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Override a JavaScript function

Last Updated : 03 Nov, 2025
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In JavaScript, function overriding means redefining an existing function with a new implementation. It allows developers to change or extend the behaviour of a user-defined or built-in function during runtime.

  • You can override both user-defined and built-in JavaScript functions.
  • The new function replaces the old one with the same name.
  • Useful for customizing existing logic or debugging without modifying original code.
  • Overriding built-in functions (like alert() or parseFloat()) should be done with caution.
  • The overridden function remains in effect for the entire scope where it’s defined.

[Approach 1]: Overriding a User-Defined Function

  • You can redefine an existing function by assigning a new function with the same name.
  • The new function replaces the old one during runtime.
  • Useful when you want to update the behaviour of your own functions.
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>

[Approach 2]: Overriding a Built-in Function

  • JavaScript allows modifying built-in functions (like parseFloat, alert, etc.).
  • You can assign a new implementation to these functions, but it’s not recommended for production code.
  • Commonly done for debugging, testing, or customization.
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>



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PranchalKatiyar
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