18 Scripts

18.1 Introduction to scripts

Scripts offer authors a means to extend HTML documents in highly active and interactive ways. For example:

• Scripts may be evaluated as a document loads to modify the contents of the document dynamically.
• Scripts may accompany a form to process input as it is entered. Designers may dynamically fill out parts of a form based on the values of other fields. They may also ensure that input data conforms to predetermined ranges of values, that fields are mutually consistent, etc.
• Scripts may be triggered by events that affect the document, such as loading, unloading, element focus, mouse movement, etc.
• Scripts may be linked to form controls (e.g., buttons) to produce graphical user interface elements.

There are two types of scripts authors may attach to an HTML document:

• Those that are executed one time when the document is loaded by the user agent. Scripts that appear within a SCRIPT element are executed when the document is loaded. For user agents that cannot or will not handle scripts, authors may include alternate content via the NOSCRIPT element.
• Those that are executed every time a specific event occurs. These scripts may be assigned to a number of elements via the intrinsic event attributes.

18.2 Designing documents for user agents that support scripting

The following sections discuss issues that concern user agents that support scripting.

18.2.1 The SCRIPT element

<!ELEMENT SCRIPT - - %Script;          -- script statements -->
<!ATTLIST SCRIPT
  charset     %Charset;      #IMPLIED  -- char encoding of linked resource --
  type        %ContentType;  #REQUIRED -- content type of script language --
  src         %URI;          #IMPLIED  -- URI for an external script --
  defer       (defer)        #IMPLIED  -- UA may defer execution of script --
  >

Start tag: required, End tag: required

The SCRIPT element places a script within a document. This element may appear any number of times in the HEAD or BODY of an HTML document.

The script may be defined within the contents of the SCRIPT element or in an external file. If the src attribute is not set, user agents must interpret the contents of the element as the script. If the src has a URI value, user agents must ignore the element's contents and retrieve the script via the URI. Note that the charset attribute refers to the character encoding of the script designated by the src attribute; it does not concern the content of the SCRIPT element.

Scripts are evaluated by script engines that must be known to a user agent.

The syntax of script data depends on the scripting language.

18.2.2 Specifying the scripting language

As HTML does not rely on a specific scripting language, document authors must explicitly tell user agents the language of each script. This may be done either through a default declaration or a local declaration.

The default scripting language  

Authors should specify the default scripting language for all scripts in a document by including the following META declaration in the HEAD:

<META http-equiv="Content-Script-Type" content="type">

where "type" is an content type naming the scripting language. Examples of values include "text/tcl", "text/javascript", "text/vbscript".

In the absence of a META declaration, the default can be set by a "Content-Script-Type" HTTP header.

    Content-Script-Type: type

where "type" is again an content type naming the scripting language.

User agents should determine the default scripting language for a document according to the following steps (highest to lowest priority):

1. If any META declarations specify the "Content-Script-Type", the last one in the character stream determines the default scripting language.
2. Otherwise, if any HTTP headers specify the "Content-Script-Type", the last one in the character stream determines the default scripting language.

Documents that do not specify a default scripting language information and that contain elements that specify an intrinsic event script are incorrect. User agents may still attempt to interpret incorrectly specified scripts but are not required to. Authoring tools should generate default scripting language information to help authors avoid creating incorrect documents.

Local declaration of a scripting language 

The type attribute must be specified for each SCRIPT element instance in a document. The value of the type attribute for a SCRIPT element overrides the default scripting language for that element.

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0//EN"
     "http://www.w3.org/TR/REC-html40/strict.dtd">
<HTML>
<HEAD>
<TITLE>A document with SCRIPT</TITLE>
<META http-equiv="Content-Script-Type" content="text/tcl">
<SCRIPT type="text/vbscript" src="http://someplace.com/progs/vbcalc">
</SCRIPT>
</HEAD>
<BODY>
<SCRIPT type="text/javascript">
...some JavaScript...
</SCRIPT>
</BODY>
</HTML>

References to HTML elements from a script 

Each scripting language has its own conventions for referring to HTML objects from within a script. This specification does not define a standard mechanism for referring to HTML objects.

However, scripts should refer to an element according to its assigned name. Scripting engines should observe the following precedence rules when identifying an element: a name attribute takes precedence over an id if both are set. Otherwise, one or the other may be used.

18.2.3 Intrinsic events

It is possible to associate an action with a certain number of events that occur when a user interacts with a user agent. Each of the "intrinsic events" listed above takes a value that is a script. The script is executed whenever the event occurs for that element. The syntax of script data depends on the scripting language.

