Eye into the Universe

Hubble Space StationNamed after the American astronomer Edwin Hubble, the Hubble telescope was conceived as a way to put a telescope into space outside of the vision-impairing atmosphere that surrounds our planet. By doing this, we can see more and learn more about the universe that surrounds us.

Hubble was not the first astronomical effort in space: that honor belongs to two Orbital Astronomical Observatories launched into space in 1968 and 1972. Because of these earlier efforts, support for a space-based telescope grew and NASA, in 1975, began the design of what was to become known as the Hubble Space Telescope.

Several groups and companies were involved with the building of Hubble, including NASA, the European Space Agency, the Marshall Space Flight Center in Huntsville, Alabama — home of Space Camp — as well as the companies that actually built Hubble: Perkin-Elmer Corporation and Lockheed Missile and Space Company. Perkin-Elmer built the optical and guidance systems, while Lockheed built the outer shell of the telescope. The European Space Agency furnished the solar arrays and one of the scientific instruments; the other instruments were furnished by major universities.

Shuttle Discovery

According to NASA’s History of Hubble, it was launched into space on April 24, 1990, via the Shuttle, Discovery.

Hubble was designed to be “modular”, with different bay areas holding different scientific instruments. With this approach, Hubble could be maintained by crews from shuttles, basically creating the first assembly line in space. Or would that more likely be, the first erector toy in space? Whatever we call it, it is the modularity of Hubble that assisted in the first real challenge to the telescope. You see, Hubble was born near-sighted.

Off to a Rocky Start

Nebula

Like most of us, Hubble wasn’t born perfect. Hubble’s primary mirror was incorrectly shaped, ground a little too flat to the edge. How flat? The mirror’s edge was off by about the same width of 1/50th of a human hair.

Because the primary mirror was ground a bit too flat, instead of focusing all light into the center of the mirror, the light ended up being spread over a larger area, creating a blurry effect of images — hence the concept of Hubble being near-sighted.

NASA knew that maintenance of Hubble would be by shuttle, but probably wasn’t expecting that maintenance would have to occur so quickly. The challenge was to find a way to fixing the near-sighted effect without having to replace the mirror, a task that was impossible.

spiral galaxy

What NASA came up with was, basically, a pair of glasses for Hubble. Except, in this case, the glasses were the Corrective Optics Space Telescope Axial Replacement (COSTAR), and the Wide Field/Planetary Camera (WF/PC-II). Both of these instruments would provide corrective optics that would compensate for the primary mirror, thereby canceling the blurry effect and providing clear images. These new optics would then be installed by shuttle using slots within Hubble’s instrument array.

On December 2, 1993, the shuttle Endeavour was launched on a unique mission: install the corrective optics and fix Hubble. NASA was making a house call.

Did the fixes work? You only have to look at many of the hundreds of images produced by Hubble to know that the corrective optics — in addition to the excellent work of the Endeavour astronauts who installed the optics — was a success.

How it works

For this section and the next, I turned to STSI’s Hubble Primer.

hubble

According to the primer, Hubble is as long as a school bus, and as tall as a 5 story building. Definitely bigger than that backyard telescope you might be familiar with. Hubble’s shape resembles a long foil wrapped tube with wings, the wings being the solar arrays necessary to provide power for the telescope.

Hubble is controlled from earth, and moves through the use of gyros or reaction wheels. These maintain Hubble’s stable position regardless of factors such as solar radiation and gravitation from other objects. In addition, when Hubble is being positioned to record information from a specific planet, galaxy, or star, a pair of star guides near to the object are located by Hubble. The telescope uses these star guides to maintain a steady fix on the target object. In fact, one of the first activities performed when reviewing a proposal to use some of Hubble’s time and resources is to see whether guide stars have been identified near the object that is the focus of the proposal (as outlined in Observing with Hubble).

Light enters the telescope and hits the primary mirror. This mirror, in turn, focuses this light on a secondary mirror, which in turn reflects the light back to a specific point on the primary mirror. This light is then recorded and analyzed, and data gleamed from the observation process is stored in onboard computers. Scientists then tap into this stored data for the data they need.

star

What are some of the instruments used to record information? Well, the most widely known is the Wide Field and Planetary Camera, responsible for the images you see at the Hubble site and in this article. Others that are currently operating are the Near-Infrared Camera and Multi-Object Spectrometer (NICMOS), for infrared and spectroscopic imaging; the Space Telescope Imaging Spectrograph (STIS); the Corrective Optics Space Telescope Axial Replacement (COSTAR), discussed earlier, which provides for correction of the primary mirror; and the Faint Object Camera (FOS), which operates over a wide wavelength range and which contains an image intensifier to enhance image light. You can read a more detailed overview of the instruments in the General Overview of the Hubble Space Telescope

What we have learned

star

Personally, just to see neighboring galaxies and see the birth of stars sounds like more than enough justification for Hubble, but others may want more. Okay, then how about the fact that with Hubble we now have a better understanding of how the Universe works and our place in it. We have now have physical evidence of black holes to provide support for our theoretical understanding of these “sinkholes” in space.

Additionally, we know more about the age of the universe, have seen Pluto for the first time, recorded evidence of the elusive Brown Dwarfs of space, and, yes, we have even seen the birth of a star.

Into the Future

So, what do the folks at STSI, NASA, and friends have in store for us next? What else but the Next Generation Space Telescope, or NGST.

Nebula

The NGST project is currently underway and hope is that it will launch into space in the year 2007. Its purpose is to search for origins: of planets, or stars, galaxies, even the universe.

