XML DOM Node List and NamedNodeMap

DOM Node List and NamedNodeMap

This chapter explains what a NodeList is, and what a NamedNodeMap is. It also explains the differences between them.

DOM Node List

When using properties or methods like childNodes or getElementsByTagName(), we receive a NodeList object.

A NodeList object represents an ordered list of nodes.

The nodes in the NodeList can be accessed through their index number (starting from 0).

Note: In a NodeList, the nodes are returned in the order in which they are specified in the XML.

Look at the following XML file: books.xml

We will now create a node list of all the <title> elements in "books.xml", by using the getElementsByTagName("title") method. The following image represents the node list returned:

The following code fragment gets the text from the first <title> element:

getElementsByTagName("title")[0].childNodes[0].nodeValue

Output:

Everyday Italian

Get the Length of a Node List

The node list keeps itself up-to-date. If an element is deleted or added, in the node list or the XML document, the list is automatically updated.

The node list has a useful property: length. The length property returns the number of nodes in a node list.

The following code fragment gets the number of <title> elements in "books.xml":

getElementsByTagName('title').length

Output:

4

When you know the length of a node list, you can easily loop through it and extract the values you want.

The following code fragment loops through all <title> elements and prints their values:

//the x variable will hold a NodeList
var x=getElementsByTagName('title')

for (i=0;i<x.length;i++)
  {
  document.write(x[i].childNodes[0].nodeValue)
  document.write("<br />")
  }

Output:

Everyday Italian
Harry Potter
XQuery Kick Start
Learning XML

DOM NamedNodeMap

When using the attributes property on an element, we receive a NamedNodeMap object.

A NamedNodeMap object represents an unordered list of attribute nodes.

The nodes in the NamedNodeMap can be accessed through their name.

Note: In a NamedNodeMap, the nodes are not returned in any particular order.

Get the Length of a NamedNodeMap

The NamedNodeMap keeps itself up-to-date. If an element is deleted or added, in the node list or the XML document, the list is automatically updated.

The NamedNodeMap also has a length property. The length property returns the number of nodes in the list.

Look at the following XML file: books.xml

The following code fragment gets the number of attributes in the first <title> element in "books.xml":

getElementsByTagName('title')[0].attributes.length

Output:

1

Get an Item's Value in a NamedNodeMap

The getNamedItem() method of the NamedNodeMap object can be used to retrieve a specified node.

The following code fragment shows how to print the value of the "category" attribute in each <book> element":

xmlDoc=loadXMLDoc("books.xml");

var x=xmlDoc.getElementsByTagName("book");

for(i=0;i<x.length;i++)
  {
  //the attlist variable will hold a NamedNodeMap
  var attlist=x.item(i).attributes;
  var att=attlist.getNamedItem("category");
  document.write(att.value + "<br />")
  }

Output:

COOKING
CHILDREN
WEB
WEB

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