1function outer() {
2  var counter = 0; // Backpack or Closure
3  function incrementCounter() {
4    return counter++;
5  }
6  return incrementCounter;
7}
8
9const count = outer();
10count(); // 0
11count(); // 1
12count(); // 2

1function OuterFunction() {
2
3    var outerVariable = 100;
4
5    function InnerFunction() {
6        alert(outerVariable);
7    }
8
9    return InnerFunction;
10}
11var innerFunc = OuterFunction();

1function makeFunc() {
2  var name = 'Mozilla';
3  function displayName() {
4    alert(name);
5  }
6  return displayName;
7}
8
9var myFunc = makeFunc();
10myFunc();
11

1var makeCounter = function() {
2  var privateCounter = 0;
3  function changeBy(val) {
4    privateCounter += val;
5  }
6  return {
7    increment: function() {
8      changeBy(1);
9    },
10    decrement: function() {
11      changeBy(-1);
12    },
13    value: function() {
14      return privateCounter;
15    }
16  }
17};
18
19var counter1 = makeCounter();
20var counter2 = makeCounter();
21alert(counter1.value()); /* Alerts 0 */
22counter1.increment();
23counter1.increment();
24alert(counter1.value()); /* Alerts 2 */
25counter1.decrement();
26alert(counter1.value()); /* Alerts 1 */
27alert(counter2.value()); /* Alerts 0 */
28

1/*A closure is the combination of a function bundled together (enclosed) with references
2to its surrounding state (the lexical environment). In other words, a closure gives you 
3access to an outer function’s scope from an inner function. In JavaScript, closures are 
4created every time a function is created, at function creation time.*/
5
6function init() {
7  var name = 'Mozilla'; // name is a local variable created by init
8  function displayName() { // displayName() is the inner function, a closure
9    alert(name); // use variable declared in the parent function
10  }
11  displayName();
12}
13init();
14

1var counter = (function() {
2  var privateCounter = 0;
3  function changeBy(val) {
4    privateCounter += val;
5  }
6  return {
7    increment: function() {
8      changeBy(1);
9    },
10    decrement: function() {
11      changeBy(-1);
12    },
13    value: function() {
14      return privateCounter;
15    }
16  };
17})();
18
19console.log(counter.value()); // logs 0
20counter.increment();
21counter.increment();
22console.log(counter.value()); // logs 2
23counter.decrement();
24console.log(counter.value()); // logs 1
25