typeof is safer as it allows the identifier to never have been declared before:
if(typeof neverDeclared === "undefined") // no errors
if(neverDeclared === null) // throws ReferenceError: neverDeclared is not defined
GeeksforGeeks
geeksforgeeks.org › javascript › variable-undefined-vs-typeof-variable-undefined-in-javascript
variable === undefined vs. typeof ...
July 12, 2025 - The type of operator does not throw an error if the variable has not been declared. Example: Here we assign two variables one is undefined and the other one is defined as "null", here null is not undefined when you put console.log it will show "null" if you check typeof then it will display the object, below program will illustrate the approach more clearly.
Top answer 1 of 8
775
typeof is safer as it allows the identifier to never have been declared before:
if(typeof neverDeclared === "undefined") // no errors
if(neverDeclared === null) // throws ReferenceError: neverDeclared is not defined
2 of 8
54
If the variable is declared (either with the var keyword, as a function argument, or as a global variable), I think the best way to do it is:
if (my_variable === undefined)
jQuery does it, so it's good enough for me :-)
Otherwise, you'll have to use typeof to avoid a ReferenceError.
If you expect undefined to be redefined, you could wrap your code like this:
(function(undefined){
// undefined is now what it's supposed to be
})();
Or obtain it via the void operator:
const undefined = void 0;
// also safe
Videos
05:39
Null vs Undefined - which one should you use in JS? - YouTube
02:15
What Is The 'undefined' Data Type In JavaScript? - JavaScript Toolkit ...
09:39
JavaScript Null & Undefined – Explained Like Never Before - YouTube
05:53
How to Check for Undefined or Null in JavaScript (2025) - YouTube
06:43
Undefined data type in JavaScript | Part 11 | JavaScript | #SmartCode ...
04:52
Solving your “undefined” problem with well-placed errors ...
GeeksforGeeks
geeksforgeeks.org › javascript › undefined-in-javascript
Undefined in JavaScript - GeeksforGeeks
March 13, 2024 - Undefined is a type of Data type in JavaScript. It is a primitive value undefined, when a variable is declared and not initialized or not assigned with any value.
Top answer 1 of 8
775
typeof is safer as it allows the identifier to never have been declared before:
if(typeof neverDeclared === "undefined") // no errors
if(neverDeclared === null) // throws ReferenceError: neverDeclared is not defined
2 of 8
54
If the variable is declared (either with the var keyword, as a function argument, or as a global variable), I think the best way to do it is:
if (my_variable === undefined)
jQuery does it, so it's good enough for me :-)
Otherwise, you'll have to use typeof to avoid a ReferenceError.
If you expect undefined to be redefined, you could wrap your code like this:
(function(undefined){
// undefined is now what it's supposed to be
})();
Or obtain it via the void operator:
const undefined = void 0;
// also safe
W3Schools
w3schools.com › Jsref › jsref_undefined.asp
JavaScript undefined Property
altKey (Mouse) altKey (Key) animationName bubbles button buttons cancelable charCode clientX clientY code ctrlKey (Mouse) ctrlKey (Key) currentTarget data defaultPrevented deltaX deltaY deltaZ deltaMode detail elapsedTime elapsedTime eventPhase inputType isTrusted key keyCode location metaKey (Mouse) metaKey (Key) newURL oldURL offsetX offsetY pageX pageY persisted propertyName relatedTarget relatedTarget screenX screenY shiftKey (Mouse) shiftKey (Key) target targetTouches timeStamp touches type which (Mouse) which (Key) view HTML Event Methods
Musing Mortoray
mortoray.com › home › the many faces of undefined in javascript
The many faces of undefined in JavaScript - Musing Mortoray
June 26, 2024 - This convention caused problems when I wrote a cross-language serialization format. To ease the Python side of things, I ended up treated any types as null | undefined in JavaScript that could have just been undefined.
BrowserStack
browserstack.com › home › guide › how to check if a variable is undefined in javascript
How to Check if a Variable is Undefined in JavaScript | BrowserStack
February 18, 2025 - Though both undefined and null represent “no value,” they are distinct types in JavaScript. undefined: This is the default value of uninitialized variables. It is automatically assigned when a variable is declared without a value.
web.dev
web.dev › learn › javascript › data-types › null-undefined
null and undefined | web.dev
The strict equality operator considers operands of different data types to be unequal. null == undefined > true null === undefined > false · Unlike the reserved keyword null, undefined is a property of the global object.
