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();

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

1function makeAdder(x) {
2  return function(y) {
3    return x + y;
4  };
5}
6
7var add5 = makeAdder(5);
8var add10 = makeAdder(10);
9
10console.log(add5(2));  // 7
11console.log(add10(2)); // 12
12
13//=====
14//add5 and add10 are both closures. 
15//They share the same function body definition, but store different lexical environments. 
16//In add5's lexical environment, x is 5, while in the lexical environment for add10, x is 10.
17