XML Selector

XML Selector is a utility for working with XML. It provides partial DOM support and a jQuery-like interface for traversing XML documents using CSS-style selectors.

Please note that as of version 0.3 XML Selector uses its own DOM implementation based on libxml2. This is different than the previous non-standard document interface. XML Selector currently implements a read-only subset of DOM Level 1. This will be extended in the future to meet, at minimum, full Level 1 support.

Why Should I Use This Thing?

The TL;DR Version

Because you want a fast, mature XML parser, and you want to write less code without hassling with a lot of complexity.

Plus, we've thought through things like namespaces, so it will be possible to do what you need to, no matter how crazy the XML you're dealing with is, and the most common cases should just work.

The Longer Version

XML Selector provides an interface for efficiently and accurately working with XML documents. To get a sense of how it helps save you coding time, consider the following XML document:

<document>
  <list>
    <item type="greeting">Hello</item>
    <item type="punctuation">,</item>
    <item type="space" />
    <item type="object">world</item>
    <item type="punctuation">!</item>
  </list>
</document>

Let's say you want to grab the greeting item text and the object item text out of this document. If you're using the DOM, it's not exactly a trivial exercise:

function find(nodeList, testFunction) {
  for (var i = 0; i < nodeList.length; i++)
    if (testFunction(nodeList[i], i, nodeList))
      return nodeList[i];
}

var items = document.getElementsByName('item');

var greeting = find(items, function(i) { return i.getAttribute('type') === 'greeting'; });
var object = find(items, function(i) { return i.getAttribute('type') === 'object'; });

var greetingText = greeting ? greeting.textContent : undefined;
var objectText = object ? object.textContent : undefined;

console.log("greeting=%s object=%s", greetingText, objectText);

Using XML Selector, this becomes:

console.log("greeting=%s object=%s",
  $doc.find('item[type="greeting"]').text(),
  $doc.find('item[type="object"]').text());

where $doc is the document in an XMLSelector instance.

But Isn't Another Alternate Solution Just As Good?

To be sure, there are a number of other viable solutions out there for easily working with XML, but there are three big advantages that XML Selector offers:

1. Selectors are simple but powerful

   The selector syntax is very simple and quick to pick up. If you know CSS, you already know the syntax. If you don't, you can pick it up in its entirety in a few minutes. Other technologies like XPath involve a steeper learning curve.

2. Simplicity doesn't come at the cost of information loss

   Some utilities simplify working with XML by converting the document into a native data structure (e.g. mapping element names to properties on an object). Unfortunately, this tends to involve information loss since native data structures generally don't have corollaries for both named attributes and named child elements. In the case of namespaces, ambiguities tend to arise. XML Selector has the advantage of preserving all of the underlying XML document, but making it simpler to find the portion you're looking for.

3. No stopping to check for null in the middle

   XML Selector follows jQuery's pattern of providing a collection of operations for sets that can be chained together. Since you're working with sets, there isn't a need to check for nulls or no results in the middle of the chain (no results just yield an empty set instead of a null). That frees you up to build a concise set of operations and, if needed, check for success or failure at the end.

Getting Started

To start using the library, require the module, and create a new instance with a string of XML or by giving it any number of nodes:

var $$ = require('xml-selector');

var xmlStr = '<items>' +
               '<item><value>Zero</value></item>' +
               '<item><value>One</value></item>' +
               '<item><value>Two</value></item>' +
             '</items>';

var $doc = $$(xmlStr);

// or

var doc = $$.parseFromString(xmlStr);
var item = doc.documentElement.firstChild;

var $doc2 = $$(item, item.nextSibling, item.nextSibling.nextSibling);

If you're already familiar with jQuery, that may be all you need to get started. Feel free to jump ahead to the API to see exactly what's supported.

Working With Selectors

XML Selector currently supports a subset of CSS selectors. The list of currently supported selectors is shown below.

• * matches any element
• E matches any E element
• E F matches any F element that is a descendant of an E element
• E > F matches any F element that is a child of an E element
• E + F matches any F element whose previous element sibling is an E element
• E[foo="warning"] matches any E element whose foo attribute is exactly equal to "warning".

