Function in Apache JMeter

JMeter functions are no different from normal languages like C, C++, Perl etc. These functions take some parameters and return some value after processing those arguments. By convention, JMeter function names always starts with '__' i.e. two underscores. 

A typical function call looks like: 

${__FunctionName(var1,var2)}

where:
FunctionName maches any function name like __XPath

NOTE: If a function parameter contains a comma, then be sure to escape this with "\", otherwise JMeter will treat it as a parameter delimiter. For example:

${__time(EEE\, d MMM yyyy)}. 

If the comma is not escaped, you will get an error such as:

ERROR - jmeter.functions.JavaScript: Error processing Javascript: [Math.max(2]
    org.mozilla.javascript.EvaluatorException: missing ) after argument list (#1)

LIST of functions:

Type of function	Name	Usage
Information	threadNum	get thread number
Information	samplerName	get the sampler name (label)
Information	machineIP	get the local machine IP address
Information	machineName	get the local machine name
Information	time	return current time in various formats
Information	log	log (or display) a message (and return the value)
Information	logn	log (or display) a message (empty return value)
Input	StringFromFile	read a line from a file
Input	FileToString	read an entire file
Input	CSVRead	read from CSV delimited file
Input	XPath	Use an XPath expression to read from a file
Calculation	counter	generate an incrementing number
Calculation	intSum	add int numbers
Calculation	longSum	add long numbers
Calculation	Random	generate a random number
Calculation	RandomString	generate a random string
Calculation	UUID	generate a random type 4 UUID
Scripting	BeanShell	run a BeanShell script
Scripting	javaScript	process JavaScript (Mozilla Rhino)
Scripting	jexl, jexl2	evaluate a Commons Jexl expression
Properties	property	read a property
Properties	P	read a property (shorthand method)
Properties	setProperty	set a JMeter property
Variables	split	Split a string into variables
Variables	V	evaluate a variable name
Variables	eval	evaluate a variable expression
Variables	evalVar	evaluate an expression stored in a variable
String	regexFunction	parse previous response using a regular expression
String	escapeOroRegexpChars	quote meta chars used by ORO regular expression
String	char	generate Unicode char values from a list of numbers
String	unescape	Process strings containing Java escapes (e.g. \n & \t)
String	unescapeHtml	Decode HTML-encoded strings
String	escapeHtml	Encode strings using HTML encoding
String	TestPlanName	Return name of current test plan

Source: http://jmeter.apache.org