1// closure in javascript
2-> A closure gives you access to an outer function’s scope
3   from an inner function
4
5const outerFun = (a) => {
6  let b = 10;
7  // inner func can use variable/parameter of outer funcion
8  const innerFun = () => {
9    let sum = a + b; 
10    console.log(sum);
11  }
12  return innerFun;
13}
14let inner = outerFun(5);
15inner();

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// global scope
2var e = 10;
3function sum(a){
4  return function(b){
5    return function(c){
6      // outer functions scope
7      return function(d){
8        // local scope
9        return a + b + c + d + e;
10      }
11    }
12  }
13}
14
15console.log(sum(1)(2)(3)(4)); // log 20
16
17// You can also write without anonymous functions:
18
19// global scope
20var e = 10;
21function sum(a){
22  return function sum2(b){
23    return function sum3(c){
24      // outer functions scope
25      return function sum4(d){
26        // local scope
27        return a + b + c + d + e;
28      }
29    }
30  }
31}
32
33var sum2 = sum(1);
34var sum3 = sum2(2);
35var sum4 = sum3(3);
36var result = sum4(4);
37console.log(result) //log 20
38