How to check a function is defined in JavaScript ?

Last Updated : 26 Dec, 2022

In this article, 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 a 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 a function then it is defined otherwise not defined by using typeof operator.

html

<body style="text-align:center;"> 
  
    <h1 style="color:green;"> 
        GeeksForGeeks 
    </h1> 
    <h3>How to check a function is defined</h3> 
  
    <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>

Output:

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

html

<body style="text-align:center;"> 
  
    <h1 style="color:green;"> 
        GeeksForGeeks 
    </h1> 
    <h3>How to check a function is defined</h3> 
  
    <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>