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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code

<!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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