JavaScript, Third Edition
Chapter 9
Introduction to the Document
Object Model (DOM)
Objectives
• Learn about dynamic Web pages
• Study the Document Object Model (DOM)
• Work with the Image object
• Create animation with the Image object
• Learn how to cache images
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Introduction
• Businesses want:
– Web sites to include Formatting and images that can
be updated without the user having to reload a Web
page from the server
– Innovative ways to use animation and interactive Web
pages to attract and retain visitors
– To make their Web sites effective and easy to navigate
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Introduction (Cont.)
• These kinds of effects:
– Cannot be created with standard Extensible Hypertext
Markup Language (XHTML)
– Needs the use of Dynamic HTML (DHTML)
• One of the most important aspects of DHTML is the
Document Object Model (DOM)
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Creating Dynamic Web Pages
•
Dynamic:
–
Web pages that respond to user requests through
buttons or other kinds of controls
–
Various kinds of effects, such as animation, that
appear automatically in a Web browser
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Creating Dynamic Web Pages
(Cont.)
• A dynamic Web page can allow a user to:
– Change the document background color
– Submit a form and process a query
– Participate in an online game or quiz
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Creating Dynamic Web Pages
(Cont.)
• To make Web pages truly dynamic, you need more
than just XHTML
– Need Dynamic HTML or (DHTML)
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Creating Dynamic Web Pages
(Cont.)
• Dynamic HTML (DHTML):
– Refers to a combination of technologies that make
Web pages dynamic
• The term DHTML is:
– Combination of JavaScript, XHTML, CSS, and the
Document Object Model
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The Document Object Model
• Is at the core of DHTML
• Represents the Web page displayed in a window
• Each element on a Web page is represented in the
DOM by its own object
• This makes it possible for a JavaScript program to:
– Access individual elements on a Web page
– Change elements individually, without having to
reload the page from the server
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Document Object Properties
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Document Object Methods
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Document Object Methods (Cont.)
• Open() method:
– Could be used to create a new document in a window
or frame
– Use the write() and writeln() methods to add content to
the new document
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Document Object Methods (Cont.)
• The close() method:
– Notifies the Web browser that
• You are finished writing to the window or frame
• The document should be displayed
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The Image Object
• Represents an image created using the <img> element
• Use to dynamically change an image displayed on a
Web page
• Image objects for each <img> element:
– Assigned to elements of images[] array in the order
they appear on the Web page
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The Image Object (Cont.)
• An Image object contains various properties and
events that you can use to manipulate your objects
• The src property:
– One of the most important parts of image object
– Allows JavaScript to dynamically change an image
– Changing assigned value also changes the src attribute
associated with an <img> element
• Dynamically changes an image displayed on a Web
page
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The Image Object (Cont.)
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The Image Object (Cont.)
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Animation with the Image Object
• You can create simple animation on a Web page:
– Combine the src attribute of the Image object with the
setTimeout() or setInterval() methods
• Create an animated sequence with JavaScript by using:
– The setInterval() or setTimeout() methods to cycle
through the frames in an animation series
– Each iteration of a setInterval() or setTimeout() method
changes the frame displayed by an <img> element
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Image Caching
• Technique for eliminating multiple downloads of the
same file
• Temporarily stores image files in memory on a local
computer
• Allows JavaScript to store and retrieve an image from
memory rather than download the image each time it is
needed
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Image Caching (Cont.)
•
Images are cached using the Image() constructor of
the Image object
–
•
Creates new Image object
Three steps for caching an image in JavaScript:
1. Create a new object using the Image() constructor
2. Assign a graphic file to the src property of the new
Image object
3. Assign the src property of the new Image object to
the src property of an <img> element
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Chapter Summary
• Dynamic HTML (DHTML):
– Combination of technologies that make Web pages
dynamic
– DHTML is a combination of JavaScript, XHTML,
CSS, and the Document Object Model
• Document Object Model, or DOM:
– At the core of DHTML
– Represents the Web page displayed in a window
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Chapter Summary (cont.)
• The open() method:
– Creates a new document in a window or frame
• The close() method:
– Notifies Web browser that you are finished writing to
the window or frame and that the document should be
displayed
• An Image object:
– Represents an image created using the <img> element
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Chapter Summary (cont.)
• Src property:
– One of the most important properties of the Image
object
– Allows JavaScript to change an image dynamically
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Chapter Summary (cont.)
• Image caching:
– Technique for eliminating multiple downloads of the
same file
– Temporarily stores image files in memory
– Allows JavaScript to store and retrieve an image from
memory rather than downloading the image each time
it is needed
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Chapter Summary (cont.)
• Onload event handler of the Image:
– Use it to be certain that all images are downloaded into
a cache before commencing an animation sequence
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