JavaScript Nested functions

In JavaScript, Functions within another function are called “Nested function.” These nested functions have access to the variables and parameters of the outer (enclosing) function, creating a scope hierarchy. A function can have one or more inner functions. 

Syntax:

// Outer function
function outerFunction() {
    // Nested function
    function nestedFunction() {
        // Function logic here
    }
    // Call the nested function
    nestedFunction();
    // Rest of the outer function logic
}
// Call the outer function
outerFunction();

Approach:

• Write one function inside another function.
• Make a call to the inner function in the return statement of the outer function.
• Call it fun(a)(b) where a is a parameter to the outer and b is to the inner function.
• Finally, return the combined output from the nested function.

Example 1: This example uses the approach discussed above. 

Javascript

function fun1(a) {
    function fun2(b) {
        return a + b;
    }
    return fun2;
}
function GFG_Fun() {
    console.log(fun1("A Online Computer Science Portal")
        (" GeeksforGeeks"));
}
GFG_Fun()

Output

A Online Computer Science Portal GeeksforGeeks

Example 2: This example uses the approach discussed above, but here the nested function is created differently than the previous one. 

Javascript

function fun1(a) {
    fun = function fun2(b) {
        return a + b;
    }
    return fun;
}
function GFG_Fun() {
    console.log(fun1("This is ")("GeeksforGeeks"));
}
GFG_Fun()

Output

This is GeeksforGeeks

Supported Browser: 

• Google Chrome
• Edge
• Firefox
• Opera
• Safari

JavaScript is best known for web page development but it is also used in a variety of non-browser environments. You can learn JavaScript from the ground up by following this JavaScript Tutorial and JavaScript Examples.