undefined
The undefined
global property represents the primitive
value undefined
. It is one of JavaScript's
primitive types.
Try it
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 Language Specification # sec-undefined |
Browser compatibility
BCD tables only load in the browser