The JavaScript Object Model



JavaScript is based on a simple object-oriented paradigm. An object is a construct with properties that are JavaScript variables. Properties can be other objects. Functions associated with an object are known as the object's methods. 

In addition to objects that are built into the Navigator client and the LiveWire server, you can define your own objects. 

•Objects and Properties •Functions and Methods •Creating New Objects 



Objects and Properties



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


objectName.propertyName





You define a property by assigning it a value. For example, suppose there is an object named myCar (we'll discuss how to create objects later-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 = 69;





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 described above as follows: 

myCar["make" = "Ford"
myCar["model" = "Mustang"
myCar["year" = 67;




Equivalently, each of these elements can be accessed by its index, as follows: 

myCar[0] = "Ford"
myCar[1] = "Mustang"
myCar[2] = 67;




This type of an 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 = "quot; 
    for (var i in obj) 
        result += obj_name + ""+ i + "= "+ obj[i] + "n" 
    return result; 
} 




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

myCar.make = Ford
myCar.model = Mustang
myCar.year = 67




Functions and Methods



Functions are one of the fundamental building blocks in JavaScript. A function is a JavaScript procedure-a set of statments that performs a specific task--that you can then call anywhere in the current application. In a Navigator application, you can use any functions defined in the current page. You use the function statement to define a function. It is generally a good idea to define all your functions in the HEAD of a page. When a user loads the page, the functions will then be loaded first. 

A function definition consists of the function keyword, followed by 

•the name of the function •a list of parameters to the function, enclosed in parentheses, and separated by commas •the JavaScript statements that define the function, enclosed in curly braces, {...} 

For example, here is the definition of a simple function named pretty_print: 


function pretty_print(string) {
   document.write("<HR><P>" + string)
}





This function takes a string as its argument, adds some HTML tags to it using the concatenation operator (+), then displays the result to the current document. 

Defining a function does not execute it. You have to call the function for it to do its work. For example, you could call the pretty_print function as follows: 


<SCRIPT>
pretty_print("This is some text to display")
</SCRIPT>





The parameters of a function are not limited to just strings and numbers. You can pass whole objects to a function, too. 

Methods



A method is a function associated with an object. You define a method in the same way as you define a standard function. Then, 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: 


object.methodname(params);





Using this for Object References



JavaScript has a special keyword, this, that you can use to refer to the current object. For example, suppose you have a function called validate that validates an object's value property, given the object, and the high and low values: 


function validate(obj, lowval, hival) {
   if ((obj.value  hival))
      alert("Invalid Value!")
}





Then, you could call validate in each form element's onChange event handler, using this to pass it the form element, as in the following example: 


<INPUT TYPE = "text" NAME = "age" SIZE = 3 onChange="validate(this, 18, 99)">





In general, in a method this refers to the calling object. 

Creating New Objects



Both client and server JavaScript have a number of predefined objects. In addition, you can create your own objects. Creating your own object requires two steps: 

•Define the object type by writing a function. •Create an instance of the object with new. 



To define an object type, create a function for the object type that specifies its name, and its 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: 


car1 = new car("Eagle", "Talon TSi", 1993);





This statement creates mycar and assigns it the specified values for its properties. Then the value of car1.make is the string "Eagle", car1.year is the integer 1993, and so on. 

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


car2 = new car("Nissan", "300ZX", 1992)





An object can have a property that is itself another object. For example, suppose I 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 McNally", 33, "M")
ken = new person("Ken Jones", 39, "M")





Then we 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)





Then if you want to find out the name of the owner of car2, you can access the following property: 


car2.owner.name





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. 

Defining Methods



You can define methods for an object type by including a method defnition in the object type definition. For example, suppose you have a set of image GIF files, and you want to define a method that displays the information for the cars, along with the corresponding image. You could define a function such as: 


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



where pretty_print is the previously defined function to display 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 this new method as follows: 


car1.displayCar()
car2.displayCar()



This will produce output like this: 


------------------------------------------------------------------------
A Beautiful 1993 Eagle Talon TSi 


------------------------------------------------------------------------
A Beautiful 1992 Nissan 300ZX