How Do Global and Local Variables Differ in XSLT?

In XSLT, variables declared using the <xsl:variable> element act as immutable bindings whose accessibility and lifespan are determined entirely by where they are placed in the stylesheet. This article breaks down the fundamental differences between global and local XSLT variables, focusing on their declaration placement, scope and visibility, evaluation timing, and shadowing rules across templates.

Declaration and Placement

The primary syntactic difference between global and local variables lies in where the <xsl:variable> element is positioned relative to other elements in the stylesheet document:

Global Variables: Declared as top-level elements directly under the root <xsl:stylesheet> or <xsl:transform> element, outside of any <xsl:template> or function definition.
Local Variables: Declared inside an instructional block, such as an <xsl:template>, <xsl:for-each>, <xsl:if>, or <xsl:choose> element.

<xsl:stylesheet version="2.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform">

    <!-- Global Variable -->
    <xsl:variable name="globalTaxRate" select="0.08" />

    <xsl:template match="order">
        <!-- Local Variable -->
        <xsl:variable name="itemTotal" select="price * quantity" />
        <total>
            <xsl:value-of select="$itemTotal + ($itemTotal * $globalTaxRate)" />
        </total>
    </xsl:template>

</xsl:stylesheet>

Scope and Visibility

Scope dictates which parts of the stylesheet can reference a variable using the $variableName syntax.

Feature	Global Variables	Local Variables
Visibility	Visible throughout the entire stylesheet, across all templates and included/imported modules.	Visible only to following sibling elements and their descendants within the enclosing container.
Order Dependency	Can be referenced before or after their physical line of declaration in the stylesheet.	Can only be referenced after their point of declaration; preceding elements cannot access them.
Parent Access	Readily accessible from inside any child instruction block.	Inaccessible outside the parent element in which they are defined.

In local scoping, placing an <xsl:variable> inside an <xsl:for-each> loop means the variable is re-bound for every iteration and cannot be read after the closing </xsl:for-each> tag.

Evaluation Timing and Context

While both global and local variables are strictly immutable once evaluated, their evaluation context differs:

Global Evaluation: Evaluated using the initial context of the transformation, typically using the root document node as the context node. They are evaluated lazily (on demand) in most processors and cached for the duration of the transformation.
Local Evaluation: Evaluated within the current dynamic context of the executing template or loop. The value of select="." inside a local variable will bind to the specific node currently being processed by that template.

Variable Shadowing Rules

XSLT defines strict rules regarding duplicate variable names:

A local variable is allowed to share the same name as a global variable. When this occurs within a template, the local variable shadows (overrides) the global one within that local scope.
You cannot declare two global variables with the exact same name and import precedence in the same stylesheet module.
You cannot declare two local variables with the exact same name within the same lexical scope or overlapping child scope.