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.

<!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>                     

Output:

Before clicking the button:
After clicking the button:

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

<!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>                     

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.

<!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>                     

Output:

Before clicking the button:
After clicking the button:

My Personal Notes

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