Functional programming uses pure functions, higher order functions and closures

• Pure functions
  • The function always returns the same result if the same arguments are passed in. It does not depend on any state, or data, change during a program’s execution. It must only depend on its input arguments.
  • The function does not produce any observable side effects such as network requests, input and output devices, or data mutation.
    • Side effects include:
      • Making a HTTP request
      • Mutating data
      • Printing to a screen or console
      • DOM Query/Manipulation
      • Math.random()
      • Getting the current time

What is a Closure? A closure is the combination of a function bundled together (enclosed) with references to its surrounding state (the lexical environment). In other words, a closure gives you access to an outer function’s scope from an inner function. In JavaScript, closures are created every time a function is created, at function creation time.