Inner (Private) Functions

Bearing in mind that a function is just like any other value, there’s nothing that stops you from defining a function inside another function.

function a(param) {
  function b(theinput) {
    return theinput * 2;
  };
  return 'The result is ' + b(param);
};

Using the function literal notation, this can also be written as:

var a = function(param) {
  var b = function(theinput) {
    return theinput * 2;
  };
  return 'The result is ' + b(param);
};

When you call the global function a(), it will internally call the local function b(). Since b() is local, it’s not accessible outside a(), so we can say it’s a private function.

a(2)
//"The result is 4"
a(8)
//"The result is 16"
b(2)
//b is not defined

The benefit of using private functions are as follows:

• You keep the global namespace clean (smaller chance of naming collisions).
• Privacy—you expose only the functions you decide to the “outside world”, keeping to yourself functionality that is not meant to be consumed by the rest of the application.