DOM nodes help you analyze relations between elements. Because of the element nesting, a node tree is created to provide better visualization. The structure and relationships between nodes will be explained as well. You will find out what a root, parent, child, and a sibling is in a node tree.
- The HTML DOM allows you to navigate the node tree by making use of the node relationships.
What are Nodes and How are They Connected
The HTML Document Object Model standard says that everything in HTML documents is a node:
- The document itself is a document node.
- Each element is an element node.
- All of the text inside the elements are text nodes.
- Each attribute assigned to an element is an attribute node.
- Each comment is a comment node.
The nodes are connected by hierarchical relationships in the DOM tree. The terms parent, child, and sibling are used to describe this relationship:
- Root is the top node of the tree.
- Parent is an element that contains other elements. Every element has one, except for the root.
- Child is below, or contained by, the parent. A parent may have any number of child nodes.
- Siblings are nodes under the same parent.
Let's illustrate this with an example:
<html> <head> <title>Amazingly Creative Title</title> </head> <body> <h2>Wonderful Heading</h2> <div>The text!</div> </body> </html>
In an example of an HTML script above, you can see that:
<html>is the root node, has no parent and is the parent of
<body>are children of
<title>is a child of
<head>and has a single child, which is a text node
Amazingly Creative Title.
<body>has two children:
<h2>has a single child text node:
<div>has a single child text node:
Navigating Between Nodes
Expecting an element node to contain text is a frequent error when working with the document object model.
For example: in the case of
<title>Amazingly Creative Title</title>, the element node
<title> itself contains no text. However, it does contain a text node
Amazingly Creative Title.
This can be accessed by using the
nodeValue property, or
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Child Nodes and Node Values
innerHTML property is handy for changing the content of an element,
nodeValue properties can be used to get the element's content.
The code in the example below gets the value of
<h2> element and copies it into a
<html> <body> <div id="el1">Amazingly Creative Heading</div> <div id="el2">Look this text DOM!</div> <script> document.getElementById("el2").innerHTML = document.getElementById("el1").childNodes.nodeValue; </script> </body> </html>
In the example above,
getElementById is a method, meanwhile
nodeValue are properties.
For the purposes of this tutorial, we are going to use the
innerHTML property. This does not mean the above method is somehow less useful for learning and understanding the DOM's underlying structure!
Keep in mind that using
firstChild has a similar effect as
<html> <body> <h2 id="el1">Amazingly Creative Title</h2> <div id="el2">the Text!</div> <script> document.getElementById("el2").innerHTML = document.getElementById("el1").firstChild.nodeValue; </script> </body> </html>
DOM Root Nodes
document.bodyis used for getting the document's body.
document.documentElementused to get the whole document.
Look at the examples below to see both of them used in code:
<html> <body> <script> alert(document.body.innerHTML); </script> <p>Amazingly Creative Title</p> <div> <p>Hail the DOM!</p> <p>Witness the power of the <strong>document.body</strong> property.</p> </div> </body> </html>
<html> <body> <script> alert(document.documentElement.innerHTML); </script> <p>Check this out!</p> <div> <p>Hail the DOM!</p> <p>Witness the power of the <strong>document.documentElement</strong> property!</p> </div> </body> </html>
Properties You Should Know
nodeName property is used to specify the node's name and is always read-only. It can return these values:
- Tag name
- Attribute name
- #text (in case of a text node).
- #document (in case of a document node).
- #comment (in case of a comment node).
Note: nodeName will always contain the HTML element name in uppercase.
nodeValue property is used to specify the value of a node. It has three possible return values:
- NULL (in case of a document or element node)
- Content (in case of a text or comment node)
- Attribute value (in case of an attribute node)
nodeType property is used to return the type of node. Review the table below to see the most important types of nodes:
- Node relationships are defined as
- There are various node properties you can use to access nodes.