Before you learn about closures, you need to understand two concepts:

• Nested Function
• Returning a function

JavaScript Nested Function

In JavaScript, a function can also contain another function. This is called a nested function. For example,

// nested function example

// outer function
function greet(name) {

    // inner function
    function displayName() {
        console.log('Hi' + ' ' + name);
    }

    // calling inner function
    displayName();
}

// calling outer function
greet('John'); // Hi John

In the above program, the greet() function contains the displayName() function inside of it.

Returning a Function

In JavaScript, you can also return a function within a function. For example,

function greet(name) {
    function displayName() {
        console.log('Hi' + ' ' + name);
    }

    // returning a function
    return displayName;
}

let g1 = greet('John');
console.log(g1); // returns the function definition
g1(); // calling the function

Output

function displayName() {
      console.log('Hi' + ' ' + name);
  }
Hi John

In the above program, the greet() function is returning the displayName function definition.

Here, 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 parenthesis.

JavaScript Closures

In JavaScript, closure provides access to the outer scope of a function from inside the inner function, even after the outer function has closed. For example,

// javascript closure example

// outer function
function greet() {

    // variable defined outside the inner function
    let name = 'John';

    // inner function
    function displayName() {

        // accessing name variable
        return 'Hi' + ' ' + name;
      
    }

    return displayName;
}

let g1 = greet();
console.log(g1); // returns the function definition
console.log(g1()); // returns the value

Output

function displayName() {
      // accessing name variable
      return 'Hi' + ' ' + name;
  }
Hi John

In the above example, when greet() function is called, it returns the function definition of displayName.

Here, g1 is a reference to the displayName function.

When g1() is called, it still has access to the greet() function.

When we run console.log(g1), it returns the function definition.

The concept of closure exists for other programming languages like Python, Swift, Ruby, etc.

Let’s have a look at another example.

// closure example

function calculate(x) {
    function multiply(y) {
        return x * y;
    }
    return multiply;
}

let multiply3 = calculate(3);
let multiply4 = calculate(4);

console.log(multiply3); // returns calculate function definition
console.log(multiply3()); // NaN

console.log(multiply3(6)); // 18
console.log(multiply4(2)); // 8

In the above program, the calculate() function takes a single argument x and returns the function definition of the multiply function. The multiply() function takes a single argument y and returns x * y.

Both multiply3 and multiply4 are closures.

The calculate() function is called passing a parameter x. When multiply3 and multiply4 are called, the multipy() function has access to the passed x argument of the outer calculate() function.

Data Privacy

JavaScript closure helps in the data privacy of the program. For example,

let a = 0;
function sum() {
    function increaseSum() {

        // the value of a is increased by 1
        return a = a + 1;
    }
    return increaseSum;
}

let x = sum();
console.log(x()); // 1
console.log(x()); // 2
console.log(x()); // 3
a = a + 1;
console.log(a); // 4

In the above example, the sum() function returns the function definition of the increaseSum function.

The a variable is increased inside the increaseSum() function. However, the value of the a variable can also be changed outside of the function. In this case, a = a + 1; changes the value of the variable outside the function.

Now, if you want the a variable to be increased only inside the function, you can use a closure. For example,

function sum() {
    let a = 0;
    function increaseSum() {

        // the value of a is increased by 1
        return a = a + 1;
    }
    return increaseSum;
}

let x = sum();
let a = 5;
console.log(x()); // 1
console.log(x()); // 2
console.log(a); // 5

In the above example, the sum() function sets the value of a to 0 and returns the increaseSum function.

Because of the closure, even though sum() is already executed, increaseSum() still has access to a and can add 1 to a every time x() is called.

And the a variable is private to the sum() function. It means that the a variable can only be accessed inside of the sum() function.

Even if you declare a and use it, it does not affect the a variable inside of the sum() function.

Note: Generally, closures are used for data privacy.