CoreUI
coreui.io › blog › what-is-the-difference-between-null-and-undefined-in-javascript
What is the Difference Between Null and Undefined in JavaScript · CoreUI
February 9, 2025 - Unlike null, undefined means JavaScript cannot find a meaningful value. In contrast, null means a developer has explicitly assigned an empty value. The value null signals the intentional absence of data or object value.
Dmitri Pavlutin
dmitripavlutin.com › 7-tips-to-handle-undefined-in-javascript
7 Tips to Handle undefined in JavaScript
March 23, 2023 - Because JavaScript is permissive, developers have the temptation to access uninitialized values. I'm guilty of such bad practice too. Often such risky actions generate undefined related errors: ... To reduce such errors, you have to understand the cases when undefined is generated. Let's explore undefined and its effect on code safety. ... Undefined: undefined. And a separated object type: {name: "Dmitri"}, ["apple", "orange"].
MDN Web Docs
developer.mozilla.org › en-US › docs › Web › JavaScript › Reference › Global_Objects › undefined
undefined - JavaScript | MDN
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 ...
freeCodeCamp
freecodecamp.org › news › javascript-check-if-undefined-how-to-test-for-undefined-in-js
JavaScript Check if Undefined – How to Test for Undefined in JS
November 7, 2024 - An undefined variable or anything without a value will always return "undefined" in JavaScript. This is not the same as null, despite the fact that both imply an empty state. You'll typically assign a value to a variable after you declare it, ...
SheCodes
shecodes.io › athena › 81408-what-does-undefined-mean-in-javascript
[JavaScript] - What does !==undefined mean in JavaScript? - | SheCodes
Learn about the !==undefined comparison operator in JavaScript and how it is used to check if a variable is not undefined. Asked over 2 years ago in JavaScript by Keamogetse ... Write a function that takes in a single number. It should return the string even if the number is even, and the string odd if the number is odd. ... on a page with just a button, how can I change the color of the whole background?
DEV Community
dev.to › sduduzog › null-vs-undefined-what-to-choose-what-to-use-11g
null vs undefined? What to choose? What to use? - DEV Community
August 23, 2023 - When a javascript object is being serialized, all undefined properties are discarded, remember 'undefined' means a property is yet to be assigned a value. But null on the other hand is known by JSON as its a valid JSON data type · That's it.
ui.dev
ui.dev › check-for-undefined-javascript
How to check for undefined in JavaScript
The way I recommend to check for undefined in JavaScript is using the strict equality operator, ===, and comparing it to the primitive undefined. ... Checking for `undefined`` this way will work in every use case except for one, if the variable ...
Sentry
sentry.io › sentry answers › javascript › how can i check for "undefined" in javascript?
How can I Check for "undefined" in JavaScript? | Sentry
December 15, 2022 - An interesting thing to note is that undefined is not a reserved word in JavaScript. A reserved word is a keyword that can’t be used as an identifier for naming things such as variables, properties, or functions. Reserved words include: import, const, and return. You can use undefined as a variable name, as long as the variable is not in the global scope. As can be seen in the example code below, you can make the typeof undefined equal to string:
JavaScript.info
javascript.info › tutorial › the javascript language › javascript fundamentals
Data types
July 9, 2024 - It makes a type of its own, just like null. The meaning of undefined is “value is not assigned”. If a variable is declared, but not assigned, then its value is undefined: ... …But we don’t recommend doing that.
W3Schools
w3schools.com › js › js_typeof.asp
JavaScript typeof
The typeof of a variable with no value is undefined. The value is also undefined. ... Any variable can be emptied, by setting the value to undefined. The type will also be undefined. let car = "Volvo"; car = undefined; Try it Yourself » · ...
Mozilla
developer.mozilla.org › en-US › docs › Web › JavaScript › Guide › Data_structures
JavaScript data types and data structures - JavaScript | MDN
The object wrapper classes' reference pages contain more information about the methods and properties available for each type, as well as detailed descriptions for the semantics of the primitive types themselves. The Null type is inhabited by exactly one value: null. The Undefined type is inhabited ...
The Valley of Code
thevalleyofcode.com › how-to-check-undefined-property-javascript
How to check if a JavaScript object property is undefined
May 26, 2018 - In a JavaScript program, the correct way to check if an object property is undefined is to use the typeof operator. typeof returns a string that tells the type of the operand.