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undefined

Baseline Widely available

This feature is well established and works across many devices and browser versions. Its been available across browsers since 2015 7.

undefined undefined , JavaScript .

In this article

function test(t) {
  if (t === undefined) {
    return "Undefined value!";
  }
  return t;
}

let x;

console.log(test(x));
// Expected output: "Undefined value!"

undefined .

Property attributes of undefined

undefined . , .

undefined , . .

undefined . undefined . undefined .

: ( ) undefined (undefined ), . .

js
// DON'T DO THIS

(() => {
  const undefined = "foo";
  console.log(undefined, typeof undefined); // foo string
})();

((undefined) => {
  console.log(undefined, typeof undefined); // foo string
})("foo");

undefined

undefined , . x , if .

js
let x;
if (x === undefined) {
  //   
} else {
  //    
}

: : x == undefined x null , ( ) . , . null undefined .

Equality comparison and sameness .

typeof undefined

typeof .

js
let x;
if (typeof x === "undefined") {
  //   
}

typeof .

js
// x   
//   true 
if (typeof x === "undefined") {
  //   
}

// ReferenceError 
if (x === undefined) {
}

. JavaScript , .

, in . ,

js
if ("x" in window) {
  // x      
}

void undefined

void .

js
let x;
if (x === void 0) {
  //   
}

// y   
if (y === void 0) {
  // throws Uncaught ReferenceError: y is not defined
}

Specification
ECMAScript 2027 LanguageSpecification
# sec-undefined

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