Javascript object undefined value

How to Check if a JavaScript Variable is Undefined

In JavaScript, checking if a variable is undefined can be a bit tricky since a null variable can pass a check for undefined if not written properly. As a result, this allows for undefined values to slip through and vice versa. Make sure you use strict equality === to check if a value is equal to undefined .

let x; const y = null; x == null && y == undefined; // true x === null || y === undefined; // false x == null && typeof x == 'object'; // false x === undefined && typeof x == 'undefined'; // true

Another alternative is checking if typeof x === ‘undefined’ . The biggest difference between these two approaches is that, if x has not been declared, x === undefined throws a ReferenceError , but typeof does not.

When using x === undefined , JavaScript checks if x is a declared variable that is strictly equal to undefined . If you want to check if x is strictly equal to undefined regardless of whether is has been declared or not, you should use typeof x === ‘undefined’ .

 x === undefined; // Throws a ReferenceError typeof x == 'undefined'; // true

However, typeof x may still throw an error if you later declare x using let or const :

typeof x; // Throws "ReferenceError: Cannot access 'x' before initialization" because of `let` let x = 42;

Checking Whether Object Properties are Undefined

Checking whether an object property is undefined is subtle, because if you access a property that doesn’t exist in the object, JavaScript will report the property’s value as undefined rather than throw a ReferenceError .

const obj = < answer: 42, question: undefined >; obj.answer; // 42 obj.question; // undefined obj.notInObject; // undefined

In JavaScript obj.propName === undefined is true if either obj has a property ‘propName’ and the property’s value is trictly equal undefined, or if obj does not have a property ‘propName’. If you want to check whether obj has a property and that property is strictly equal to undefined , you should use the in operator.

const obj = < answer: 42, question: undefined >; ('question' in obj && obj.question === undefined); // true ('notInObject' in obj && obj.notInObj === undefined); // false

More Fundamentals Tutorials

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undefined

The undefined global property represents the primitive value undefined . It is one of JavaScript’s primitive types.

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Value

Property attributes of undefined
Writable	no
Enumerable	no
Configurable	no

Description

undefined is a property of the global object. That is, it is a variable in global scope.

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 .

Note: While you can use undefined 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 (() =>  const undefined = "foo"; console.log(undefined, typeof undefined); // foo string >)(); ((undefined) =>  console.log(undefined, typeof undefined); // foo string >)("foo"); 

Examples

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 initialized, and the if statement evaluates to true.

let x; if (x === undefined)  // these statements execute > else  // these statements do not execute > 

Note: The strict equality operator (as opposed to the standard equality operator) must be used here, because x == undefined also checks whether x is null , while strict equality doesn’t. This is because null is not equivalent to undefined .

typeof operator and undefined

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 // evaluates to true without errors if (typeof x === "undefined")  // these statements execute > // Throws a ReferenceError if (x === undefined)  > 

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 > 

void operator and undefined

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 > 

Specifications

Browser compatibility

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See also

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This page was last modified on Apr 28, 2023 by MDN contributors.

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