression cond is true.

if (cond) x = {hi:"there"}

The following example creates myHonda with three properties. Note that the engine property is also an object with its own properties.

myHonda = {color:"red",wheels:4,engine:{cylinders:4,size:2.2}}

You can also use object initializers to create arrays. See "Array Literals" on page 28.

JavaScript 1.1 and earlier. You cannot use object initializers. You can create objects only using their constructor functions or using a function supplied by some other object for that purpose. See Using a Constructor Function.

Using a Constructor Function

Alternatively, you can create an object with these two steps:

  1. Define the object type by writing a constructor function.

  2. Create an instance of the object with new.

To define an object type, create a function for the object type that specifies its name, properties, and methods. For example, suppose you want to create an object type for cars. You want this type of object to be called car, and you want it to have properties for make, model, year, and color. To do this, you would write the following function:

function car(make, model, year) {
   this.make = make;
   this.model = model;
   this.year = year;

Notice the use of this to assign values to the object's properties based on the values passed to the function.

Now you can create an object called mycar as follows:

mycar = new car("Eagle", "Talon TSi", 1993);

This statement creates mycar and assigns it the specified values for its properties. Then the value of mycar.make is the string "Eagle", mycar.year is the integer 1993, and so on.

You can create any number of car objects by calls to new. For example,

kenscar = new car("Nissan", "300ZX", 1992);
vpgscar = new car("Mazda", "Miata", 1990);

An object can have a property that is itself another object. For example, suppose you define an object called person as follows:

function person(name, age, sex) {
   this.name = name
   this.age = age
   this.sex = sex

and then instantiate two new person objects as follows:

rand = new person("Rand McKinnon", 33, "M");
ken = new person("Ken Jones", 39, "M");

Then you can rewrite the definition of car to include an owner property that takes a person object, as follows:

function car(make, model, year, owner) {
   this.make = make;
   this.model = model;
   this.year = year;
   this.owner = owner

To instantiate the new objects, you then use the following:

car1 = new car("Eagle", "Talon TSi", 1993, rand);
car2 = new car("Nissan", "300ZX", 1992, ken);

Notice that instead of passing a literal string or integer value when creating the new objects, the above statements pass the objects rand and ken as the arguments for the owners. Then if you want to find out the name of the owner of car2, you can access the following property:


Note that you can always add a property to a previously defined object. For example, the statement

car1.color = "black"

adds a property color to car1, and assigns it a value of "black." However, this does not affect any other objects. To add the new property to all objects of the same type, you have to add the property to the definition of the car object type.

Indexing Object Properties

In JavaScript 1.0, you can refer to an object's properties by their property name or by their ordinal index. In JavaScript 1.1 or later, however, if you initially define a property by its name, you must always refer to it by its name, and if you initially define a property by an index, you must always refer to it by its index.

This applies when you create an object and its properties with a constructor function, as in the above example of the Car object type, and when you define individual properties explicitly (for example, myCar.color = "red"). So if you define object properties initially with an index, such as myCar[5] = "25 mpg", you can subsequently refer to the property as myCar[5].

The exception to this rule is objects reflected from HTML, such as the forms array. You can always refer to objects in these arrays by either their ordinal number (based on where they appear in the document) or their name (if defined). For example, if the second <FORM> tag in a document has a NAME attribute of "myForm", you can refer to the form as document.forms[1] or document.forms["myForm"] or document.myForm.

Defining Properties for an Object Type

You can add a property to a previously defined object type by using the prototype property. This defines a property that is shared by all objects of the specified type, rather than by just one instance of the object. The following code adds a color property to all objects of type car, and then assigns a value to the color property of the object car1.


See the prototype property of the Function object in the Core JavaScript Reference for more information.

Defining Methods

A method is a function associated with an object. You define a method the same way you define a standard function. Then you use the following syntax to associate the function with an existing object:

object.methodname = function_name

where object is an existing object, methodname is the name you are assigning to the method, and function_name is the name of the function.

You can then call the method in the context of the object as follows:


You can define methods for an object type by including a method definition in the object constructor function. For example, you could define a function that would format and display the properties of the previously-defined car objects; for example,

function displayCar() {
   var result = "A Beautiful " + this.year + " " + this.make
      + " " + this.model;

where pretty_print is function to display a horizontal rule and a string. Notice the use of this to refer to the object to which the method belongs.

You can make this function a method of car by adding the statement

this.displayCar = displayCar;

to the object definition. So, the full definition of car would now look like

function car(make, model, year, owner) {
   this.make = make;
   this.model = model;
   this.year = year;
   this.owner = owner;
   this.displayCar = displayCar;

Then you can call the displayCar method for each of the objects as follows:


This produces the output shown in the following figure.