Control elements such as INPUT, SELECT, BUTTON, TEXTAREA, and LABEL all respond to certain intrinsic events. When these elements do not appear within a form, they may be used to augment the graphical user interface of the document.

For instance, authors may want to include press buttons in their documents that do not submit a form but still communicate with a server when they are activated.

The following examples show some possible control and user interface behavior based on intrinsic events.

<INPUT NAME="userName" onblur="validUserName(this.value)">

<INPUT NAME="num"
    onchange="if (!checkNum(this.value, 1, 10)) 
        {this.focus();this.select();} else {thanks()}"
    VALUE="0">

    <INPUT name="edit1" size="50">    
    <SCRIPT type="text/vbscript">
      Sub edit1_changed()
        If edit1.value = "abc" Then
          button1.enabled = True
        Else
          button1.enabled = False
        End If
      End Sub
    </SCRIPT>

    <INPUT name="edit1" size="50">
    <SCRIPT type="text/tcl">
      proc edit1_changed {} {
        if {[edit value] == abc} {
          button1 enable 1
        } else {
          button1 enable 0
        }
      }
      edit1 onChange edit1_changed
    </SCRIPT>

    
<BUTTON type="button" name="mybutton" value="10">
<SCRIPT type="text/javascript">
      function my_onclick() {
         . . .
      }
    document.form.mybutton.onclick = my_onclick
 </SCRIPT>
 </BUTTON>

    
<SCRIPT type="text/javascript">
      function my_onload() {
         . . .
      }

      var win = window.open("some/other/URI")
      if (win) win.onload = my_onload
</SCRIPT>

 <SCRIPT type="text/tcl">
     proc my_onload {} {
       . . .
     }
     set win [window open "some/other/URI"]
     if {$win != ""} {
         $win onload my_onload
     }
 </SCRIPT>

18.2.4 Dynamic modification of documents

The dynamic modification of a document may be modeled as follows:

1. All SCRIPT elements are evaluated in order as the document is loaded.
2. All script constructs within a given SCRIPT element that generate SGML CDATA are evaluated. Their combined generated text is inserted in the document in place of the SCRIPT element.
3. The generated CDATA is re-evaluated.

HTML documents are constrained to conform to the HTML DTD both before and after processing any SCRIPT elements.

 <TITLE>Test Document</TITLE>
 <SCRIPT type="text/javascript">
     document.write("<p><b>Hello World!<\/b>")
 </SCRIPT>

 <TITLE>Test Document</TITLE>
 <P><B>Hello World!</B>

18.3 Designing documents for user agents that don't support scripting

The following sections discuss how authors may create documents that work for user agents that don't support scripting.

18.3.1 The NOSCRIPT element

<!ELEMENT NOSCRIPT - - (%block;)+
  -- alternate content container for non script-based rendering -->
<!ATTLIST NOSCRIPT
  %attrs;                              -- %coreattrs, %i18n, %events --
  >

Start tag: required, End tag: required

• The user agent is configured not to evaluate scripts.
• The user agent doesn't support a scripting language invoked by a SCRIPT element earlier in the document.

User agents that do not support client-side scripts must render this element's contents.

<SCRIPT type="text/tcl">
 ...some Tcl script to insert data...
</SCRIPT>
<NOSCRIPT>
 <P>Access the <A href="http://someplace.com/data">data.</A>
</NOSCRIPT>

18.3.2 Hiding script data from user agents

User agents that don't recognize the SCRIPT element will likely render that element's contents as text. Some scripting engines, including those for languages JavaScript, VBScript, and Tcl allow the script statements to be enclosed in an SGML comment. User agents that don't recognize the SCRIPT element will thus ignore the comment while smart scripting engines will understand that the script in comments should be executed.

Another solution to the problem is to keep scripts in external documents and refer to them with the src attribute.

<SCRIPT type="text/javascript">
<!--  to hide script contents from old browsers
  function square(i) {
    document.write("The call passed ", i ," to the function.","<BR>")
    return i * i
  }
  document.write("The function returned ",square(5),".")
// end hiding contents from old browsers  -->
</SCRIPT>

   <SCRIPT type="text/vbscript">
     <!--
       Sub foo()
        ...
       End Sub
     ' -->
    </SCRIPT>

<SCRIPT type="text/tcl">
<!--  to hide script contents from old browsers
  proc square {i} {
    document write "The call passed $i to the function.<BR>"
    return [expr $i * $i]
  }
  document write "The function returned [square 5]."
# end hiding contents from old browsers  -->
</SCRIPT>