I’m fascinated by the thought of how much we have learned from Hubble and the promise of how much more we will learn from NGST. Who knows? Maybe someday while we are looking outward, we’ll actually spot someone looking inward…at us.

XML Expectations

Originally published at Netscape Enterprise Developer, now archived at Wayback Machine

Extensible Markup Language is a language that defines other languages. It also has the potential to give structure and meaning to the information contained in HTML documents or any other data form — making such information naturally as searchable and structured as the information locked into a database. Such capabilities mean that XML can turn our current view of data upside down — instead of a static, impenetrable lump of information, a file that uses XML suddenly has a logical structure that can be manipulated, queried, and changed without delving down into the data itself. The potential of such a meta-language is huge — if it’s implemented as an open standard. Right now, XML remains such a standard, and if it continues to evolve along open lines, it could drastically improve Web-based development.XML is similar to SQL in a number of ways; SQL is also an example of a multi-purpose language used to define data structures and query those same data structures without concern as to how the information is displayed or used. The only guarantees are that the information is defined in structures, the structures follow certain rules, and the information contained within the structures can be accessed automatically or manually. Both SQL and XML define structures for information in the form of elements, element attributes, and element content. The main difference is that instead of defining data that is stored in a physical storage medium usually only accessible by a database engine, XML describes data that is stored and accessed from within documents.

XML’s parent: SGML
XML is a subset of SGML (Standardized General Markup Language), a generalized markup language that was passed as an ISO standard in the 1980s. Rather than specifying a language’s elements directly, SGML is used to define the rules that constrain the elements of a specific language.

SGML grew out of the need to define a document’s structure and to define rules used to determine whether the document is valid and well formed. The document’s structure is defined through the use of markup tags, which delimit the elements, and Document Type Definition (DTD) files that define each element’s structure and content, providing a sort of grammar for the document.

For example, using SGML, a customer element within a document could have the following structure:

<CUSTOMER name="Shelley Powers" id="CUST011A1">
<PO id="PO23349008">
<POITEM id="POI1">
<ITEM id="14453">
Item ID: 14453
Item Desc: some description
</ITEM>
</POITEM>
</PO>
</CUSTOMER>

To validate the markups used to define the structure of the document, an associated DTD would have to be created, with statements similar to the following:

<!ELEMENT customer - - (POITEM)+><!ATTLIST customer    name CDATA   id CDATA>

This extremely simplified and abbreviated DTD uses an Extended Backus-Naur Form (EBNF) syntactic notation to create the grammar.

Using a standardized meta-language to define entities within a document allows SGML parsers to pull out the individual entities (such as the customer entity just described) and any associated attributes and content. An application can then use that information for a number of purposes, including the following possibilities:

  • To define information in a database-neutral format for transport between unlike databases.
  • To provide a search engine that allows a person to query on the entity type as well as the data.
  • For report generation, or even an online hypertext order processing form that allows the reader to drill down within the document to find the desired information.
  • To define a standard language for a specific industry or science, such as the petroleum industry or chemistry, which includes special notational conventions.

The concept of SGML is very attractive: Define a language that in turn defines a document structure used for a specific group of documents and which can be extended without impacting the underlying language generation mechanism. Unfortunately, the downside to SGML is that it is far from trivial to define the DTD for a language. SGML is a complex standard that is difficult to implement.

HTML, a derivative of SGML
SGML did, however, provide the roots of the first Web document specification, HTML. HTML was derived from SGML, except that it predefined a group of elements that controlled the delivery of a Web page’s content. In addition, the original HTML elements were expanded to include suggested presentation elements that controlled the appearance of the Web page. SGML does not control the presentation of elements, only the element structure and semantics.

The following code defines an HTML unnumbered list element, which is defined by the DTD associated with the HTML 4.0 specification as having a start and end tag and containing at least one list item:

<!ELEMENT UL - - (LI)+>

According to the EBNF associated with SGML, this DTD states that UL is an element, the double dashes assert that the element requires a start and end tag, and the element consists of at least one, and possibly more than one, list item (LI). When a user agent such as a browser parses an HTML element, it knows to look for both beginning and ending UL tags and at least one LI element contained within those tags.

Associated with the DTD for HTML 4.0 is an implied visual presentation of an unnumbered list, which is that each list item has a specified list graphic, each list item is on a separate line, and each list item lines up beneath the previous list item. However, not all user agents (such as browsers) are visual, so presentation can only be a suggestion, not a mandate.

After the first releases of the HTML specification, new elements crept into the language to provide control over page presentation. One such element is the FONT element, which controls the size, color, type, and font family of any text the element contains. The problem with using a specific tag like FONT, however, is that non-standardized tags can lead to different Web page presentations depending on the user agents.

To help differentiate an element’s structure and its presentation in HTML, the W3C issued a recommendation for CSS1, or Cascading Style Sheet Level 1, a specification that provides presentation information for HTML elements.

The real advantage of HTML was that it was relatively easy to code and display, even in different browsers. The ease of HTML was directly responsible for the massive growth of the Web. If Web document access had begun with XML, chances are you wouldn’t be reading this article right now and Web access would probably be limited to the scientific community. We initially needed a simple mechanism to create Web documents, and HTML was it. The very lack of flexibility was the language’s strength.

Now that the Web and Internet-based technologies have matured to a certain extent, enterprise developers are increasingly demanding a way to build flexibility into documents like Web pages in order to increase their effectiveness and ease of access.

