undefined
Глобальна властивість undefined
представляє просте значення undefined
. Це один з простих типів JavaScript.
Атрибути поля undefined |
|
---|---|
Доступний для запису | ні |
Доступний для переліку | ні |
Доступний для налаштування | ні |
The source for this interactive example is stored in a GitHub repository. If you'd like to contribute to the interactive examples project, please clone https://github.com/mdn/interactive-examples and send us a pull request.
Синтаксис
undefined
Опис
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
.
While it is possible to use it 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
// logs "foo string"
(function() { var undefined = 'foo'; console.log(undefined, typeof undefined); })();
// logs "foo string"
(function(undefined) { console.log(undefined, typeof undefined); })('foo');
Приклади
Сувора рівність та undefined
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
}
Note: The strict equality operator rather than the standard equality operator must be used here, because x == undefined
also checks whether x
is null
, while strict equality doesn't. null
is not equivalent to undefined
. See comparison operators
for details.
Typeof
operator and undefined
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).
Void
operator and undefined
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
}
Специфікації
Специфікація | Статус | Коментар |
---|---|---|
ECMAScript 1st Edition (ECMA-262) The definition of 'undefined' in that specification. |
Standard | Первинне визначення. Реалізовано у JavaScript 1.3. |
ECMAScript 5.1 (ECMA-262) The definition of 'undefined' in that specification. |
Standard | |
ECMAScript 2015 (6th Edition, ECMA-262) The definition of 'undefined' in that specification. |
Standard | |
ECMAScript (ECMA-262) The definition of 'undefined' in that specification. |
Living Standard |
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