JAVASCRIPT

closure

function outer() {
  const outerVar = "Hi i am outerVar";
  function inner() {
    const innerVar = "Hi i am innerVar";
    console.log(outerVar);
    console.log(innerVar);
  }
  return inner;
}

const innerFn = outer();
innerFn(); 
// Hi i am outerVar
// Hi i am innerVar

closure

function makeFunction() {
  const name = 'TK';
  function displayName() {
    console.log(name);
  }
  return displayName;
};

Closure ()

// Closures
// In JavaScript, closure is one of the widely discussed and important concepts.
// A closure is a function that has access to the variable from another function’s scope which is accomplished by creating a function inside a function. As defined on MDN:
// “Closures are functions that refer to independent (free) variables. In other words, the function defined in the closure ‘remembers’ the environment in which it was created.”
// In JavaScript, closures are created every time a function is created, at function creation time. Most JavaScript developers use closure consciously or unconsciously — but knowing closure provides better control over the code when using them.
// Example:

function Spellname(name) {
var greet = "Hi, " + name + "!";
var sName = function() {
var welc = greet + " Good Morning!";
console.log(greet);
};
return sName;
}
var Myname = SpellName("Nishi");
Myname();  // Hi, Nishi. Good Morning!

// In the above example, the function sName() is closure; it has its own local scope (with variable welc) and also has access to the outer function’s scope. After the execution of Spellname(), the scope will not be destroyed and the function sName() will still have access to it.

what are closures

Closures are related and often confused with lambda functions.
The 2 concepts are wonderfully explained and distinguished 
in this StackOverflow answer :
https://stackoverflow.com/a/36878651/13574304

closure

var counter = (function() {  //exposed function references private state (the outer function’s scope / lexical environment) after outer returns. 
 var privateCounter = 0;
 function changeBy(val)   { privateCounter += val; }
 return {  increment: function() {changeBy(1); },
           decrement: function() {changeBy(-1);},
           value: function() {return privateCounter; }
  };
})();

counter.increment(); counter.increment();
counter.decrement();
counter.value();      //  1

closures

// Closures
//Almost mystical like feature that many developers fail to fully understand.
//We cannot create closures manually like how we create arrays and functions.
//Rather closures happen in certain  situations. We just need to recognize 
//those situations.

//Example:
const secureBooking = function () {
   let passengerCount = 0; //Variable of the secureBooking Function
   
  //returns a function
   return function () {
     passengerCount++; //Adding to the passengerCount
     console.log(passengerCount);
  };
 };

const book = secureBooking(); //capture that function
book();

//In the above example we have a function called secureBooking
//That function returns another function, which we stored in book variable
//We then call the book() function and it adds to the passengerCount.
//But you may be wondering? How can it add to the passengerCount
//if the secureBooking has finished executing, shouldn't it not exist?
//This works because all functions have access to the variable environment
// in which they were created in. Meaning since secureBooking created
// the function which we stored in book. The book function now has 
// access to the variable environment of secureBooking function.

Closure Example

	 function Counter() {
    var counter = 0;
alert("XXXXX");
function increaseCounter()
{
return counter +=1;
} 

return increaseCounter;
}

/***/
const counter = new Counter();
console.log(counter());
console.log(counter());


/*note that alert("XXXX") only executes once*/
/*think of counter() = new Counter() declaration as storing the value of ONE Counter function execution and 
the remaining times you use counter(), you reexecute only the returned function*/
/*use counter() instead of Counter() if you want alert("XXXX") to execute only once AND for you to be able to return an actual value otherwise you only console.log a function and alert executes multiple times*/

what is closure

outer = function() {
  var a = 1;
  var inner = function() {
    console.log(a);
  }
  return inner; // this returns a function
}

var fnc = outer(); // execute outer to get inner 
fnc();

closure

func main() {
  x := 0
  increment := func() int {
    x++
    return x
  }
  fmt.Println(increment())
  fmt.Println(increment())
}

closure

function add(a) {
     return function(b) {
         return a + b;
     };
 }

 let addTen = add(10);
 let addSeven = addTen(7);

 console.log(addSeven); // 17

closure

def say():
    greeting = 'Hello'

    def display():
        print(greeting)

    display()
Code language: Python (python)

closure examples

function makeFunc() {
  const name = 'Mozilla';
  function displayName() {
    console.log(name);
  }
  return displayName;
}

const myFunc = makeFunc();
myFunc();

closure example

const add = (function () {
  let counter = 0;
  return function () {counter += 1; return counter}
})();

add();
add();
add();

// the counter is now 3