If you're not already familiar with CSS selectors, the rules are pretty simple. A selector with just a name: "item" will search the document (or current context) for all elements with that name. So, that selector would return all of the item elements from the document in our example at the start of this page.

You can combine multiple names for more complex searches. Separating the names with only a space means any number of elements may come between those, but the elements to the right must be descendants of those to the left. So, a selector like "document item" would also match all of the item elements in our example at top since all of those elements are descendants of the document element. The items element that comes between them doesn't have to appear in the selector because the space separator doesn't require the elements to be direct descendants.

The > combinator, however, does require elements to be direct descendants. So a selector of "document > item" wouldn't match anything in our example. A selector of either "items > item" or "document > items > item" would match those same item elements.

You can further limit elements that match by specifying attribute values in square brackets. As shown in the example code in the introduction, you can limit the item elements to specific types, for example, only type attributes with a value of "greeting", like so: 'item[type="greeting"]'.

A second combinator, the + sign, can also be used to specify a different relationship between elements. That symbol requires the matching element on the right to be the next sibling of the element on the left of the + sign. So, a selector like 'item[type="space"] + item' would match the <item type="object">world</item> element, since it is the next sibling of the space type item.

Lastly, * matches any element. By itself it can be used to return every element in the document. It's most useful, though, for expressions where you may want to match multiple element names, such any element of type punctuation: '*[type="punctuation"]' or any next sibling of an item element: "item + *".

Namespaces

Namespaces are frequently a source of problems. The flexibility of declaring namespaces in different ways in XML combines with varying support in available XML tools to create a whole lot of headache for developers.

Consider the following XML. This is a simplified document similar to a real-life API response I once encountered:

<foo:ApiResponse xmlns:foo="urn:acmecompany:foo:api:v1.0" version="1.0">
 <foo:Status Code="urn:acmecompany:foo:api:rest:1.0:status:Incorrect" Message="" />
</foo:ApiResponse>

That's what an invalid response looks like. Now consider the corresponding valid response:

<ApiResponse xmlns="urn:acmecompany:foo:api:v1.0" version="1.0">
  <Value><!-- some result here --></Value>
  <Status Code="urn:acmecompany:foo:api:rest:1.0:status:Success"/>
</ApiResponse>

Both of these documents have all elements in the same namespace (urn:acmecompany:foo:api:v1.0), but they specify it in completely different ways that mean exactly the same thing. If you're using a library with good namespace support to parse the document, this is hopefully made transparent to you, otherwise you're going to have to struggle with the fact that in some cases the status code can be found in an element named "foo:Status" and in other cases it's in an element named just "Status".

XML Selector is a bit opinionated about this. It assumes that most of the time, you don't care about the namespace at all, you just want the Status element. In the cases where you do care about the namespace, you don't care whether the document author declared a namespace prefix or set the default namespace. You just want to access elements from the right one.

Using XML Selector you can access the Code attribute of the Status element from both of the above documents either by not specifying a namespace:

q.find('Status').attr('Code');

or by explicitly ensuring you match elements with the correct namespace URI:

$doc.addNamespace('acme', 'urn:acmecompany:foo:api:v1.0');
$doc.find('acme:Status').attr('Code');

Chaining Operations

Most of the methods on XMLSelector return a new XMLSelector instance with their results. This allows you to chain together operations and easily build more complex searches from the basic ones provided by XML Selector. For an example of how this is useful, let's consider this XML:

<catalog>
  <books>
    <book>
      <title>Bean Trees, The</title>
      <author>Kingsolver, Barbara</author>
      <published>1988</published>
      <languages><english/></languages>
      <isbn-10>0061097314</isbn-10>
      <isbn-13>978-0061097317</isbn-13>
      <publisher>HarperTorch</publisher>
    </book>
    <book>
      <title>Cien años de soledad</title>
      <author>García Márquez, Gabriel</author>
      <published>1967</published>
      <languages><spanish/></languages>
      <isbn-10>0307474720</isbn-10>
      <isbn-13>978-0307474728</isbn-13>
      <publisher>Vintage Espanol</publisher>
    </book>
    <book>
      <title>San Manuel Bueno, mártir</title>
      <author>de Unamuno, Miguel</author>
      <published>1931</published>
      <languages><spanish/></languages>
      <isbn-10>8437601851</isbn-10>
      <isbn-13>978-8437601854</isbn-13>
      <publisher>Catedra</publisher>
    </book>
    <book>
      <title>To Kill a Mockingbird</title>
      <author>Lee, Harper</author>
      <published>1960</published>
      <languages><english/></languages>
      <isbn-10>0446310786</isbn-10>
      <isbn-13>978-0446310789</isbn-13>
      <publisher>Grand Central Publishing</publisher>
    </book>
  </books>
</catalog>

