18. Scripts

18.1 Introduction toscripts

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

• Scripts may be evaluated as a document loads to modify the contents of thedocument dynamically.
• Scripts may accompany a form to process input as it is entered. Designersmay 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 ofvalues, that fields are mutually consistent, etc.
• Scripts may be triggered by events that affect the document, such asloading, unloading, element focus, mouse movement, etc.
• Scripts may be linked to form controls (e.g., buttons) to produce graphicaluser interface elements.

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

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

18.2 Designing documents for user agents that supportscripting

The following sections discuss issues that concern user agents that supportscripting.

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 mayappear 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 srcattribute is not set, user agents must interpret the contents of the element asthe script. If the src has a URI value, user agents must ignore the element'scontents and retrieve the script via the URI. Note that the charset attribute refers to the characterencoding of the script designated by the src attribute; it does notconcern the content of the SCRIPT element.

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

The syntax of script data depends onthe scripting language.

18.2.2 Specifying the scripting language

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

The default scriptinglanguage  

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

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

where "type" is a content typenaming 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 a contenttype naming the scripting language.

User agents should determine the default scripting language for a documentaccording 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 scriptinglanguage.
2. Otherwise, if any HTTP headers specify the "Content-Script-Type", the lastone in the character stream determines the default scripting language.

Documents that do not specify default scripting language information andthat contain elements that specify an intrinsic eventscript are incorrect. User agents may still attempt to interpret incorrectlyspecified scripts but are not required to. Authoring tools should generatedefault scripting language information to help authors avoid creating incorrectdocuments.

Local declaration of a scriptinglanguage 

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 defaultscripting language for that element.

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" "http://www.w3.org/TR/html4/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 HTMLelements from a script 

Each scripting language has its own conventions for referring to HTMLobjects from within a script. This specification does not define a standardmechanism 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 whenidentifying an element: a name attribute takes precedence overan 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 thatoccur 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 wheneverthe 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. Whenthese elements do not appear within a form, they may be used to augment thegraphical user interface of the document.

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

The following examples show some possible control and user interfacebehavior 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 isloaded.
2. All script constructs within a given SCRIPT element that generate SGMLCDATA are evaluated. Their combined generated text is inserted in the documentin place of the SCRIPT element.
3. The generated CDATA is re-evaluated.

HTML documents are constrained to conform to the HTML DTD both before andafter 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 thatdon't support scripting

The following sections discuss how authors may create documents that workfor 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 SCRIPTelement earlier in the document.

User agents that do not support client-side scripts must render thiselement'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 useragents

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

Another solution to the problem is to keep scripts in external documents andrefer 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>