JavaScript for Ajax developers in .NET framework

Paint PDF 417 in .NET framework JavaScript for Ajax developers

properties to the new object. The use of the new operator causes the constructor to implicitly return the newly created object. As result, the cat variable in the previous statement holds an object with two properties: _name and _age.

Often, some properties of an object are prefixed with an underscore as is the case with _name and _age to suggest that they should be considered private. However, this remains a naming convention only because properties of objects can t have a private scope. Despite what happens in Java or C#, where you can use the private modifier to prevent external objects from accessing a member of a class, in JavaScript the properties of an object are always publicly accessible. By using closures, you can treat local variables defined in a function as private members. But the convention offers a number of advantages, including the ability to inspect members from a debugger.

Every JavaScript object has a property called prototype that returns a reference to an internal object called the prototype. The prototype object plays a major role in JavaScript because it s used to define the template of an object and to implement inheritance.

In a JavaScript object, the purpose of the prototype object is to hold all the properties that will be inherited by all the instances. The prototype object defines the structure of an object, in a manner similar to what is done with classes in many object-oriented languages. In the previous section, you saw how a function the constructor can be used to create custom objects and to add properties to the instances. Listing 3.3 shows how you can use the constructor s prototype object to add additional properties and methods to instances.

function Cat() { this._name; this._age; } Cat.prototype.speak = function() { alert("Meeeeooow!"); }

In listing 3.3, you access the prototype of the Cat function and add a speak method. The speak method calls the alert function to display a string with the voice of a (hungry) cat. What are the consequences of adding a method to the prototype object of the constructor First, whenever you create an object with the new operator and the Cat constructor, the new instance inherits the speak method, as shown in the following code:

var cat = new Cat(); cat.speak();

Second, all references to objects and arrays added to the prototype object are shared between all the instances.

Never store objects or arrays in the prototype object, unless you want to share them across all instances. Instead, store references to objects or arrays in the constructor. This way, each instance has its own copy of the object.

Adding methods to the prototype object is safe, because you re sharing the same function objects between different instances. This can yield some advantages in terms of memory used to store multiple instances, because you re sharing the same function objects. But accessing functions in the prototype is slightly slower than accessing them in the constructor, because they re searched first in the current instance and then in the prototype. A common approach is to declare members in the constructor and methods in the prototype object; this is the approach we ll follow in this book. Now that we ve introduced the prototype object, we ll examine object extensibility. In the next section, we ll recap the most common ways of adding properties to JavaScript objects.

In the previous sections, we explained how to add properties to objects. JavaScript s dynamic features let you add a property to an object at any time by accessing a nonexistent property and assigning it a value, as shown in the following code:

var book = {}; book.title = 'ASP.NET AJAX in Action'; book.publisher = 'Manning';

In addition, you can extend instances of the built-in types by adding new properties to them. For example, you could expand an object of type String as follows:

var str = new String(); str.createdOn = new Date();