elasticsearch/docs/painless/painless-functions.asciidoc

[[painless-functions]]
=== Functions

A function is a named piece of code comprised of one-to-many statements to
perform a specific task. A function is called multiple times in a single script
to repeat its specific task. A parameter is a named type value available as a
<<painless-variables, variable>> within the statement(s) of a function. A
function specifies zero-to-many parameters, and when a function is called a
value is specified per parameter. An argument is a value passed into a function
at the point of call. A function specifies a return type value, though if the
type is <<void-type, void>> then no value is returned.  Any non-void type return
value is available for use within an <<painless-operators, operation>> or is
discarded otherwise.

You can declare functions at the beginning of a Painless script, for example:

[source,painless]
---------------------------------------------------------
boolean isNegative(def x) { x < 0 }
...
if (isNegative(someVar)) {
  ...
}
---------------------------------------------------------