Enter XML
XML arose from a need to create more generalized markup languages without having to follow the large and complex SGML standard. The XML standard still demands that a markup language be defined as well-formed, but it makes the validation step optional, which means that an associated DTD is not required (though one can be included). Additionally, XML uses only a subset of the rules for SGML, letting developers understand the principles and implementation of the technology more quickly.

Like SGML, XML is a meta-language that provides rules to define a set of tags that can be used within a document. These tags are then used to delimit an XML entity, its attributes, and its contents, and to define the elements’ syntax. These tags are read by a XML processor, which in turn provides an application with access to the entities. The application can then perform one or more actions on the XML entities.

XML processors can either be validating, which means that they make use of an associated DTD in order to ensure valid structures, or non-validating. Regardless of whether or not they are validated, XML documents can be considered to be well-formed as long as they match the XML syntax overall and as long as each entity within the document meets the syntax for a well-formed XML entity.

The main requirements for a well-formed Extensible Markup Language include the following:

  • The language may begin with a valid XML declarative statement or prolog.
  • There is one element that acts as the root element and which acts as parent to all other elements.
  • Elements are either not empty or, if they are empty, they have a “hint” encoded within the element that defines this information to the XML parser.
  • Non-empty elements must have start and ending tags.
  • All elements except the root element are contained within some element, referred to as the element’s parent; all contained elements are referred to as the parent element’s children.
  • Elements can contain character data, other elements, CData sections, processing instructions or comments.
  • Each parsed element within the document is well-formed.
  • Character data that may be processed as XML is enclosed within CData sections.
  • Documents can include comments, white space, and processing instructions.

Consider that a valid and well-formed XML document consists of the following EBNF format non-terminating symbols (non-terminating meaning that the symbols are themselves expanded elsewhere):

document::= prolog element Misc*

A complying document could be as simple as:

<?XML VERSION="1.0" ENCODING="UTF-8"?>
<ARTICLE name="XML" author="Shelley Powers"/>

This document consists of the prolog section which includes the XML declaration (“<?XML”) and includes the version number of the XML definition, as well as the encoding declaration. It also contains one element, ARTICLE, which has two attributes, NAME and AUTHOR. Since the element is an empty element, it ends with a backslash to signal to the processor that the element contains no other content. This is necessary for a non-DTD (non-validating) document. Otherwise the XML processor would not know when to look ahead in the parsing for required element content. This is one of the key features of XML: Forward processing information is embedded directly within the document, negating the necessity of creating an associated DTD.

The example just provided is a well-formed document, but not a valid one, since no DTD is provided for validation. The example also demonstrates the simplicity of XML. An even simpler version of the language would be:

<ARTICLE name="XML" author="Shelley Powers"/>

To make the document a valid one, I could have added a DTD for the ARTICLE element directly into the document, or linked to a DTD external file:

<?XML VERSION="1.0" ENCODING="UTF-8"?>
<!DOCTYPE article SYSTEM "article.dtd">

<ARTICLE name="XML" author="Shelley Powers"/>

XML in action
Though the standard is relatively new, there are several XML parsers that validate whether an XML document and its associated DTD fit the rules for a valid XML document. In addition, these same parsers may return the elements within a document exposed in their tree-like form — a form that can be used by applications.

XML is being used in the real world already. For example, Microsoft has defined an XML application it terms “Channel Definition Format,” or CDF. CDF files contain entities that describe the contents of an active channel. Following the accepted technique for XML, CDF files do not contain reference to a DTD file and instead use clues embedded within the tags and tag definitions to provide forward-looking information for the XML parser.

CDF’s purpose is to provide a document that defines the use of push technology at a specific Web site, including which pages are to be displayed as channels, what icons to display, what the update schedules are, etc. With this information, the XML processor provides the key elements that a channels-based application can use to control channel access on the Web site.

The following code shows the CDF file I have defined for use at my personal Web site. The root element for the file is the CHANNEL element. It is the parent element for several other elements, such as an ICON element, an ITEM element, and an ABSTRACT element. Each of the elements within the document may or may not have attributes, and a child element may in turn be the parent for another element:

<?XML VERSION="1.0" ENCODING="UTF-8"?>
<CHANNEL HREF="http://www.yasd.com/plus/index.htm" 
        BASE="http://www.yasd.com/plus/">
    <TITLE>YASD+</TITLE>
    <ABSTRACT>YASD+ pages, using the newest technologies</ABSTRACT>
    <LOGO HREF="http://www.yasd.com/mm/wide_logo.gif" STYLE="IMAGE-WIDE"/>
    <LOGO HREF="http://www.yasd.com/mm/logo.gif" STYLE="IMAGE"/>
    <LOGO HREF="http://www.yasd.com/mm/icon.gif" STYLE="ICON"/>
    <SCHEDULE>
        <INTERVALTIME DAY="1"/>
        <EARLIESTTIME HOUR="0"/>
        <LATESTTIME HOUR="12"/>
    </SCHEDULE>
    <ITEM HREF="http://www.yasd.com/samples/bytes/daily.htm">
        <LOGO HREF="http://www.yasd.com/mm/icon.gif" STYLE="ICON"/>
        <ABSTRACT>YASD Code Byte</ABSTRACT>
    </ITEM>
    <ITEM HREF="http://www.yasd.com/samples/bytes/cheap.htm">
        <LOGO HREF="http://www.yasd.com/mm/icon.gif" STYLE="ICON"/>
        <ABSTRACT>Cheap Page Tricks</ABSTRACT>
    </ITEM>
</CHANNEL>

