Difference Between xsl:value-of and xsl:copy-of

When transforming XML documents using XSLT, understanding how to extract and output data correctly is essential. This article explains the fundamental differences between the <xsl:value-of> and <xsl:copy-of> elements, demonstrating how each instruction handles XML nodes, child elements, and attributes so you can choose the correct element for your transformation.

Key Difference at a Glance

The primary difference between <xsl:value-of> and <xsl:copy-of> lies in how they handle XML markup:

• <xsl:value-of> extracts only the string value (plain text) of the selected node and its descendants, discarding all XML tags and attributes.
• <xsl:copy-of> creates a deep copy of the selected node, preserving the node itself, its child elements, attributes, and internal XML structure intact.

How xsl:value-of Works

The <xsl:value-of> element evaluates an XPath expression, converts the result into a string, and outputs only the textual content.

• Markup Stripping: Any child elements inside the selected node lose their XML tags.
• Attributes: Attribute tags are omitted; only the string value is extracted if an attribute is directly selected.
• Primary Use Case: When you need to insert plain text into HTML elements, attributes, or text files without carrying over source XML tags.

How xsl:copy-of Works

The <xsl:copy-of> element performs a deep duplication of the nodes matched by the XPath expression.

• Markup Preservation: All child tags, nested elements, and namespaces remain completely intact.
• Attributes: All attributes belonging to the selected elements are copied into the result tree.
• Primary Use Case: When you want to replicate an entire XML subtree, carry over formatted markup (such as embedded XHTML tags), or pass a node-set directly to the output.

Practical Example

Consider the following source XML snippet:

<product id="101">
    <description>Heavy-duty <b>steel</b> wrench</description>
</product>

Applying xsl:value-of

<xsl:value-of select="product/description"/>

Output:

Heavy-duty steel wrench

Note: The <b> and </b> tags are completely stripped away.

Applying xsl:copy-of

<xsl:copy-of select="product/description"/>

Output:

<description>Heavy-duty <b>steel</b> wrench</description>

Note: The entire element structure, including the <description> and <b> tags, is preserved.

Summary Comparison