Now let's say we'd like to list the titles of all the books in Spanish. We don't have selectors sophisticated enough to match that pattern, but we can do something like this:

$doc.find('spanish').closest('book').find('title').map(function(t) { return $$(t).text(); });

// produces: ['Cien años de soledad', 'San Manuel Bueno, mártir']

Non-Matching Operations

One of the advantages we gain from building operations around sets is that we don't have to introduce a lot of checks and special cases for operations that don't produce a match. Because the result of an operation that doesn't match anything is an empty set, and performing operations on an empty set simply produces another empty set, a set of chained operations can remain very concise and still work correctly in the case of non-matching operations.

Let's say, for example, we want to find the titles of books in French instead:

$doc.find('french').closest('book').find('title').map(function(t) { return $$(t).text(); });

// produces: []

This works as desired, even though we have three different operations that follow a failed match. Again, that works because a non-match produces an empty set and not a value like null, false, or undefined.

Higher-Order Functions

It's possible to produce even more customized search operations by using some of the higher-order functions which allow you to pass in callbacks for filtering or finding elements. Continuing with the book example, let's say we wanted the titles of the books by Unamuno:

$doc.find('author')
    .filter(function(elem) {
      return /^de Unamuno,/.test($$(elem).text());
    })
    .closest('book')
    .find('title')
    .map(function(elem) {
      return $$(elem).text();
    });

// produces: ['San Manuel Bueno, mártir']

Using the same approach you could just as easily create a function to return the publication year for an ISBN:

function isbnPubYear(isbn) {
  return $doc.find(String(isbn).length == 10 ? 'isbn-10' : 'isbn-13')
             .filter(function(elem) { return $$(elem).text() === isbn; })
             .closest('book')
             .find('published')
             .text();
}

The functions filter() and find() both accept callbacks for this purpose, and several other functions allow you to iterate over, test, or modify the set using callbacks. See the API section, below, for details.

API

• $$ (XML Selector)
• CharacterData
• Document
• Element
• Node

$$

This is the function exported by the xml-selector module. It may be invoked directly to obtain a selector instance. It also provides a utility function for direct DOM usage.

[new] $$(xmlString)

[new] $$(node1[, ... nodeN])

[new] $$(nodeArray)

Accepts an XML String, one or more nodes, an array of nodes, or any combination thereof. It returns a new selector instance whose context is the provided node list. It may be optionally be called with new, but that is not required.

Instance Properties

$selector.length

The number of nodes in this instance's list.

Instance Methods

$selector[index]

• index: Number Zero-based index

Instances of a selector can be accessed by numerical index like an array, so you can do things like this:

var $$ = require('xml-selector');

var $doc = $$('<items>' +
                '<item content="Zero" />' +
                '<item content="One" />' +
                '<item content="Two" />' +
              '</items>');

var $items = $doc.find('item');

console.log($items[1].getAttribute('content'));
// outputs 'One'

Array access returns a Node or undefined.

$selector.addNamespace(prefix, uri)

• prefix: String Namespace prefix to use in selector expressions
• uri: String Corresponding namespace URI

Associates a prefix to use in your selectors with a namespace URI. Returns this selector instance.

$selector.attr(name)

• name: String Attribute name

Access the value of the named attribute from the first node in the list. Returns a String.

$selector.children([selector])

• selector: String Optional selector expression

Return a new XML Selector instance containing the children of the nodes in this set, optionally filtered by selector.

$selector.closest(selector)

• selector: String Selector expression to match

Return a new XML Selector instance with the closest ancestor of each node in the list that matches selector.

