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How to override a JavaScript function ?

Last Updated : 12 Jul, 2025
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In this article, we are given an HTML document and the task is to override the function, either a predefined function or a user-defined function using JavaScript. 

Approach: When we run the script then Fun() function is called. After clicking the button the GFG_Fun() function is called and this function contains another function that will run. 

Example 1: In this example, a function written by the user is overridden. 

html 
<body style="text-align:center;">

    <h1 style="color:green;" id="h1">
        GeeksForGeeks
    </h1>

    <p id="GFG_UP" style="font-size: 15px; 
                          font-weight: bold;">
    </p>

    <button onclick="GFG_Fun()">
        click here
    </button>

    <p id="GFG_DOWN" style="color:green; 
                            font-size: 20px; 
                            font-weight: bold;">
    </p>

    <script>
        var up = document.getElementById('GFG_UP');    
        up.innerHTML = "Click on the button to override"
                    + " the function.";
        
        var down = document.getElementById('GFG_DOWN');
        
        function Fun() {
            return "This is from old function";
        }
        
        down.innerHTML = Fun();
        
        function GFG_Fun() {
            var newFun = Fun;
            Fun = function() {
                return "This is from overridden function";
            }
            down.innerHTML = Fun();
        }
    </script>
</body>

Output:

 

Example 2: In this example, a parseFloat method is overridden using the above-mentioned approach. 

html 
<body style="text-align:center;">

    <h1 style="color:green;" id="h1">
        GeeksForGeeks
    </h1>

    <p id="GFG_UP" style="font-size: 15px; 
                          font-weight: bold;">
    </p>

    <button onclick="GFG_Fun()">
        click here
    </button>

    <p id="GFG_DOWN" style="color:green; 
                            font-size: 20px; 
                            font-weight: bold;">
    </p>

    <script>
        var up = document.getElementById('GFG_UP');    
        
        up.innerHTML = "Click on the button to override"
                    + " the function.";
        
        var down = document.getElementById('GFG_DOWN');
        
        down.innerHTML = "Floor value of '2.345' is ";
        
        function GFG_Fun() {
            
            // Override
            parseFloat = function(x) {
                return "Floor value of '2.345' is "
                            + Math.floor(x);
            }
            
            // Overriding the parseFloat function and
            // use it as Math.floor
            down.innerHTML = parseFloat(2.345);
        }
    </script>
</body>

Output:

 

P
PranchalKatiyar
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