Notice that the first line contains the XML declaration element, a version number, and an encoding declaration. The main entity within the document is the CHANNEL entity, enclosing other elements such as TITLE, ITEM, ABSTRACT, and LOGO. Each of these elements falls within the allowable XML definition for elements:

element ::= EmptyElemTag | STag content ETag 
EmptyElemTag ::= '<' Name (S Attribute)* S? '/>'
STag :: = '<' Name (S Attribute)* S? '>'
ETag::= '</' Name S? '>'
content ::= (element | CharData | Reference | CDSect | PI | Comment )*

Without continuing to resolve the non-terminating references, what the syntax just shown states is that each element is either an empty element, in which case it ends with a backslash/angle bracket combination (‘/>’), or it has start and end tags which enclose content. A “well-formed” constraint is placed on the start and end tags in that the NAME used in both is the same. The enclosed content can include other elements, comments, processing instructions, or other well-formed XML entities. Both empty and non-empty elements can have zero or more attributes, as the following demonstrates:

<CHANNEL HREF="http://www.yasd.com/plus/index.htm" 
        BASE="http://www.yasd.com/plus/">
...
</CHANNEL>

or

<INTERVALTIME DAY="1"/>

Internet Explorer 4.0 has an associated XML parser that pulls the element information out of the document. IE 4.0 uses this parsed element information to create the channel for the Web site, including the two sub-channel items, as shown below:

This site has a main Web page channel, denoted by the top-level graphic, and two sub-channels, with the second sub-channel loaded into the browser.

Accessing the CDF file directly with IE 4.0 opens a dialog box asking the individual how they would like to subscribe to the site’s channel, and allowing the reader to determine how and when the channel contents are downloaded to their client machine.

Other uses of XML
In addition to CDF, Microsoft and Marimba, Inc. have also proposed XML-based technology called the Open Software Description (OSD) format, which can be used to control software downloads and installations over a corporate network. A major IS expense for larger corporations, especially those that are geographically distributed, is installing and maintaining software upgrades on employees’ desktops. One small upgrade to a popular piece of software can take days of planning and weeks of actual implementation (i.e., walking around to each person’s desk and installing the upgrade). During the upgrade rollout, employees will have different versions of the same software, which can create problems. With OSD, software upgrades can be handled automatically using push technology, reducing both IS staff hours and logistical problems.

SGML and XML have both been used to create a Chemical Markup Language (CML) for the chemistry community. With the CML vocabulary, molecular structures can be defined within a document and the information can be either posted or transmitted. XML processors can pull out the CML elements and pass these to applications that perform actions like preparing a print-out of the information, either textually or graphically, or creating an online three-dimensional model of the information using VRML or some other 3D technology.

Netscape, Apple, and others have proposed a Meta Content Framework (MCF) created in XML that can expose a Web site structure for navigation or online exploration. MCF can be used to do such things as generating a three-dimensional site map which can be used for Web site publication and administration. The technology is currently used by Apple’s ProjectX/HotSauce browser, and “Xspace”-compatible content can also be viewed using a plug-in available from Apple.

XML can also be used to define a relational database meta-language, which can then be used to describe documents containing relational database information. These same documents can be easily generated from the relational database dictionaries, which are repositories of information about the information stored in the database. The extensible markup language can then be used to create context-centered documents like “all information pertaining to any purchases, week of January 16 through January 23,” rather than using the context-neutral database format. In addition, supporting information that is not part of the data in the database, like images or reference material, can be pulled into the document.

An XML processor can process this context-based data document and use the information therein to present reports, perform online research and queries, or even to create interactive three-dimensional models of the data. Instead of issuing a SQL statement such as:

select customer_name, customer_address, city, state, zip_code from customer, 
purchase_orderwhere purchase_order.order_id = 32245 and 
customer.customer_id = purchase_order.customer_id;

I could enter a three-dimensional VRML world at a purchase_order portal and scan a virtual filing cabinet for my purchase order number. Once I find it, I can open the door into another room with doors labeled “Purchase Order items” and “Customer” and open the Customer door into another room containing the information I am looking for. Best of all, the documents containing the context-based data could be generated automatically, processed automatically, and presented automatically. This means a change in the database table could be handled automatically.

Besides three-dimensional database applications, defining data in an XML document could be used as a method to convert database data in one format, such as relational data, into another format, such as object- based database records. The resources section at the end of this article has a reference to a preliminary XML representation of a relational database.

In addition, with XML processors (or XML parsers, if you prefer), the most difficult aspect of XML has already been implemented: pulling the entities out of the document.

Returning to the CDF example, not only can the XML document be used by Internet Explorer 4.0 to provide information about the structure of a Web site’s channels, I can also access the XML entities using JavaScript, C++, or Java and use the information for other purposes. For example, the following JavaScript functions open a CDF file, pull out information about the elements contained within the CDF file, and print out this information in a newly opened window.

