JavaScript undefined Property

Below is the example of the undefined Property.

Example:

<script> 
    var a ="Geeksforgeeks"
    function test() { 
        if (typeof a === "undefined") { 
            txt = "'a' is undefined"; 
        } else { 
            txt = "'a' is defined"; 
        } 
        document.write(txt); 
    } 
    test(); 
</script> 

Output:
```
'a' is defined
```

The undefined property is used to check if a value is assigned to a variable or not.

Syntax:

var x;
if (typeof x === "undefined") {
  txt = "x is undefined";
} else {
  txt = "x is defined";
}

Return Value: It returns ‘defined’ if the variable is assigned any value and ‘undefined’ if the variable is not assigned any value.

More example code for the above property are as follows:
Program 1:

<!DOCTYPE html> 
<html> 
  
<body> 
    <center> 
        <h1 style="color: green"> 
        GeeksforGeeks 
    </h1> 
        <button onclick="test()"> 
        Press 
    </button> 
        <h4> 
        Click on the Press button to  
        check if "a" is defined or undefined. 
    </h4> 
        <p id="gfg"></p> 
        <script> 
            function test() { 
                if (typeof a === "undefined") { 
                    txt = "'a' is undefined"; 
                } else { 
                    txt = "'a' is defined"; 
                } 
                document.getElementById( 
                "gfg").innerHTML = txt; 
            } 
        </script> 
    </center> 
</body> 
  
</html> 

Output:

Supported Browsers:

Google Chrome
Firefox
Internet Explorer
Opera
Safari

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