Although the Document object's write()and writeln() methods are part of the DOM, they cannot be used to change content after a Web page has been rendered. You can write code that executes the write()and writeln() methods in the current document after it is rendered, but they replace the content that is currently displayed in the Web browser window.

You can, however, use the open() method to create a new document in a window or frame, and then use the write() and writeln() methods to add content to the new document. The close() method notifies the Web browser that you are finished writing to the window or frame and that the document should be displayed. Although later versions of Internet Explorer and Netscape do not require you to use the open() and close() methods with the write() and writeln() methods, some older browsers do not display any content in the window until you execute the close() method. In addition, some browsers, including Firefox, do not stop the spinning icon in the upper-right browser corner that indicates a document is loading until the close() method executes. Because Firefox is the second most widely used browser, you should always use the open() and close() methods when dynamically creating document content.

You should always use the open() and close() methods when you want to use the write() and writeln() methods to update the text displayed in an existing window or frame. Specifically, if you do not use the close() method to notify the Web browser that you are finished writing to the window or frame, then any new calls to the write() and writeln() methods are appended to the existing text that is currently displayed in the window or frame. For example, consider the links in the following code. Each link uses a write() method to print a property of the Navigator object in another frame of a frame-based document. When you click a link, the contents in the right frame should be replaced. However, each time you click a link, the Navigator object property value is appended to the frame; the entire contents of the frame are not replaced. Figure 10-2 shows how the target frame appears after clicking the appCodeName,

Next, you start working on a Web site for a flight-training school called Al's Aviation. You will find three prewritten Web pages, Aviation.html, Pilot.html, and Inst.html, in your Chapter folder for Chapter 10. The Aviation.html file is the home page, the Pilot.html file contains information on private pilot training, and the Inst.html file contains information on flight instrument training.

You will modify these Web pages throughout the chapter.

The Al's Aviation Web pages do not contain h1 elements. You will write code that uses the title property of each Al's Aviation Web page as its h1 element.