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XML DOM Traverse Node Tree


Traversing means looping through or traveling across the node tree.


Traversing the Node Tree

Often you want to loop an XML document, for example: when you want to extract the value of each element.

This is called "Traversing the node tree"

The example below loops through all child nodes of <book>, and displays their names and values:

Example

<!DOCTYPE html>
<html>
<body>

<p id="demo"></p>

<script>
var x, i ,xmlDoc;
var txt = "";
var text = "<book>" +
"<title>Everyday Italian</title>" +
"<author>Giada De Laurentiis</author>" +
"<year>2005</year>" +
"</book>";

parser = new DOMParser();
xmlDoc = parser.parseFromString(text,"text/xml");

// documentElement always represents the root node
x = xmlDoc.documentElement.childNodes;
for (i = 0; i < x.length ;i++) {
    txt += x[i].nodeName + ": " + x[i].childNodes[0].nodeValue + "<br>";
}
document.getElementById("demo").innerHTML = txt;
</script>

</body>
</html>

Output:

title: Everyday Italian
author: Giada De Laurentiis
year: 2005
来试试吧 »

Example explained:

  1. Load the XML string into xmlDoc
  2. Get the child nodes of the root element
  3. For each child node, output the node name and the node value of the text node


Browser Differences in DOM Parsing

All modern browsers support the W3C DOM specification.

However, there are some differences between browsers. One important difference is:

  • The way they handle white-spaces and new lines

DOM - White Spaces and New Lines

XML often contains new line, or white space characters, between nodes. This is often the case when the document is edited by a simple editor like Notepad.

The following example (edited by Notepad) contains CR/LF (new line) between each line and two spaces in front of each child node:

<book>
  <title>Everyday Italian</title>
  <author>Giada De Laurentiis</author>
  <year>2005</year>
  <price>30.00</price>
</book>

Internet Explorer 9 and earlier do NOT treat empty white-spaces, or new lines as text nodes, while other browsers do.

The following example will output the number of child nodes the root element (of books.xml) has. IE9 and earlier will output 4 child nodes, while IE10 and later versions, and other browsers will output 9 child nodes:

Example

function myFunction(xml) {
var xmlDoc = xml.responseXML;
    x = xmlDoc.documentElement.childNodes;
    document.getElementById("demo").innerHTML =
    "Number of child nodes: " + x.length;
}
来试试吧 »

PCDATA - Parsed Character Data

XML parsers normally parse all the text in an XML document.

When an XML element is parsed, the text between the XML tags is also parsed:

<message>This text is also parsed</message>

The parser does this because XML elements can contain other elements, as in this example, where the <name> element contains two other elements (first and last):

<name><first>Bill</first><last>Gates</last></name>

and the parser will break it up into sub-elements like this:

<name>
  <first>Bill</first>
  <last>Gates</last>
</name>

Parsed Character Data (PCDATA) is a term used about text data that will be parsed by the XML parser.


CDATA - (Unparsed) Character Data

The term CDATA is used about text data that should not be parsed by the XML parser.

Characters like "<" and "&" are illegal in XML elements.

"<" will generate an error because the parser interprets it as the start of a new element.

"&" will generate an error because the parser interprets it as the start of an character entity.

Some text, like JavaScript code, contains a lot of "<" or "&" characters. To avoid errors script code can be defined as CDATA.

Everything inside a CDATA section is ignored by the parser.

A CDATA section starts with "<![CDATA[" and ends with "]]>":

<script>
<![CDATA[
function matchwo(a,b) {
    if (a < b && a < 0) {
        return 1;
    } else {
        return 0;
    }
}
]]>
</script>

In the example above, everything inside the CDATA section is ignored by the parser.

笔记 on CDATA sections:

A CDATA section cannot contain the string "]]>". Nested CDATA sections are not allowed.

The "]]>" that marks the end of the CDATA section cannot contain spaces or line breaks.





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