Closure function in JS

#javascript #beginners #programming #tutorial

JavaScript Nested Function :

A function can also contain another function. This is called nested function.

function greet(name) {

        function displayName() {
                console.log("Hi " + name)
        }

        displayName();

}

greet("Varun");

Output :
Hi Varun

Returning a function :

In JavaScript, it is allowed to return a function within a function.

function greet(name) {

        function displayName() {
                console.log("Hi " + name)
        }

        return displayName; //returning a function

}

const g1 = greet("Varun");
console.log(g1);
g1();

Output :
displayName() {
                console.log("Hi " + name)
        }
Hi Varun

In the above program, the greet() function is returning the displayName function definition .
The returned function definition is assigned to the g1 variable. When you print g1 using console.log(g1), you will get the function definition.
To call the function stored in the g1 variable, we use g1() with parentheses.

What is a Closure?

A closure is a function that remembers the variables from its outer scope even after that outer scope has finished executing.

function outer(){
        let count = 0; // This variable lives in outer scope

        function inner(){
                count++
                console.log(count);
        }
        return inner
}

const counter1 = outer(); // outer() finishes, but count isn't gone!
counter1();
counter1();
counter1();

const counter2 = outer();
counter2();
counter2();
counter2();

Output :
1
2
3
1
2
3

outer() runs once.
But count is not destroyed.
Because inner() remembers it.
That memory = closure.
Even though outer() has returned, inner still holds a live reference to count. That's a closure.

function bank(name,totalAmt)
{
        return {
                deposit : function(amount){
                        totalAmt = totalAmt+amount;
                        return totalAmt
                },
                withdraw : function(amount){
                        totalAmt = totalAmt-amount;
                        return totalAmt
                },
                checkbal : function(){
                        console.log(`Hi ${name}, Your total amount is ${totalAmt}`);
                }       
        }


}

const varunacc = bank("Varun",5000)
console.log(varunacc.deposit(1000));
console.log(varunacc.withdraw(500));
varunacc.checkbal();

Output :
6000
5500
Hi Varun, Your total amount is 5500