JavaScript | undefined Property

This 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.

Example:

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code

<!DOCTYPE html>
<html>
  
<body>
    <center>
        <h1 style="color: green">
          Geeks
      </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>

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Output:
Before clicking on the button:

After clicking on the button:

Browser Support: The listed browsers support JavaScript Undefined Property-

  • Google Chrome
  • Firefox
  • Internet Explorer
  • Opera
  • Safari


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