--- title: undefined slug: Web/JavaScript/Reference/Global_Objects/undefined tags: - JavaScript translation_of: Web/JavaScript/Reference/Global_Objects/undefined ---
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The global undefined property represents the primitive value {{Glossary("Undefined", "undefined")}}. It is one of JavaScript's {{Glossary("Primitive", "primitive types")}}.

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語法

undefined

描述

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 {{Glossary("Undefined", "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 {{jsxref("Statements/return", "returned")}}.

While it is possible to use it as an {{Glossary("Identifier", "identifier")}} (variable name) in any scope other than the global scope (because undefined is not a {{jsxref("Reserved_Words", "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');

範例

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
}

備註：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 {{jsxref("Operators/Comparison_Operators", "comparison operators")}} for details.

Typeof operator and undefined

Alternatively, {{jsxref("Operators/typeof", "typeof")}} can be used:

var x;
if (typeof x === 'undefined') {
   // these statements execute
}

One reason to use {{jsxref("Operators/typeof", "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 {{jsxref("Operators/in", "in")}} operator, for instance).

Void operator and undefined

The {{jsxref("Operators/void", "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
}

規範

Specification Status Comment
{{SpecName('ES1', '#sec-4.3.9', 'undefined')}} {{Spec2('ES1')}} Initial definition. Implemented in JavaScript 1.3.
{{SpecName('ES5.1', '#sec-15.1.1.3', 'undefined')}} {{Spec2('ES5.1')}}  
{{SpecName('ES6', '#sec-undefined', 'undefined')}} {{Spec2('ES6')}}  
{{SpecName('ESDraft', '#sec-undefined', 'undefined')}} {{Spec2('ESDraft')}}  

瀏覽器相容性

{{Compat("javascript.builtins.undefined")}}