<script language="jscript">
<!--
var doc = new ActiveXObject("msxml");
var wndw = null;

// display elements in CDF file
// file reference must be fully resolved Internet reference
function DisplayElements(cdffile)
{
// Display this with an appropriate message in a popup window
wndw = window.open("","CDFFile",
"resizable,scrollbars=yes");
wndw.document.open();
doc.URL = cdffile;

// begin displaying elements at root
displayElement(doc.root);

wndw.document.write("</body>");
wndw.document.close();

}

// display element tagname, if any
// and information about element such as any attributes (even 
// if undefined for element) and text and element type
function displayElement(elem) {
if (elem == null) return;
wndw.document.writeln("<p>");
if (elem.type == 0)
    wndw.document.writeln("Document contains element with 
                           tagname: " + elem.tagName);
else
    wndw.document.writeln("Document contains element with no tagname");
wndw.document.writeln("<br>Element is of type: " + 
                                GetType(elem.type) +"<br>");
wndw.document.writeln("Element text: " 
                                + elem.text + "<br>");
wndw.document.writeln("Element href: " 
                                + elem.getAttribute("href") + "<br>");
wndw.document.writeln("Element base: " 
                                + elem.getAttribute("base") + "<br>");
wndw.document.writeln("Element style: " 
                                + elem.getAttribute("style") + "<br>");
wndw.document.writeln("Element day: " 
                                + elem.getAttribute("day") + "<br>");
wndw.document.writeln("Element hour: " 
                                + elem.getAttribute("hour") + "<br>");
wndw.document.writeln("Element minute: " 
                                + elem.getAttribute("min") + "<br>");

// check to see if element has children
var elem_children = elem.children;
if (elem_children != null)
   for (var i = 0; i < elem_children.length; i++) {
      element_child = elem_children.item(i);
        displayElement(element_child);
   }

}

// element type
function GetType(type) { 
if (type == 0) 
        return "ELEMENT"; 
if (type == 1) 
        return "TEXT"; 
if (type == 2) 
        return "COMMENT"; 
if (type == 3) 
        return "DOCUMENT"; 
if (type == 4) 
        return "DTD"; 
else 
        return "OTHER";
}

//-->
</script>

See the Resources section for a pointer to an XML demonstration.

Creating an XML document
A key to the true usefulness of XML is that once an XML parser has been created to process an XML document, you can use it to parse out entity information from any document containing any well-formed XML content.

In the last section, I used Internet Explorer’s ability to parse XML entities, attributes, and content to create a Web page that listed the entities, their attributes, and some content. An interesting example, but not really useful. But what if I were to define my own XML document, including my own XML entities and attributes, and then use IE’s built-in XML parser to create my own graphic menu Web page application? This is fairly simple and only took a couple of hours of playing around to accomplish.

First, I defined my own CDF file and created my own entities, as shown here:

<?XML VERSION="1.0" ENCODING="UTF-8"?>

<DOCUMENT >
    <TITLE>YASD+</TITLE>
    <STYLESHEET HREF="http://www.yasd.com/css/daily.css" />
    <ITEM HREF="http://www.yasd.com/plus/plus.htm">
        <IMAGE HREF="http://www.yasd.com/plus/logo.jpg">
        <ALT>YASD+ Main Page</ALT>
        </IMAGE>
    </ITEM>
    <ITEM HREF="http://www.yasd.com/samples/bytes/daily.htm">
        <IMAGE HREF="http://www.yasd.com/plus/logo.jpg">
        <ALT>YASD Code Byte</ALT>
        </IMAGE>
    </ITEM>
    <ITEM HREF="http://www.yasd.com/samples/bytes/cheap.htm">
        <IMAGE HREF="http://www.yasd.com/plus/logo.jpg">
        <ALT>YASD Cheap Page Tricks</ALT>
        </IMAGE>
    </ITEM>
</DOCUMENT>

I redefined what ITEM is, created a new root element called “DOCUMENT,” and added some new elements of IMAGE, STYLESHEET, and ALT. I followed the XML convention for well-formed entities — opening up this document for parsing within IE 4.0 generates no errors.

I then created an application, consisting of two frames, that uses the images associated with the items to create a graphical menu bar in the top frame of the window and set the link associated with each image to open in the bottom frame of the window. The window originally opens with the form to access the CDF file and process its contents. This form is then overwritten with the processing results. The code for the form and to process the form contents is as follows:

 
<script language="jscript">
<!--
var doc = new ActiveXObject("msxml");
var wndw = null;

var title = "";
var stylesheet = "";
items = new Array();
itemimages = new Array();
itemalts = new Array();
ct = -1;

function createWindow(cdffile)
{
doc.URL = cdffile;

// find main document and any associated item documents
findElements(doc.root);

// if associated documents
if (ct > 0) {
  var strng = "<HTML><HEAD><TITLE>" + title + 
        "</TITLE><LINK REL=STYLESHEET TYPE='text/css'" +  
        " HREF='" + stylesheet + "'></HEAD><BODY>";
  for (var i = 0; i <= ct; i++) 
     strng+="<a href='" + items[i] + 
                "' target='Body'><IMG src='" + itemimages[i] + "' ALT='" + 
                itemalts[i] + "' border=0>" + 
                "</a>"; 
  strng+="</BODY></HTML>";
  document.open();
  document.writeln(strng);
  document.close();
  }
}

// display element tagname, if any
// and information about element such as any attributes (even if undefined for element)
// and text and element type
function findElements(elem) {
if (elem == null) return;
if (elem.type == 0) {
    if (elem.tagName == "TITLE")
        title = elem.text;
    if (elem.tagName == "STYLESHEET")
        stylesheet = elem.getAttribute("href");
    if (elem.tagName == "ITEM") {
        ct++;
          items[ct] = elem.getAttribute("href");
        }
    if (elem.tagName == "ALT") 
        itemalts[ct] = elem.text;
    if (elem.tagName == "IMAGE")
        itemimages[ct] = elem.getAttribute("href");
    }
        
// check to see if element has children
var elem_children = elem.children;
if (elem_children != null)
   for (var i = 0; i < elem_children.length; i++) {
      element_child = elem_children.item(i);
        findElements(element_child);
   }
}
//-->
</script>

I could have defined any elements within the XML document as long as I used well-formed XML entities, and I could process the results in virtually any way I desired just by using simple scripting techniques.

