sys.application

        If you enabled the Microsoft Ajax Library, a new lifecycle starts on the client
        side. As soon as the browser loads the main script file, MicrosoftAjax.js, the client
        runtime creates a global JavaScript object—the Application object—and
        stores it in a global variable called Sys.Application.This new object becomes the brains of a web page in the browser. Despite its
        name, it plays a role similar to the Page object on the server side. Once the Page
        object is done on the server side, the processing on the client side is delegated to
        Sys.Application.
        <br />
        Providing a centralized place to execute the client code—This goal is reached by
        defining a custom page lifecycle on the client. As you’ll see in a moment,
        the client page lifecycle starts when the browser loads the page and ends
        when the user navigates away from the page or the page is reloaded. When
        each stage in the lifecycle is entered, the Application object raises a corresponding
        event.
        <br>
        Hosting the client components instantiated in the page—Once instantiated, client
        components become children of the Application object and can be easily
        accessed through the Application object. Also, they’re automatically disposed
        by the Application object when the web page is unloaded by the browser.
        <br />
        The primary tenet behind components is code reusability. Components implement
        a well-defined set of interfaces that allows them to interact with other components
        and to be interchanged between applications. Thanks to the base
        interfaces, the code encapsulated by components can change at any time without
        affecting the other processing logic.
        <br />
        The Microsoft Ajax Library provides specialized client classes that simplify the
        authoring of client components. The group of classes related to component
        development is called the client component model and closely mirrors the model in
        use in the .NET framework. In this way, you can write component-oriented client
        applications using JavaScript code.