Live Demo: Determine If a Variable Exists in JavaScript

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<html lang="en">
<head>
<meta charset="utf-8">
<title>Determine If a Variable Exists in JavaScript</title>
</head>
<body>
    <p><strong>Note:</strong> Press F12 key on the keyboard and see the console tab for errors. Click the "Show Output" button or refresh the page to run the test again.</p>
    <hr>
	<script>
        var x;
        var y = 10;
        
        if(typeof x !== 'undefined'){
            // this statement will not execute
            document.write("Variable x is defined.");
        }
        
        if(typeof y !== 'undefined'){
            // this statement will execute
            document.write("Variable y is defined.");
        }
        
        // Attempt to access an undeclared z variable
        if(typeof z !== 'undefined'){
            // this statement will not execute
            document.write("Variable z is defined.");
        }
        
        /* Throws Uncaught ReferenceError: z is not defined,
        and halt the execution of the script */
        if(z !== 'undefined'){
            // this statement will not execute
            document.write("Variable z is defined.");
        }
        
        /* If the following statement runs, it will also
        throw the Uncaught ReferenceError: z is not defined */
        if(z){
            // this statement will not execute
            document.write("Variable z is defined.");
        }
    </script>
</body>
</html>