How to check a function is defined in JavaScript ?

The job is to identify whether a function is defined or not. The JavaScript typeof operator is used to solve the problem described below:

Javascript typeof Operator: This operator can be used to find the type of a JavaScript variable. This operator returns the type of a variable or an expression:

Syntax: 



typeof var

Parameter: It contains single value var which is a Javascript variable.

Return value: It returns the type of a variable or an expression:

Example 1: This example checks the type of the function, If it is function then it is defined otherwise not defined by using typeof operator.

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<!DOCTYPE HTML> 
<html> 
    <head> 
        <title> 
            How to check a function is defined
        </title>
    </head> 
      
    <body style = "text-align:center;"> 
      
        <h1 style = "color:green;" > 
            GeeksForGeeks 
        </h1> 
          
        <p id = "GFG_UP" style = "font-size: 16px; font-weight: bold;">
        </p>
          
        <button onclick = "gfg_Run()"> 
            Click here
        </button>
          
        <p id = "GFG_DOWN" style = 
            "color:green; font-size: 20px; font-weight: bold;">
        </p>
          
        <script>
            var el_up = document.getElementById("GFG_UP");
            var el_down = document.getElementById("GFG_DOWN");
              
            el_up.innerHTML = 
                "Function named 'fun' which is not defined";
                  
            function gfg_Run() {
                var defined = 'Not defined';
                if(typeof fun == 'function') {
                    defined = "Defined";
                }
                el_down.innerHTML = defined;
            }         
        </script> 
    </body> 
</html>                    

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Output:


    

  • 
Before clicking on the button:
    


    • 

  • 
After clicking on the button:
    


    • 



Example 2: This example checks the type of the function, If it is function then it is defined otherwise not defined by using typeof operator by creating a function.











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<!DOCTYPE HTML>  


<html>  


    <head>  


        <title>  


            How to check a function is defined 


        </title> 


    </head>  


      


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


      


        <h1 style = "color:green;" >  


            GeeksForGeeks  


        </h1>  


          


        <p id = "GFG_UP" style = "font-size: 16px; font-weight: bold;"> 


        </p> 


          


        <button onclick = "gfg_Run()">  


            Click here 


        </button> 


          


        <p id = "GFG_DOWN" style =  


            "color:green; font-size: 20px; font-weight: bold;"> 


        </p> 


          


        <script> 


            var el_up = document.getElementById("GFG_UP"); 


            var el_down = document.getElementById("GFG_DOWN"); 


              


            el_up.innerHTML = "Function named 'fun' which is defined"; 


              


            function isFunction(possibleFunction) { 


                return typeof(possibleFunction) === typeof(Function); 


            } 


              


            function fun() { 


                  


            } 


              


            function gfg_Run() { 


                var defined = 'Not defined'; 


                if(isFunction(fun)) { 


                    defined = "Defined"; 


                } 


                el_down.innerHTML = defined; 


            }          


        </script>  


    </body>  


</html>                     



















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Output: