Monday, July 4, 2011

HTML DOM Access Nodes



With the DOM, you can access every node in an HTML document.

Accessing Nodes

You can access a node in three ways:
  1. By using the getElementById() method
  2. By using the getElementsByTagName() method
  3. By navigating the node tree, using the node relationships

The getElementById() Method

The getElementById() method returns the element with the specified ID:

Syntax

node.getElementById("id");
The following example gets the element with id="intro":

Example

document.getElementById("intro");
Note: The getElementById() method doesn't work in XML.

The getElementsByTagName() Method

getElementsByTagName() returns all elements with a specified tag name.

Syntax

node.getElementsByTagName("tagname");
The following example returns a nodeList of all <p> elements in the document:

Example 1

document.getElementsByTagName("p");
The following example returns a nodeList of all <p> elements that are descendants of the element with id="main":

Example 2

document.getElementById('main').getElementsByTagName("p");

DOM Node List

The getElementsByTagName() method returns a node-list. A node-list is an array of nodes.
The following code selects all <p> nodes in a node-list:

Example

x=document.getElementsByTagName("p");
The nodes can be accessed by index number. To access the second <p> you can write:
y=x[1];

Note: The index starts at 0.
You will learn more about node-lists in a later chapter of this tutorial.

DOM Node List Length

The length property defines the number of nodes in a node-list.
You can loop through a node-list by using the length property:

Example

x=document.getElementsByTagName("p");

for (i=0;i<x.length;i++)
{
document.write(x[i].innerHTML);
document.write("<br />");
}
Example explained:
  1. Get all <p> element nodes
  2. For each <p> element, output the value of its text node

Navigating Node Relationships

The three properties; parentNode, firstChild, and lastChild, follow the document structure and allow short-distance travel in a document.
Look at the following HTML fragment:
<html>
<body>

<p>Hello World!</p>
<div>
  <p>The DOM is very useful!</p>
  <p>This example demonstrates node relationships.</p>
</div>

</body>
</html>
In the HTML code above, the first p element is the first child node (firstChild) of the body element, and the div element is the last child node (lastChild) of the body element. The parent node (parentNode) of the first p element and the div element, is the the body element, and the parent node of the p elements inside the div element, is the div element.
The firstChild property can also be used to access the text of an element:

Example

<html>
<body>

<p id="intro">Hello World!</p>

<script type="text/javascript">
x=document.getElementById("intro");
document.write(x.firstChild.nodeValue);
</script>

</body>
</html>

DOM Root Nodes

There are two special document properties that allow access to the tags:
  • document.documentElement - returns the root node of the document
  • document.body - gives direct access to the <body> tag
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