JavaScript undefined

Variable with no value:

let x;
if (x === undefined) {
text = "x is undefined";
} else {
text = "x is defined";
}

let x;
if (typeof x === "undefined") {
text = "x is undefined";
} else {
text = "x is defined";
}

More examples below.

Definition and Usage

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

undefined() is an ECMAScript1 (ES1) feature.

ES1 (JavaScript 1997) is fully supported in all browsers:


Chrome	Edge	Firefox	Safari	Opera	IE
Yes	Yes	Yes	Yes	Yes	Yes

Variable not declared:

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