Parsing and serializing XML

At times, you may need to parse XML content and convert it into a DOM tree, or, conversely, serialize an existing DOM tree into XML. In this article, we'll look at the objects provided by the web platform to make the common tasks of serializing and parsing XML easy.

XMLSerializer

Serializes DOM trees, converting them into strings containing XML.

DOMParser

Constructs a DOM tree by parsing a string containing XML, returning a XMLDocument or Document as appropriate based on the input data.

XMLHttpRequest

Loads content from a URL; XML content is returned as an XML Document object with a DOM tree built from the XML itself.

XPath

A technology for creating strings that contain addresses for specific portions of an XML document, and locating XML nodes based on those addresses.

Creating an XML document

Using one of the following approaches to create an XML document (which is an instance of Document).

Parsing strings into DOM trees

This example converts an XML fragment in a string into a DOM tree using a DOMParser:

js

const xmlStr = '<q id="a"><span id="b">hey!</span></q>';
const parser = new DOMParser();
const doc = parser.parseFromString(xmlStr, "application/xml");
// print the name of the root element or error message
const errorNode = doc.querySelector("parsererror");
if (errorNode) {
  console.log("error while parsing");
} else {
  console.log(doc.documentElement.nodeName);
}

Parsing URL-addressable resources into DOM trees

Using XMLHttpRequest

Here is sample code that reads and parses a URL-addressable XML file into a DOM tree:

js

const xhr = new XMLHttpRequest();

xhr.onload = () => {
  dump(xhr.responseXML.documentElement.nodeName);
};

xhr.onerror = () => {
  dump("Error while getting XML.");
};

xhr.open("GET", "example.xml");
xhr.responseType = "document";
xhr.send();

The value in the xhr object's responseXML field is a Document constructed by parsing the XML.

If the document is HTML, the code shown above will return a Document. If the document is XML, the resulting object is actually a XMLDocument. The two types are essentially the same; the difference is largely historical, although differentiating has some practical benefits as well.

Note: There is in fact an HTMLDocument interface as well, but it is not necessarily an independent type. In some browsers it is, while in others it is an alias for the Document interface.

Serializing an XML document

Given a Document, you can serialize the document's DOM tree back into XML using the XMLSerializer.serializeToString() method.

Use the following approaches to serialize the contents of the XML document you created in the previous section.

Serializing DOM trees to strings

First, create a DOM tree as described in How to Create a DOM tree. Alternatively, use a DOM tree obtained from XMLHttpRequest.

To serialize the DOM tree doc into XML text, call XMLSerializer.serializeToString():

js

const serializer = new XMLSerializer();
const xmlStr = serializer.serializeToString(doc);

Serializing HTML documents

If the DOM you have is an HTML document, you can serialize using serializeToString(), but there is a simpler option: just use the Element.innerHTML property (if you want just the descendants of the specified node) or the Element.outerHTML property if you want the node and all its descendants.

js

const docInnerHtml = document.documentElement.innerHTML;

As a result, docInnerHtml is a string containing the HTML of the contents of the document; that is, the <body> element's contents.

You can get HTML corresponding to the <body> and its descendants with this code:

js

const docOuterHtml = document.documentElement.outerHTML;

See also