undefined is a property of the global object. That is, it is a variable in global scope. The initial value of undefined is the primitive value undefined.

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.

//  DON'T DO THIS

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

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

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
if (typeof x === 'undefined') { //  evaluates to true without errors
  //  these statements execute
}

if (x === undefined) { //  throws a ReferenceError

}

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
}

• JavaScript's primitive types
• null