Skip to content
Related Articles

Related Articles

Improve Article
Save Article
Like Article

JavaScript | Check the existence of variable

  • Difficulty Level : Easy
  • Last Updated : 30 Aug, 2021

JavaScript has a built-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. 
 

html




<!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. 
 

html




<!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. 
 

html




<!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 arrow_drop_up
Recommended Articles
Page :

Start Your Coding Journey Now!