undefined

Глобальна властивість undefined представляє просте значення undefined. Це один з простих типів JavaScript.

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undefined

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

In modern browsers (JavaScript 1.8.5 / Firefox 4+), undefined is a non-configurable, non-writable property per the ECMAScript 5 specification. 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"
(function() { var undefined = 'foo'; console.log(undefined, typeof undefined); })();

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

Виконавши строге порівняння змінної із значенням undefined можна перевірити, чи змінна має значення. У прикладі нижче змінна x не ініціалізована, тому умовний операторif буде оцінено як істина (true)

var x;
if (x === undefined) {
   // код у цьому блоці буде виконано
}
else {
   // код у цьому блоку не буде виконано
}

Alternatively, typeof can be used:

var 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, this kind of technique should be avoided. 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 only exception is the global scope, but 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).

The void operator is a third alternative.

var x;
if (x === void 0) {
   // these statements execute
}

// y has not been declared before
if (y === void 0) {
   // throws a - Uncaught ReferenceError: y is not defined
}