Linking and style information
In addition to the XML specification, other efforts are currently underway to add supporting specifications. The first is XML part 2, which includes linking. Another is XSL, the Extensible Style Language, which defines an XML stylesheet.

Linking has been extended considerably with XML. You can specify an attribute that determines how a resource is displayed, specify whether the resource is displayed automatically, and even specify multiple layers of linkage. Of particular interest is the capability to define a group of links, associating documents together in such a way that the person following the links does not have to hunt around for related documents. If you have ever jumped to a Web site page by following a link from another site, you know how frustrating it can be to try establish the context of the link in order to find related documents.

XSL would be specified using XML and would provide a way to define presentation elements, such as those used currently in HTML. For example, HTML includes the Emphasis element, delimited with <EM> </EM> tags, the Strong element, delimited with <STRONG> </STRONG>, and others. With XSL, you could create styles to provide recommendations for how an XML entity is rendered.

The downside to XML
While XML’s implementation-neutral technique allows parsed information to be used for multiple purposes in multiple applications, it is this same flexibility that may cause problems.

Returning one last time to my CDF example, I created a simple JavaScript application that opens the main channel page and all the associated pages into a frames-based Web page. The main page opens into the top-most frame, and each individual CDF ITEM element opens into one of the smaller frames located along the bottom of the document.

This isn’t a problem for my own CDF file, which is relatively simple. Applying the same application to another CDFfile, however — one I neither created nor control — creates a Web page that probably does not meet the expectations of the page’s designer. The following screen shot shows the result of using the frames-generation application on the IDG.net channel:

To create this page, I used a publicly accessible file, IDG.net’s CDF file, and exposed the XML elements to create a presentation neither Microsoft nor IDG.net intended. Even with the new effort on XSL, currently only a W3C proposal, there is no guarantee that the information exposed with XML will be used for anything approaching the intended purpose of the XML document’s original creator.

Another potential problem area with XML is the CDF specification. CDF’s potential is great; you could use it to build an XML-based document that could be used by different push technology vendors with relatively comparable results. But what happens if a vendor supports channels but doesn’t want to use CDF? Do we end up with different “flavors” of channels? Does the W3C then create a different standards specification for channels, another for chemistry, another for math, another for finance, and so on in order to ensure that only one specification for each “topic” or “business” is created? Or can we design tools for translating between each of the XML document definitions?

In conclusion
Even with these issues at stake, XML is a terrific addition to Web and other application development. One of the most difficult aspects in application programming is extracting the structure as well as the contents of documents. XML has made this process a whole lot easier.

During the recent XML/SGML conference in Washington DC (December10-12), XML became a proposed recommendation of the W3C, the last remaining step before becoming a real recommendation. It may be only a matter of time before XML is just as common as SQL is today.

Netscape Navigator’s JavaScript 1.1 vs Microsoft Internet Explorer’s JScript

Originally published in Netscape World, archived at the Wayback Machine

Prior to Netscape implementing JavaScript in Netscape Navigator, web developers had few tools to create interactive content for their web pages. Now this scripting language gives developers the ability to do things such as check form contents, communicate with the user based on their actions, and modify the web page dynamically without the web page being re-loaded and without the use of Java, plug-ins or ActiveX controls.

Unfortunately, JavaScript was not usable by any other browser until Microsoft released Internet Explorer (IE) 3.0. With this release web developers could deliver interactive content that would at least work with the two most widely used browsers. Or could they?

On the surface, the JavaScript supported by both companies is identical. They both provide the same conditional control statements, have defined objects such as window or document,and can be used directly in HTML documents. They both support events based on user actions and support functions in a similar manner. However, this article will demonstrate that though the languages may look the same on the surface, there are differences that can trip the unwary developer.

JavaScript Objects

To understand the differences between the two implementations of JavaScript you must examine the objects both support. As an example, with Navigator 3.0 Netscape provides a new object image which is an array (defined as images that contains the images in the document currently loaded. This object allows the developer to change the images of a document without re-loading the document or using Java or other technique. With this capability the developer can write the following JavaScript code section without error:


sSelected = "http://www.some.com/some.gif"
document.images[iIndex].src = sSelected

This code will change the src property of the image that is contained in the array at the index given in the variable iIndex. If this variable contained the value 2, the 3rd image as loaded in the document (the array indices begin at 0) would be changed to the image located in the given URL. An example using images can be found here.

Running the same example with Internet Explorer will result in a alert message that states that “‘images’ is not an object”.

Other examples of objects that are defined for Netscape Navigator but not for Microsoft Internet Explorer are:

  • The Area object, which is an array of links for an image map that allows the developer to capture certain events for the image map that can be used to provide additional information to the user. With this, the developer can capture a mouseOver event to write out information about the link in the status bar.
  • The FileUpload object which provides a text like control and a button marked with “Browse” that will allow a reader to enter a file name. The JavaScript can then access the name of the file.
  • The Function object which allows the developer to define and assign a function to a variable which can then be assigned to an event.
  • The mimeTypes Array of supported MIME types.
  • The option object which is an array of the options implemented for SELECT and which allows the developer to change the text of the option at runtime
  • The applet object, which is an array of Applets in the document (read-only)
  • The plugin object, which is an array called embeds that contains the plug-ins contained in the document (read-only)
  • The plugins array, which is an array of plug-ins currently installed in the client browser

At this time there are no JavaScript or JScript objects defined for Internet Explorer that are also not defined for Netscape. However, as JScript is an implementation script for IE and Microsoft has defined their own IE scripting model, this could change in the future.

