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undefined - JavaScript | MDN

Baseline Widely available

The undefined global property represents the primitive value undefined. It is one of JavaScript's primitive types.

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

let x;

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

The primitive value undefined.

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");

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.

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
}

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
}

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