XSLT supports all functions defined in XPath. In addition, it defines 10 extra functions. Most XSLT processors also make several extension functions available and allow you to write your own extension functions in Java or other languages. The extension API is nonstandard and processor-dependent.
XPath and XSLT functions are weakly typed. Although one type or another is occasionally preferred, the processor normally converts any type you pass in to the type the function expects. Functions that only take node-sets as arguments are an exception to the weak-typing rule. Other data types, including strings, numbers, and Booleans, cannot be converted to node-sets automatically.
XPath and XSLT functions also use optional arguments, which are filled in with defaults if omitted. In the following sections, we list the most common and useful variations of each function.