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How to create nested functions in Javascript?

Nested functions in JavaScript.

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

            Nested functions in JavaScript.
    <body id = "body" style = "text-align:center;"> 
        <h1 style = "color:green;" > 
        <p id = "GFG_UP" style = 
            "font-size: 15px; font-weight: bold;"> 
        <button onclick = "GFG_Fun()">
        click here
        <p id = "GFG_DOWN" 
           style = "font-size: 24px; 
                    font-weight: bold; 
                    color: green;"> 
            var up = document.getElementById('GFG_UP');
            var down = document.getElementById('GFG_DOWN');
            up.innerHTML = 
               "Click on the button to call nested function.";     
            function fun1(a) { 
            function fun2(b) { 
                return a + b;
            return fun2;
            function GFG_Fun() {
                down.innerHTML = 
                      fun1("A Online Computer Science Portal")
                      (" GeeksforGeeks");


Before clicking on the button:

After clicking on the button:


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