Internet Explorer 5.0 supports persistence, a very important requirement in every computer programming environment. Persistence is a generic term programmers have used for years, to describe the action of capturing an active session, for the purpose of restarting it later from the same state. The term "persistence" is a relatively new one and is actively promoted by Microsoft all over its array of products. "Freeze Files" is a less-esoteric name for the same capability. The essence of this feature is twofold. For CPU-intensive applications, such as complex computer simulations like weather or airflow prediction, persisting an application may be needed for restarting it from an intermediate step, saving the costly calculations up to this step. For user-interaction-intensive applications, such as a request to open a bank account, persisting an application may be needed in order to allow continuation of the session some other time, without the tedious re-filling of the application form.

Microsoft implemented persistence using Behaviors, the new JavaScript feature of Internet Explorer 5.0. We introduced you to Behaviors in Column 22, DHTML Behaviors and explained that there are two types of Behaviors in Part II, More on DHTML Behaviors . Behaviors defined by the user are the most general ones. The other type of Behaviors are predefined by Internet Explorer 5.0 and are used for persistence:

• saveFavorite persists data of a Web page when the user adds it to its Favorites and then access it through the Favorites list.
• saveHistory persists data of a Web page when the user goes forward and backward in his history of pages.
• saveSnapshot persists data of a Web page when the user saves the page to an HTML file and then reopen it.
• userData persists data of a Web page when the user saves it in the browser environment and then retrieves it from there.

Each of the above Behaviors provides a different type of persistence. We'll use a modified version of our Connect Three board game from Column 22, DHTML Behaviors, to demonstrate three of these persistence capabilities.

In this column you'll learn:

• How to recognize the benefits of persistence Behaviors.
• How to write the Connect Three board game.
• How to identify the limitations of Internet Explorer 4.x that persistence removes from version 5.0..
• How to use the saveFavorite persistence Behavior.
• How to use the saveHistory persistence Behavior.
• How to use the saveSnapshot persistence Behavior.
• How to use the userData persistence Behavior.
• How to use the persistence Behavior's methods and events.

At this point you may also view the codes for the non-persistent Connect Three board game, its Favorites-persistent version, its History-persistent version, and its user-data-persistent version.