JavaScript | Check the existence of variable

JavaScript has a bulit-in function to check whether a variable is defined/initialized or undefined.

Note:

  • The typeof operator will check whether a variable is defined or not.
  • The typeof operator doesn’t throw a ReferenceError exception when it is used with an undeclared variable.
  • The typeof null will return an object. So, check for null also.

Example 1: This example checks whether a variable is defined or not.



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<!DOCTYPE html> 
<html> 
  
<head> 
    <title> 
        JavaScript to check existence of variable
    </title>
</head> 
          
<body style = "text-align:center;"> 
      
    <h1 style = "color:green;" > 
        GeeksForGeeks 
    </h1> 
              
    <p>
        variable-name : GFG_Var
    </p>
  
    <button onclick="myGeeks()"> 
        Check
    </button> 
      
    <h3 id = "div" style="color:green;"></h3>
              
    <!-- Script to check existence of variable -->
    <script> 
        function myGeeks() {
              
            var GFG_Var;
            var h3 = document.getElementById("div");
              
            if (typeof GFG_Var === 'undefined') {
                h3.innerHTML = "Variable is Undefined";
            }
            else {
                h3.innerHTML = "Variable is defined and"
                    + " value is " + GFG_Var;
            }
        }
    </script> 
</body> 
  
</html>                    

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Output:


    

  • Before clicking the button:
    

    • 

  • After clicking the button:
    

    • 



Example 2: This example also checks whether a variable is defined or not.











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<!DOCTYPE html>  


<html>  


  


<head>  


    <title>  


        JavaScript to check existence of variable 


    </title> 


</head>  


          


<body style = "text-align:center;">  


      


    <h1 style = "color:green;" >  


        GeeksForGeeks  


    </h1>  


              


    <p> 


        variable-name : GFG_Var 


    </p> 


  


    <button onclick="myGeeks()">  


        Check 


    </button>  


      


    <h3 id = "div" style="color:green;"></h3> 


              


    <!-- Script to check existence of variable -->


    <script>  


        function myGeeks() { 


          


            var GFG_Var = "GFG"; 


            var h3 = document.getElementById("div"); 


              


            if (typeof GFG_Var === 'undefined') { 


                h3.innerHTML = "Variable is Undefined"; 


            } 


            else { 


                h3.innerHTML = "Variable is defined and" 


                    + " value is " + GFG_Var; 


            } 


        } 


    </script>  


</body>  


  


</html>                     



















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Output:


    

  • Before clicking the button:
    

    • 

  • After clicking the button:
    

    • 



Example 3: The previous example doesn’t check for null value of variable. This example also checks a variable is null or not.











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<!DOCTYPE html>  


<html>  


  


<head>  


    <title>  


        JavaScript to check existence of variable 


    </title> 


</head>  


          


<body style = "text-align:center;">  


      


    <h1 style = "color:green;" >  


        GeeksForGeeks  


    </h1>  


              


    <p> 


        variable-name : GFG_Var 


    </p> 


  


    <button onclick="myGeeks()">  


        Check 


    </button>  


      


    <h3 id = "div" style="color:green;"></h3> 


              


    <!-- Script to check existence of variable -->


    <script>  


        function myGeeks() { 


       


            var GFG_Var = null; 


            var h3 = document.getElementById("div"); 


              


            if (typeof GFG_Var === 'undefined' || GFG_Var === null ) { 


                h3.innerHTML = "Variable is Undefined"; 


            } 


            else { 


                h3.innerHTML = "Variable is defined and value is " 


                        + GFG_Var; 


            } 


        } 


    </script>  


</body>  


  


</html>                     



















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Output:


    

  • Before clicking the button:
    

    • 

  • After clicking the button:
    

    • 




          
          
          
          

            

    

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