A function is a named block of code that performs a specific task, possibly acting upon a set of values given to it, or parameters, and possibly returning a single value. Functions save on compile time no matter how many times you call them, functions are compiled only once for the page. They also improve reliability by allowing you to fix any bugs in one place, rather than everywhere you perform a task, and they improve readability by isolating code that performs specific tasks.
This chapter introduces the syntax of function calls and function definitions and discusses how to manage variables in functions and pass values to functions (including pass-by-value and pass-by-reference). It also covers variable functions and anonymous functions.