undefined
Baseline
Widely available
This feature is well established and works across many devices and browser versions. It’s been available across browsers since July 2015.
The undefined global property represents the primitive
value undefined. It is one of JavaScript's
primitive types.
Try it
function test(t) {
if (t === undefined) {
return "Undefined value!";
}
return t;
}
let x;
console.log(test(x));
// Expected output: "Undefined value!"
Value
The primitive value undefined.
Property attributes of undefined | |
|---|---|
| Writable | no |
| Enumerable | no |
| Configurable | no |
Description
undefined is a property of the global object. That is, it is a variable in global scope.
In all non-legacy browsers, undefined is a non-configurable, non-writable property. Even when this is not the case, avoid overriding it.
A variable that has not been assigned a value is of type undefined. A
method or statement also returns undefined if the variable that is being
evaluated does not have an assigned value. A function returns undefined if
a value was not returned.
Note:
While you can use undefined as an identifier (variable name) in any scope other than the global scope (because undefined is not a reserved word), doing so is a very bad idea that will make your code difficult to maintain and debug.
// DON'T DO THIS
(() => {
const undefined = "foo";
console.log(undefined, typeof undefined); // foo string
})();
((undefined) => {
console.log(undefined, typeof undefined); // foo string
})("foo");
Examples
Strict equality and undefined
You can use undefined and the strict equality and inequality operators to
determine whether a variable has a value. In the following code, the variable
x is not initialized, and the if statement evaluates to true.
let x;
if (x === undefined) {
// these statements execute
} else {
// these statements do not execute
}
Note:
The strict equality operator (as opposed to the
standard equality operator) must be used here, because
x == undefined also checks whether x is null,
while strict equality doesn't. This is because null is not equivalent to
undefined.
See Equality comparison and sameness for details.
typeof operator and undefined
Alternatively, typeof can be used:
let x;
if (typeof x === "undefined") {
// these statements execute
}
One reason to use typeof is that it does not throw an
error if the variable has not been declared.
// x has not been declared before
// evaluates to true without errors
if (typeof x === "undefined") {
// these statements execute
}
// Throws a ReferenceError
if (x === undefined) {
}
However, there is another alternative. JavaScript is a statically scoped language, so knowing if a variable is declared can be read by seeing whether it is declared in an enclosing context.
The global scope is bound to the global object, so
checking the existence of a variable in the global context can be done by checking the
existence of a property on the global object, using the
in operator, for instance:
if ("x" in window) {
// These statements execute only if x is defined globally
}
void operator and undefined
The void operator is a third alternative.
let x;
if (x === void 0) {
// these statements execute
}
// y has not been declared before
if (y === void 0) {
// throws Uncaught ReferenceError: y is not defined
}
Specifications
| Specification |
|---|
| ECMAScript® 2026 Language Specification # sec-undefined |
Browser compatibility
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