JavaScript Object Properties

JavaScript undefined Property

Last Updated : 08 Feb, 2023

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:

Below are examples of the undefined Property.

Example 1:

JavaScript

var a ="Geeksforgeeks"
function test() {
    if (typeof a === "undefined") {
        txt = "'a' is undefined";
    } else {
        txt = "'a' is defined";
    }
console.log(txt);
}
test();

Output:

'a' is defined

Example 2:

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>

Output:

Supported Browsers:

Google Chrome
Firefox
Internet Explorer
Opera
Safari

JavaScript Object Properties

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