JavaScript Object Behavior and Ownership

Internet Explorer may not have additional objects but it has defined a different hierarchy and ownership for some of the objects that are used by both it and Navigator. All objects are contained within the window object in the IE scripting model, which can be viewed here, but not all objects are owned by window with the Navigator model, which can be viewed with the JavaScript Authoring Guide. The object Navigator, which is the object that stores information about the browser currently being used, is an example of an owned object by window in IE but not in Navigator.

This will not present an incompatibility problem between the two browsers as the developer will usually not preface the object with the term “window” as the following code demonstrates:


<FORM NAME="form1">
<input type=button value='press' onClick="alert(navigator.appName)">
</FORM>

The above code will work with both browsers.

The differences between the ownership can become a problem when an object is owned by different levels of objects. An example is the history object, which is owned by the windowobject in IE, but by the document object in Navigator. When used in the current window and document, the object will work the same as the following code will demonstrate:


<FORM NAME="form1">
<input type=button value='press' onClick="history.back()">
</FORM>

The reason the same code can work for both is that window is assumed for both IE and Netscape and document is assumed for Navigator, at least in this instance because history is also an object in its own right. In the case of a document being opened as part of a frame, the differences then become noticeable. The following code will work when the document is opened as a frame in IE, but will not work in Navigator:


<body>
<script>
function clicked() {
history.back()
}
</script>
<FORM NAME="form1">
<input type=button value='press' onClick="clicked()">
</FORM>
</body>

Clicking on the button from the code above will work for IE. The previous document in the History list will be opened, but the same code will not work for Navigator. Clicking on the button will result in neither a change of document nor an error. Prefacing the history object with parent will enable this code to work with both browsers.

JavaScript Properties

Even when IE and Navigator share a common object and a common object ownership, they can differ on the properties for an object. An example is the document object. The properties for both implementations are the same except for an URL property for the Navigator implementation and a location property for the IE implementation. However, if you examine both properties, they are identical! Both contain the URL of the document. Both are read-only. The following code will work with Navigator, but results in an empty Alert message box for IE:


<FORM NAME="form1">
<input type=button value='press' onClick="alert(document.URL)">
</FORM>

According to Microsoft documentation, the equivalent for IE would be to use document.location.href. However, though this does not result in an error, it also results in an empty alert box. The following code achieves the desired results and, happily, works in both browsers:


<FORM NAME="form1">
<input type=button value='press' onClick="alert(location.href)">
</FORM>

The above example does demonstrate another area of caution when using the JavaScript language: this language is unstable in both environments and is changing continuously. Don’t assume something will work because the documentation states it will, and don’t assume it will work the same on all operating systems. Both browsers can have different behaviors across different operating systems, sometimes because of operating system differences and sometimes because of bugs that were missed during testing on that specific OS.

JavaScript Methods and Events

A web developer can find ways of working around differences in objects and properties, but working around differences in methods may not be so easy. When we develop we expect a certain behavior to result when we call a specific function and pass to it certain parameters. With Microsoft Internet Explorer and Netscape Navigator, the best case scenario is that we can use a pre-defined object method with both browsers and have the same result. The worst case scenario is that the method works with both browsers, but the result is different.

An example of the best case scenario is to use JavaScript to validate form field contents which have changed or to use these contents to calculate a value used elsewhere. Calling a JavaScript function from the onChange event to process the changed contents as demonstrated below will work with both browsers:


<!--- Form fields
<p>Item Quantity: <INPUT TYPE="text" Name="qty">
Item Cost: <INPUT TYPE="text" Name="cost"
                onChange="NewCost()">
Total Cost: <INPUT TYPE="text" Name="total" Value=0>
…
<!--- Function
<SCRIPT LANGUAGE="javascript">
<!--- hide script from old browsers

// NewCost will
// calculate cost of qty and item
function NewCost() {
        var iCost = parseFloat(document.Item.cost.value)
        var iQty = parseInt(document.Item.qty.value)

        var iTotal = iCost * iQty
        document.Item.total.value = iTotal
        }

The above will work as expected with both IE and Navigator. When the user enters a quantity and a cost, the onChange event will fire for the cost field and a JavaScript function called NewCost() will be called. This function will call two built-in JavaScript functions, parseFloat() and parseInt(), to access and convert the form field values. These will then be used to compute a total which is placed in the total field.

So far so good. Another JavaScript function in the web page will be processed when the user presses the submit button. This pre-defined button style will normally submit the form. The developer can capture the submission and perform validation on the fields. Coding this for Navigator would look like the following:


<FORM NAME="Item"
        ACTION="some.cgi" onSubmit="return SendOrder()">
…
<!--- Function
// submit order
function SendOrder() {

        // validate data
        if (document.Item.Name.value == "") {
                alert("You must enter your name")
                return false
                }
        return true
        }

Capturing the onSubmit event will allow the developer to call a function to process the form fields. If they choose, they can perform validation in this function. If the validation fails, say the user did not provide a name, the function notifies the reader and returns false, preventing the form from being submitted. If the user did provide a name, the function would return true, and the form would be submitted.

Following the documentation that can be found at the Microsoft site, the developer would expect something like this to work for IE as well as Navigator. It does, to a point.