$selector.every(predicate[, thisArg])

• predicate: Function Callback function for testing items, takes three arguments:
  • item: Node The item to test
  • index: Number The index of the item in the list
  • $selector: XMLSelector This selector instance
• thisArg: Mixed Optional value to use as this when executing callback

Returns true if all items in this set pass the given predicate (a user-supplied callback which should return a boolean for each item supplied).

$selector.filter(selector)

• selector: String Selector expression

Return a new XML Selector instance containing the nodes from this set that match the given selector.

$selector.filter(filterFunction[, thisArg])

• filterFunction: Function Callback function for filtering items, takes three arguments:
  • item: Node The item to test
  • index: Number The index of the item in the list
  • $selector: XMLSelector This selector instance
• thisArg: Mixed Optional value to use as this when executing callback

Return a new XML Selector instance containing only the nodes from this set for which the supplied callback returns true.

$selector.find(selector)

• selector: String Selector expression to search for

Alias of search. Searches this set for descendants matching selector and returns a new XML Selector instance with the result.

$selector.find(predicate[, thisArg])

• predicate: Function Callback function for testing items, takes three arguments:
  • item: Node The item to test
  • index: Number The index of the item in the list
  • $selector: XMLSelector This selector instance
• thisArg: Mixed Optional value to use as this when executing callback

Find the first Node in this selector instance for which the user-supplied callback returns true. Returns a Node or undefined.

$selector.findIndex(predicate[, thisArg])

• predicate: Function Callback function for testing items, takes three arguments:
  • item: Node The item to test
  • index: Number The index of the item in the list
  • $selector: XMLSelector This selector instance
• thisArg: Mixed Optional value to use as this when executing callback

Iterate over the nodes in this selector instance and return the index of the first node for which the user-supplied callback returns true. Returns the numerical index of the matching node, or -1 in the case of no match.

$selector.first()

Returns a new XML Selector instance containing the first node from this set.

$selector.forEach(iterator[, thisArg])

• iterator: Function Callback function, takes three arguments:
  • item: Node The item
  • index: Number The index of the item in the list
  • $selector: XMLSelector This selector instance
• thisArg: Mixed Optional value to use as this when executing callback

Iterate over the nodes in this selector instance (behaves like Array.forEach). Returns this selector instance.

$selector.last()

Returns a new XML Selector instance containing the last node from this set.

$selector.map(iterator[, thisArg])

• iterator: Function Callback function, takes three arguments:
  • item: Node The item to map
  • index: Number The index of the item in the list
  • $selector: XMLSelector This selector instance
• thisArg: Mixed Optional value to use as this when executing callback

Iterates over the nodes in this selector instance and returns a new Array containing the values returned by each invocation of iterator.

$selector.next([selector])

• selector: String Optional selector expression

Returns a new XML Selector instance containing the next siblings of the nodes in this set, optionally filtered by selector.

$selector.nextAll([selector])

• selector: String Optional selector expression

Returns a new XML Selector instance containing all the next siblings of the nodes in this set, optionally filtered by selector.

$selector.nextUntil(selector)

• selector: String Selector expression

Returns a new XML Selector instance containing all the next siblings of the nodes in this set up to siblings matching selector.

$selector.not(selector)

• selector: String Selector expression

Returns a new XML Selector instance containing the nodes in this set not matching selector.

$selector.parent([selector])

• selector: String Optional selector expression

Returns a new XML Selector instance containing the parent of the nodes in this set, optionally filtered by selector.

$selector.parents([selector])

• selector: String Optional selector expression

Returns a new XML Selector instance containing all the ancestors of the nodes in this set, optionally filtered by selector.

$selector.parentsUntil(selector)

• selector: String Selector expression

Returns a new XML Selector instance containing all the ancestors of the nodes in this set up to the ancestor matching selector.

$selector.prev([selector])

• selector: String Optional selector expression

Returns a new XML Selector instance containing the previous siblings of the nodes in this set, optionally filtered by selector

$selector.prevAll([selector])

• selector: String Optional selector expression

Returns a new XML Selector instance containing all the previous siblings of the nodes in this set, optionally filtered by selector.

$selector.prevUntil(selector)

• selector: String Selector expression

Returns a new XML Selector instance containing all the previous siblings of the nodes in this set up to siblings matching selector.

