avaScript is based on a simple object-oriented 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 built into the Navigator client and the LiveWire 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.
objectName.propertyNameBoth 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"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.model = "Mustang"
myCar.year = 69;
myCar["make"] = "Ford"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:
myCar["model"] = "Mustang"
myCar["year"] = 67
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 = 67
For example, here is the definition of a simple function named pretty_print:
function pretty_print(str) {
document.write("<HR><P>" + str)
}
This function takes a string, str, as its argument, adds some HTML tags to it using the concatenation operator (+), and then displays the result to the current document using the write method.In addition to defining functions as described here, you can also define Function objects, as described in "Function object".
<SCRIPT>The arguments of a function are not limited to strings and numbers. You can pass whole objects to a function, too. The show_props function (defined in "Objects and properties") is an example of a function that takes an object as an argument.
pretty_print("This is some text to display")
</SCRIPT>
function factorial(n) {
if ((n == 0) || (n == 1))
return 1
else {
result = (n * factorial(n-1) )
return result
}
}
You could then display the factorials of one through five as follows:for (x = 0; x < 5; x++) {
document.write("<BR>", x, " factorial is ", factorial(x))
}
The results are:
0 factorial is 1
1 factorial is 1
2 factorial is 2
3 factorial is 6
4 factorial is 24
5 factorial is 120
functionName.arguments[i]where functionName is the name of the function and i is the ordinal number of the argument, starting at zero. So, the first argument passed to a function named myfunc would be myfunc.arguments[0]. The total number of arguments is indicated by the variable arguments.length.
Using the arguments array, you can call a function with more arguments than it is formally declared to accept using. This is often useful if you don't know in advance how many arguments will be passed to the function. You can use arguments.length to determine the number of arguments actually passed to the function, and then treat each argument using the arguments array.
function list(type) {
document.write("<" + type + "L>") // begin list
// iterate through arguments
for (var i = 1; i < list.arguments.length; i++)
document.write("<LI>" + list.arguments[i])
document.write("</" + type + "L>") // end list
}
You can pass any number of arguments to this function, and it will then display each argument as an item in the indicated type of list. For example, the following call to the functionlist("o", "one", 1967, "three", "etc., etc...")
results in this output:
1.one
2.1967
3.three
4.etc., etc...
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)
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:car2.owner.nameNote 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.
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].
Car.prototype.color=null
car1.color="black"
birthday.description="The day you were born"
object.methodname = function_namewhere 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);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
pretty_print(result)
}
where pretty_print is the function (defined in "Functions") 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:car1.displayCar()This will produce output like:
car2.displayCar()
function validate(obj, lowval, hival) {
if ((obj.value < lowval) || (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 = 3In general, this refers to the calling object in a method.
onChange="validate(this, 18, 99)">
When combined with the form property, this can refer to the current object's parent form. In the following example, the form myForm contains a Text object and a button. When the user clicks the button, the value of the Text object is set to the form's name. The button's onClick event handler uses this.form to refer to the parent form, myForm.
<FORM NAME="myForm">
Form name:<INPUT TYPE="text" NAME="text1" VALUE="Beluga">
<P>
<INPUT NAME="button1" TYPE="button" VALUE="Show Form Name"
onClick="this.form.text1.value=this.form.name">
</FORM>