With IE the onSubmit event is captured and the SendOrder() function is called. If the user did not enter a name value, an alert would occur. The behavior is the same for both browsers at this point. However, if the user does provide a value, Navigator would then submit the form and a follow-up form would be displayed. This does NOT occur with IE IF you are testing the page locally, probably due to a bug missed during the testing. It does work if you run the web page documents through a web server.

However, without knowing that the difference between the two results was a matter of document location rather than document coding the web page developer could have spent considerable time trying to get the same behavior for both browsers.

Aside from the differences already noted, the browsers may process code in a funtionally identical manner and yet perform quite differently. This can be demonstrated with another popular use of JavaScript which is to open a secondary window for some purpose and to maintain communication between the two windows. This is widely used by Netscape for their tutorials.

Both browsers support a property for the window object called opener. This property can be used to contain a reference to the window that opened the secondary window.

The following code demonstrates using JavaScript to open a secondary window and to set the opener property to the original window:


<HTML>
<HEAD><TITLE> Original Window </TITLE>
<SCRIPT LANGUAGE="JavaScript">
<!--- hide script from old browsers
// OpenSecondary will open a second window
//
function OpenSecond(iType, sMessage) {
        // open window
        newWindow=window.open("second.htm","",
                "toolbar=no,directories=no,width=500,height=300")
        // modify the new windows opener value to
        // point to this window
        if (newWindow != null && newWindow.opener == null)
                newWindow.opener=window

        }
// end hiding from old browsers -->
</SCRIPT>
</HEAD>
<BODY>
<H1> Open a new Window and get information</H1>

<FORM NAME="CallWindow">
<p>Click to open Window:
<p>
<INPUT TYPE="button" NAME="OpenWin" VALUE="Click Here"
        onClick="OpenSecond()">
<p>
</FORM>
</BODY>
</HTML>

The above HTML and JavaScript creates a simple document with one button. Pressing the button opens a new window (creates a new instance of the browser) that does not have a toolbar or directories and is set to a certain width and height. The HTML source document that is opened into this new window is “second.htm”.

The HTML and JavaScript in the secondary window is shown below:


<HTML>
<HEAD><TITLE> Information</TITLE>

<SCRIPT LANGUAGE="JavaScript">
<!--- hide script from old browsers
// SetInformation
// function will get input values
// and set to calling window
function SetInformation() {
        opener.document.open()
        opener.document.write("<BODY>")
        opener.document.write("<H1> Return from Secondary</h1>")
        opener.document.write("</BODY>")
        opener.document.close()
        opener.document.bgColor="#ff0000"
        window.close()
        }
// end hiding from old browsers -->
</SCRIPT>
</HEAD>
<BODY>
<p><form>
Click on the button:<p>
<INPUT type="button" Value="Click on Me"
     onClick="SetInformation()">
</CENTER>
</FORM>
</BODY>
</HTML>

This code will display a button labeled “Click on Me”. Pressing this button will result in the document page for the original window being modified to display the words “Return from Secondary”. The JavaScript will also change the background color of the original document and then will close the secondary window. When run in Navigator, the sample behaves as expected. Not so, however, with IE.

First, when you open the secondary window in IE you will notice that the document in the original window seems to blank out for a moment. In addition, the secondary window opens at the top left hand side of the desktop with IE but opens directly over the original window with Navigator, no matter where that original window is.

When you press the button that modifies the original document, IE does modify the document with the new header, as Navigator does, and the background on the original document will change briefly. However, when the secondary window is closed the background color of the original document returns to the original color!

What is worse is that running this sample for a few times with IE will cause the browser to crash! How soon it will crash seems to suggest that resources are not being freed, or are being freed incorrectly. Whatever the cause, this behavior should make a developer feel cautious about opening up a secondary window within IE.

Solutions to working with both Internet Explorer and Navigator

JavaScript is a useful tool for creating interactive content without using Java or some other technique. As we have seen, problems arise out of the incompatibility between Navigator and IE. What are ways to avoid these problems?

  • Code for one browser only. This is not really a viable solution. While Netscape Navigator is the most popular browser in use today, Microsoft Internet Explorer is gaining quite a following. Using Navigator-only JavaScript that will limit your web page’s audience.
  • Code only to the least common denominator. By limiting the JavaScript to that which works for both Navigator and IE, developers will have access to most of the functionality they need. However, with this approach developers will need to be aware of the gotchas that have been discussed in this article. In other words, the developer has to test with both browsers; the tests will have to occur via the web server as well as run locally; the tests should be repeated several times to ensure that no adverse effects show up over time; the code should be tested with all possible environments.
  • Code for each browser within the same page. This is actually my favorite approach though it will require more effort by the developer. Both browsers will process JavaScript labeled as <SCRIPT LANGUAGE=”javascript”>, however only Navigator will process script labeled as <SCRIPT LANGUAGE=”javascript1.1″>. With this approach, the developer can place JavaScript common to both browsers in a SCRIPT tag labeled with “javascript” and Navigator specific code in a SCRIPT tag labeled with “javascript1.1”. To add in IE specific code, the developer would probably want to use VBScript code or use the tag JScript whenever applicable. At this time, JScript is implemented for scripting with events only, not as a language specification for the SCRIPT tag.

Summary

JavaScript is a very effective tool, and a relatively simple method for adding interactive content to a web page. With version 3.0 of Internet Explorer, using JavaScript is viable for both of the most popular browsers in use. However, developersneeds to be aware of the differences between Netscape Navigator and Microsoft Internet Explorer and should test thoroughly with both before posting the page to their site.