JavaScript Document Object

In the JavaScript Window Object chapter we looked at how perform such tasks as opening and closing browser windows and controlling the appearance and position of those windows. Whilst the window object gives us the ability to control various aspects of the browser window it does not provide us any control over the content of the window. In order to manipulate the content that appears in the browser we need to turn to the JavaScript Document object.

What is the JavaScript Document Object?

The JavaScript Document object essentially contains the HTML elements that are contained the <head> and <body> sections of a web page. It provides the hooks (via a set of object methods and properties) that enable basic changes to be made to the content of a browser window from within a JavaScript script.

The Document object gets the initial value of its title property from the HTML <title> tag and a number of color and event values from the <body> element:


<body
   [alink="activated link color"]
   [background="background image file"]
   [bgcolor="background color"]
   [link="unvisited link color"]
   [onload="page load functionName"]
   [onunload="page unload functionName"]
   [text="foreground color"]
   [vlink="visited link color"]>
</body>

It is important to note that these are only optional initial values. Regardless of whether these are declared in the <body> element they can still be set, or modified from JavaScript via the Document object.

JavaScript Document Object Methods and Properties

The JavaScript Document contains a wide range of methods and properties that are considerable use to developers. The following table lists the key methods and properties provided by the Document object:

Document Object Methods

Method	Description
open()	Opens the output stream to the document for writing
close()	Closes the output stream to the document to prevent further writing
write()	Appends text to the document.
writeln()	Appends text to the document followed by a newline character

Document Object Properties

Property	Description
alinkcolor	The color to be used when displaying activated links in the document
anchors	An array of anchor objects present in the document
applets	An array of Java Applet objects embedded into the document
bgColor	The background color of the document
fgColor	The foreground color (i.e. the color of the text in the document)
cookie	The cookie, if aany, assosiated with the document. See Understanding JavaScript Cookies
domain	The current domain of the document
embeds	An array of objects embedded into the document
forms	An array of the Forms objects contained in the document. See JavaScript Forms and HTML
images	An Array of Image objects contained the document
lastModified	The date that the document was last modified
linkColor	The color to be used when displaying links in the document
links	An array of links contained in the document
referrer	The URL of the document from which the current document was linked (i.e. how the visitor arrived at this page)
title	The title of the docuemnt (corresponds to the HTML <title> tag)
URL	The URL of the current document
vlinkColor	The color to be used when displaying links in the document which have been visited by the current user

Using the JavaScript Document Object

Now that we have seen the methods and properties that are avialable with the Document object we can begin explore how to use them:

Writing text to a document

The document object open() and write() methods can be used to dynamically write text to a web page. The following example contains both some text contained in the HTML file, togetehr with some JavaScript to write additional text into the page:


<html>
<head>
<title>JavaScript Document Object Example</title>
</head>
<body>
<p>
This is a line of text specified in the HTML file
</p>
<script language="javascript" type="text/javascript">
document.open()
document.write("This is a line of text written to the document using JavaScript");
</script>

</body>
</html>

The above example will display the following in the browser window:

This is a line of text specified in the HTML file
This is a line of text written to the document using JavaScript

Writing Text to a document in a different Window

the writing of text to a document not restricted to writing to the document in the current window. It is also possible to write content to a different window simply by referencing the window in tyhe JavaScript command (for details on opening new windows see the JavaScript Window Object chapter). The following example opens a new window and then writes text into that new window:


<html>
<head>
<title>JavaScript Document Object Example</title>
<script language="javascript" type="text/javascript">

function writeContent(content, windowObj)
{
       windowObj.document.write(content);
}
</script>
</head>

<body>

<script language="javascript" type="text/javascript">

newWindowObj = window.open ("", "MyWindow");

</script>

<form action="">
      <input type=button value="Write to New Window" onclick="writeContent('This is new content in the new window', newWindowObj)"/>
</form>
</body>
</html>

Changing the Document Title

Even when the title of a document has been specified using the HTML <title> tag it is possible to dynamically change the current setting using the title property of the document object. The following example intially sets the document title to JavaScript Document Object Example and provides a button which, when pressed, change sthe title to a new value:


<html>
<head>
<title>JavaScript Document Object Example</title>
</head>
<body>
<form action="">
      <input type=button value="Press to change title" onclick="document.title='hello'"/>
</form>
</body>
</html>

Changing the Document Colors

A similar approach can be used to change the colors using in the document. As outlined in the document object properties table there a number of color setting raninging from foreground and background colrs through to colors used for links (such as visited and unvisted link colors). The following example changes the foreground and background colors when the "Change colors" button is pressed:


<html>
<head>
<title>JavaScript Document Object Example</title>
<script language="javascript" type="text/javascript">

function changeColors()
{
        document.fgColor="red";
        document.bgColor="blue";
}
</script>
</head>

<body>
<p>
Sample text to show color change
</p>
<form action="">
      <input type=button value="Change colors" onclick="changeColors()"/>
</form>
</body>
</html>

Getting a List of Objects in a Document

A typical web page can contain a number of other object types, such as links, anchors, images and Java applets. A number of the properties listed in the table above consist of arrays containing a list of objects of a particular type (for details on arrays see JavaScript Arrays). These array based properties can be accessed to obtain a list of objects present in the document. For example the links property of the document object can be used to obtain a list of Link objects (in other words, links to other web pages) in the current document. To demonstrate this, the following example consists of a simple HTML page containing two links to other sites (Amazon.com and Yahoo. The JavaScript code accesses the links property of the document object and iterates through the array elements writing out the value of each to the document using the document object write() method:


<html>
<head>
<title>JavaScript Document Object Example</title>
</head>

<body>
<p>
Sample text to show color change
</p>
<a href="http://www.amazon.com">Buy more books</a>
<a href="http://www.yahoo.com">Stop making Google rich</a>

for (i in document.links)
{
        document.write( document.links[i] + "<br>" );
}
</script>

</form>
</body>
</html>

As one might expect the JavaScript in the above HTML file outputs the following for the two links found in the document:


http://www.amazon.com/
http://www.yahoo.com/

The same approach can be taken obtain the contents of any of the object type array properties of the document object (embeds, applets, forms, and images).