ECMAscript for XML

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Draft

ECMAscript for XML (ak E4X) is an extension to ECMAscript.

It is available in

• Some JavaScript engines, e.g.
  • Firefox SpiderMonkey (the default JS engine)
• ActionScript 3

Using E4X is considerably simpler than using the DOM. Maybe some day we will be back to the elegance of languages like LISP instead of fighting against obscure C/Java syntax. But for now there are two problems with E4X: (1) It can't read the DOM, in otherwords you only can manipulate XML as (external) data. (2) There seem to be some inconsistencies that developers complain about.

Usage patterns

Creating an E4X XML object

(1) Using the XML literal syntax

The easiest way is to create a new variable and just create the XML structure like this. In ECMAScript 4 (e.g. ActionScript 3) you should define the type of the variable.

var instructions:XML  = <stepbystep>
  <doctitle></doctitle>
  <steps>
    <title></title>
  </steps>
 </stepbystep>;

Don't forget the ";" at the end since this instruction extends over several lines.

(1b) In JavaScript 1.6 (ECMAScript 3) you can't define types and you simply use:

 var instructions  = <stepbystep> ....

This the same principle as creating an array or an object like this:

 var arr = [item1,item2,item3];
 var obj = {a:"item 1",b:"item 2",c:"item 3"}

Using the XML constructor

(2) You also can create an XML E4X object from a string with the new XML() constructor.

var instruction_string = "<stepbystep>

 <doctitle></doctitle>
 <steps>
   <title></title>
 </steps>
</stepbystep>";

var instruction2:XML = new XML(instruction_string);

(3) Dynamic values Using the XML literal syntax is very practical when you want to create markup dynamically.

Variables and expressions can be used to create XML contents by simply wrapping them with braces ({}). Note: To create attribute values, omit the quotation marks (as in the example below)!

var URL = "http://edutechwiki.unige.ch/en/ECMAscript_for_XML";
var URL_text = "ECMAscript for XML";
var A   = <a href={URL}>{URL_text}</a>;

The value of A is:

 <a href="http://edutechwiki.unige.ch/en/ECMAscript_for_XML">ECMAscript for XML</a>

XML lists

XML lists are the other important datastructure in E4X. Lists are needed for example to hold results of queries (like NodeLists in the DOM model).

Here is a little example of an XML list:

    <step>
      <title>step 1</title>
    </step>
    <step>
      <title>step 2</title>
    </step>
    <step>
      <title>step 3</title>
    </step>

Accessing elements and attributes

Example XML code

We will use this XML fragment in the examples below.

var instructions:XML =
 <stepbystep>   
  <doctitle>Sample document</doctitle>
  <info>
    <para>See <a href="http://edutechwiki.unige.ch/en/ECMAscript_for_XML">ECMAscript for XML</a></para>
  </info>
  <steps>
    <title>List of steps</title>
    <step>
      <title>step 1</title>
    </step>
    <step>
      <title>step 2</title>
    </step>
    <step>
      <title>step 3</title>
      <para>That was easy !</para>
    </step>
  </steps>
 </stepbystep>;

Again, there is a similarity between E4X objects, arrays, and traditional Object.

(1) Getting en element with the "." operator

To access an element you simply can use an expression like a.b.c to walk down the tree. The result is either an XML fragment or an XMLList depending on whether there is only one child or several children.

 var step_list = instructions.steps.step;

Result, i.e. step_list is now:

 <step> <title>step 1</title> </step>
 <step> <title>step 2</title></step>
 <step> <title>step 3</title>  <para>That was easy !</para></step>

 var para = instructions.steps.step[2].para;

Result, i.e. para is:

 <para>That was easy !</para>

(1) Getting en attribute with the "@" operator

About namespaces

E4X can handle namespaces: E.g. to extract an xlink:href attribute in the following kind of fragment: xml = <STORY>

       <INFOS>
   	<a xmlns:xlink="http://www.w3.org/1999/xlink"

xlink:href="http://edutechwiki.unige.ch/en/ECMAscript_for_XML">ECMAscript for XML</a> </INFOS>

   </STORY>;

You will have to define a Namespace object

 var NS = new Namespace("http://www.w3.org/1999/xlink");

and then use the variable as prefix

 @NS::href;

function show_URL() {
      var NS = new Namespace("http://www.w3.org/1999/xlink");
      var URL = xml..a;
      var link = URL.@NS::href;
      alert ("URL (content of the 'a' tag)=" + URL + "\n" + "NameSpace=" + NS + "\nxlink:href=" + link);
      }

A little example

• Story (look at the source of this file)

Links

Overviews

• ECMAScript for XML (Wikipedia)

Introductions and tutorials

JavaScript

• E4X: JavaScript on steroids by Grace Walker, IBM Developer Works. (2008).

• E4X Tutorial, W3Schools.

• Introducing E4X (by Kurt Cagle, xml.com, nov 2007)

• E4X by Mark. Useful as short manual !

• At Mozilla (Firefox)
  • E4X Tutorial (Mozilla Developer center)
  • Processing XML with E4X (JavaScript 1.6 +)
  • E4X, Mozilla Developer Center.

Flash/ActionScript/Flex

• The E4X approach to XML processing (Adobe, part of the Flex manual/Programming ActionScript 3.0)

• Using an E4X expression in a data binding expression (Adobe)

• E4X (Adobe, Flex, Getting Started)

• Intro to E4X (Actionscript.org) by Hasan Otuome

• E4X: Beginner to Advanced by Josh Tynjala, Yahoo Flash Developer Center.

• Common E4X pitfalls by Mike Morearty (2007).

Php

• PHP5 quick database E4X queries (Adobe, Flex cookbook) by Sand Wyrm.

Namespaces

• How does E4X deal with XML with namespaces? by Martin Honnen (2005)
• How can I access elements or attributes that are in a namespace? by Martin Honnen (2005)

Ajax

• Fremantle, Paul and Anthony Elder (1005). AJAX and scripting Web services with E4X, Part 1, IBM Works, HTML

Standard and Manuals

• ECMA-357 standard
• E4X Short tutorial by Mark. Also useful as short manual !