How to override a JavaScript function ?

Given an HTML document and the task is to override the function, either predefined function or user-defined function using JavaScript.

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

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



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<!DOCTYPE HTML> 
<html> 
  
<head> 
    <title> 
        How to override a JavaScript function ?
    </title>
</head>
  
<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> 
  
</html>

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Output:


    

  • 
Before clicking on the button:
    


    • 

  • 
After clicking on the button:
    


    • 



Example 2: In this example, a parseFloat method is override.











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<!DOCTYPE HTML>  


<html>  


  


<head>  


    <title>  


        How to override a JavaScript function ? 


    </title> 


</head> 


  


<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>  


  


</html> 



















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Output:


    

  • Before Click the Button:
    

    • 

  • After Click the Button:
    

    • 




          
          
          
          

            

    

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