JavaScript undefined
Examples
Variable with no value:
let x;
if (x === undefined) {
text = "x is undefined";
} else {
text = "x is defined";
}
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let x;
if (typeof x === "undefined") {
text = "x is undefined";
} else {
text = "x is defined";
}
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More examples below.
Description
The undefined property indicates that a variable has not been assigned a value, or not declared at all.
Browser Support
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 |
More Examples
Example
Variable not declared:
if (typeof y === "undefined") {
txt = "y is undefined";
} else {
txt = "y is defined";
}
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