element-available()

Description

element-available() tests whether the XSLT processor supports a named instruction element and returns a boolean. The argument is a string containing a QName; if the QName is in the xsl: namespace, it tests for a standard XSLT instruction. If it is in another namespace, it tests for a processor-specific extension element.

The function is intended for use inside xsl:choose/xsl:when or xsl:if to branch between implementations depending on what the current processor supports. Combined with xsl:fallback, it provides a portable way to use extension elements with graceful degradation.

Only elements that appear as children of the stylesheet (i.e. XSLT instructions and extension elements, not result elements) are tested. Testing for an arbitrary user-defined element name that is not an instruction always returns false.

Parameters

Return value

xs:boolean — true if the element is available, false otherwise.

Examples

Test for a standard XSLT instruction

Input XML:

<?xml version="1.0" encoding="UTF-8"?>
<data><item>A</item><item>B</item></data>

Stylesheet:

<?xml version="1.0" encoding="UTF-8"?>
<xsl:stylesheet version="1.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
  <xsl:output method="xml" indent="yes"/>

  <xsl:template match="/data">
    <info>
      <for-each-available>
        <xsl:value-of select="element-available('xsl:for-each')"/>
      </for-each-available>
      <nonexistent-available>
        <xsl:value-of select="element-available('xsl:nonexistent')"/>
      </nonexistent-available>
    </info>
  </xsl:template>
</xsl:stylesheet>

Output:

<info>
  <for-each-available>true</for-each-available>
  <nonexistent-available>false</nonexistent-available>
</info>

Guard use of an extension element

Stylesheet:

<?xml version="1.0" encoding="UTF-8"?>
<xsl:stylesheet version="1.0"
  xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
  xmlns:ext="http://exslt.org/common"
  extension-element-prefixes="ext">

  <xsl:output method="xml" indent="yes"/>

  <xsl:template match="/data">
    <result>
      <xsl:choose>
        <xsl:when test="element-available('ext:document')">
          <!-- Use EXSLT extension to write multiple output files -->
          <message>Multi-document output supported.</message>
        </xsl:when>
        <xsl:otherwise>
          <message>Multi-document output not supported; writing single file.</message>
        </xsl:otherwise>
      </xsl:choose>
    </result>
  </xsl:template>
</xsl:stylesheet>

Notes

• element-available() only tests XSLT instructions and extension elements. It does not test for result element names (elements in no namespace or a non-XSLT namespace that become part of the output tree).
• The prefix in the QName string must be declared in the stylesheet’s namespace context; otherwise the function raises an error.
• Standard XSLT 1.0 instructions (e.g. xsl:for-each, xsl:if, xsl:choose) always return true in a conformant XSLT 1.0 processor.
• In XSLT 2.0+, the function is unchanged. It can be used to test for XSLT 2.0 instructions (e.g. xsl:for-each-group) when running under a processor that may be in XSLT 1.0 compatibility mode.

See also

• function-available()
• system-property()
• xsl:fallback