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JavaScript undefined Property

❮ JavaScript Global Functions

Example

Test if a variable is undefined:

var x;

if (typeof x === "undefined") {
    txt = "x is undefined";
} else {
    txt = "x is defined";
}
Try it Yourself »

More "Try it Yourself" examples below.


Definition and Usage

The undefined property indicates that a variable has not been assigned a value, or not declared at all.


Browser Support

Property
undefined Yes Yes Yes Yes Yes


Technical Details

JavaScript Version: ECMAScript 1

More Examples

Example

Test if a not declared variable is undefined:

if (typeof y === "undefined") {
    txt = "y is undefined";
} else {
    txt = "y is defined";
}
Try it Yourself »

❮ JavaScript Global Functions