IIFE allows us to have data privacy because it creates a new scope that is not visible from the outside scope.

app.js

// immediately invoked function expression
// its an anonymous function wrapped in parentheses
// Step 1: JS Runtime executes this line and this anonymous
// function is declared and immediately called because
// of (); operator below.
var budgetController = (function() {
  // Step 2: Then this is declared and it returns an
  // object below it.
  // this part of the code is private
  var x = 23;

  // this is a private function
  var add = function(a) {
    return x + a;
  }

  // this part of the code is public
  // return an object containing a method
  return {
    publicTest: function(b) {
      // calling the add function with b
      console.log(add(b));
    }
  }
// invoking the function
})();