undefined

BaselineWidely available

This feature is well established and works across many devices and browser versions. It’s been available across browsers since July 2015.

Theundefined global property represents the primitivevalueundefined. It is one of JavaScript'sprimitive 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 valueundefined.

Property attributes of`undefined`
Writable	no
Enumerable	no
Configurable	no

Description

undefined is a property of theglobal 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 typeundefined. Amethod or statement also returnsundefined if the variable that is beingevaluated does not have an assigned value. A function returnsundefined ifa value was notreturned.

Note:While you can useundefined as anidentifier (variable name) in any scope other than the global scope (becauseundefined is not areserved 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 useundefined and the strict equality and inequality operators todetermine whether a variable has a value. In the following code, the variablex is not initialized, and theif statement evaluates to true.

let x;if (x === undefined) {  // these statements execute} else {  // these statements do not execute}

Note:Thestrict equality operator (as opposed to thestandard equality operator) must be used here, becausex == undefined also checks whetherx isnull,while strict equality doesn't. This is becausenull is not equivalent toundefined.

SeeEquality 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 usetypeof is that it does not throw anerror if the variable has not been declared.

// x has not been declared before// evaluates to true without errorsif (typeof x === "undefined") {  // these statements execute}// Throws a ReferenceErrorif (x === undefined) {}

However, there is another alternative. JavaScript is a statically scoped language, soknowing if a variable is declared can be read by seeing whether it is declared in anenclosing context.

The global scope is bound to theglobal object, sochecking the existence of a variable in the global context can be done by checking theexistence of a property on theglobal object, using thein operator, for instance:

if ("x" in window) {  // These statements execute only if x is defined globally}

void operator and undefined

Thevoid operator is a third alternative.

let x;if (x === void 0) {  // these statements execute}// y has not been declared beforeif (y === void 0) {  // throws Uncaught ReferenceError: y is not defined}