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undefined მნიშვნელობა არის კიდევ ერთი სტანდარტული მნიშვნელობა რომელიც გენერირდება განუსაზღვრელი ცვლადის პროგრამაში გამოყენების შემდეგ

undefined ეს არის javascript-ის  primitive types.

Property attributes of undefined
Writable no
Enumerable no
Configurable no

სინტაქსი

undefined

განმარტება

undefined არის პრიმიტიული მნისვნელობა რომელიც ენიჭებათ განუსაზღვრელ ცვლადებს სხვა დაპროგრამების ენებში განუსაზღვრელი ცვლადების პროგრამაში გამოყენების შემდეგ გენერირდება შეცდომა, ხოლო javascript-ის შემთხვევაში განუსაზღვრელ ცვლადებს ავტომატურად ენიჭებათ undefined მნიშვნელობა 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.

ცვლადებს რომელთაც არ აქვთ მნიშვნელობა მინიჭებული ასევე ფუნქციები და კონსტრუქტორები რომელთაც არ აქვთ ტანი  არ არიან განსაზღვრული ენიჭებათ undefined მნიშვნელობა 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');

მაგალითი

Strict equality and undefined

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 defined, and the if statement evaluates to true.

var x;
if (x === undefined) {
   // these statements execute
}
else {
   // these statements do not execute
}

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 ReferenceError (in contrast to `typeof`)
}

Specifications

Specification Status Comment
ECMAScript 1st Edition (ECMA-262)
The definition of 'undefined' in that specification.
Standard Initial definition. Implemented in 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 2017 Draft (ECMA-262)
The definition of 'undefined' in that specification.
Draft  

Browser compatibility

Feature Chrome Firefox (Gecko) Internet Explorer Opera Safari
Basic support (Yes) (Yes) (Yes) (Yes) (Yes)
Feature Android Chrome for Android Firefox Mobile (Gecko) IE Mobile Opera Mobile Safari Mobile
Basic support (Yes) (Yes) (Yes) (Yes) (Yes) (Yes)

Document Tags and Contributors

 Contributors to this page: Gio_rgi
 Last updated by: Gio_rgi,