$selector.reduce(iterator, initialValue[, thisArg])

• iterator: Function Callback function, takes four arguments:
  • accumulator: Mixed The value returned by the previous invocation of the callback
  • item: Node The current item to process
  • index: Number The index of the item in the list
  • $selector: XMLSelector This selector instance
• initialValue: Mixed Initial value for accumulator
• thisArg: Mixed Optional value to use as this when executing callback

Calls a function for each value in the collection (left-to-right) and passes the result to the next iteration. Returns the result of the final call to iterator.

$selector.reduceRight(iterator, initialValue[, thisArg])

• iterator: Function Callback function, takes four arguments:
  • accumulator: Mixed The value returned by the previous invocation of the callback
  • item: Node The current item to process
  • index: Number The index of the item in the list
  • $selector: XMLSelector This selector instance
• initialValue: Mixed Initial value for accumulator
• thisArg: Mixed Optional value to use as this when executing callback

Calls a function for each value in the collection (right-to-left) and passes the result to the next iteration. Returns the result of the final call to iterator.

$selector.search(selector)

• selector: String Selector expression

Searches this set for descendants matching selector and returns a new XML Selector instance with the result.

$selector.some(predicate[, thisArg])

• predicate: Function Callback function for testing items, takes three arguments:
  • item: Node The item to test
  • index: Number The index of the item in the list
  • $selector: XMLSelector This selector instance
• thisArg: Mixed Optional value to use as this when executing callback

Iterates over the Nodes in this selector instance and returns true if predicate returns true for at least one of the Nodes. Returns false otherwise.

$selector.text()

Returns a String containing the text content of the first element in the list. In the case of an empty set, an empty String is returned.

$selector.xml()

Returns a String containing the XML representation of the first element in the list. In the case of an empty set, an empty String is returned.

Utility Functions

$$.parseFromString(xmlString)

• xmlString: String A string of XML to parse

Parses a string of XML and returns a Document.

CharacterData extends Node. It does not represent an actual node type, but serves as the base class for Text and Comment node types. This interface is defined in DOM Level 1. Currently a subset of that interface is implemented.

Instance Properties

characterData.data

A String containing the character data content of this Node.

characterData.length

A Number indicating the length of the data property.

Document extends Node. It is the top-level node in a document tree. This interface is defined in DOM Level 1. Currently a subset of that interface is implemented.

Instance Properties

document.documentElement

The top-level Element from the document.

Element extends Node. It represents an XML element in the document. This interface is defined in DOM Level 1. Currently a subset of that interface is implemented.

Instance Properties

element.tagName

A String containing the name of the element.

Instance Methods

element.getAttribute(name)

• name: String Attribute name

Return the value of the named attribute. Returns an empty string if the attribute is not set.

Node is the base type for all objects in the document tree. This interface is defined in DOM Level 1. Currently a subset of that interface is implemented.

Instance Properties

node.firstChild

The first child Node of this Node or null for none.

node.lastChild

The last child Node of this Node or null for none.

node.nextSibling

The sibling Node immediate following this Node in the document tree. This property is null in the case of no next sibling.

node.nodeName

A String containing the node name as defined in the DOM specification. For Elements this is the same as the tag name. For other classes, the property value varies according to the node type.

node.nodeType

An integer indicating the node type. Values follow the DOM specification:

• 1 - Element
• 2 - Attr
• 3 - Text
• 4 - CDATASection
• 5 - EntityReference
• 6 - Entity
• 7 - ProcessingInstruction
• 8 - Comment
• 9 - Document
• 10 - DocumentType
• 11 - DocumentFragment
• 12 - Notation

node.ownerDocument

The Document that contains this Node. This property is null for the Document itself.

node.parentNode

The parent Node of this Node. If this Node has no parent, this property is null. The property will only be null for top-level nodes such as Documents or DocumentFragments, attribute (Attr) nodes, or detached nodes.

node.previousSibling

The sibling Node immediate preceding this Node in the document tree. This property is null in the case of no previous sibling.

Instance Methods

node.hasChildNodes()

Returns a boolean. Returns true if this node has children, false otherwise.