undefined means a variable has been declared but has not yet been assigned a value :
var testVar;
console.log(testVar); //shows undefined
console.log(typeof testVar); //shows undefinednull is an assignment value. It can be assigned to a variable as a representation of no value :
var testVar = null;
console.log(testVar); //shows null
console.log(typeof testVar); //shows objectFrom the preceding examples, it is clear that undefined and null are two distinct types: undefined is a type itself (undefined) while null is an object.
Proof :
console.log(null === undefined) // false (not the same type)
console.log(null == undefined) // true (but the "same value")
console.log(null === null) // true (both type and value are the same)and
null = 'value' // Uncaught SyntaxError: invalid assignment left-hand side
undefined = 'value' // 'value'
MDN Web Docs
developer.mozilla.org › en-US › docs › Web › JavaScript › Reference › Operators › null
null - JavaScript | MDN
October 28, 2025 - The keyword null is a literal for the null value. Unlike undefined, which is a global variable, null is not an identifier but a syntax keyword.
Top answer 1 of 16
1651
undefined means a variable has been declared but has not yet been assigned a value :
var testVar;
console.log(testVar); //shows undefined
console.log(typeof testVar); //shows undefinednull is an assignment value. It can be assigned to a variable as a representation of no value :
var testVar = null;
console.log(testVar); //shows null
console.log(typeof testVar); //shows objectFrom the preceding examples, it is clear that undefined and null are two distinct types: undefined is a type itself (undefined) while null is an object.
Proof :
console.log(null === undefined) // false (not the same type)
console.log(null == undefined) // true (but the "same value")
console.log(null === null) // true (both type and value are the same)and
null = 'value' // Uncaught SyntaxError: invalid assignment left-hand side
undefined = 'value' // 'value'
2 of 16
803
The difference can be explained with toilet tissue holder:
A non-zero value is like a holder with roll of toilet tissue and there's tissue still on the tube.
A zero value is like a holder with an empty toilet tissue tube.
A null value is like a holder that doesn't even have a tissue tube.
An undefined value is similar to the holder itself being missing.
Undefined vs null
One reason to favor undefined over null is how javascript handle default values: const withDefault = (a = true) => { console.log(a); }; withDefault(); // logs true withDefault(undefined); // logs true withDefault(null); // logs null More on reddit.com
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46
February 27, 2023'null' or 'undefined': What should I use if I want to clear the variable from the memory?
As NateDzMtz says, the memory considerations are the same. null and undefined are unique values and don't involve any references into the heap. In this regard, false would have the same effect. As far as which is convenient for programming, since indexing an object with a key that is not found in the object returns undefined, storing undefined as the value almost simulates absence of the key. Of course, a query can be made to distinguish the case that foo has no key bar from the case where it has the key bar but undefined is stored as the value at that key. But if your design is such that those cases don't have different meanings, it's convenient to stifle slots by putting undefined in them. Note that delete can be inefficient in some engines and they are not required by the standard to make it efficient. I think that the conventional meanings of the special values are, more or less: undefined -- maybe was never initialized; isn't associated to any particular data type. null -- no object, where an object might be expected. NaN -- no number, where a number might be expected. false -- just not true, no other meaning. Note that typeof null is "object", even though you can't index null. typeof undefined is "undefined". typeof NaN is "number", even though NaN explicitly and exactly means "Not a Number"! More on reddit.com
9
4
June 7, 2023Is my interpretation of null vs. undefined correct?
This does not completely cover all cases.
Undefined doesn't necessarily mean that a variable exists but hasn't been given a value yet. It's also possible to explicitly give a variable the value of "undefined", and it is sometimes returned when trying to look up an object member that doesn't exist. E.g. when looking up myObj.thisDoesntExist.
Variables that are set to null do return a value: the value being null.
They need to both be taken into account because they behave slightly differently. For example, if you have a function with default arguments, the default argument will be used if you pass undefined (or no argument at all) - but if you pass null, then your null will be used.
5
2
June 23, 2018null != undefined != 0
So where does NaN fit into this analogy?
More on reddit.com 25
666
May 6, 2019Videos
01:12
Here’s the difference between null and undefined in JavaScript ...
05:39
Null vs Undefined - which one should you use in JS? - YouTube
09:39
JavaScript Null & Undefined – Explained Like Never Before - YouTube
05:53
How to Check for Undefined or Null in JavaScript (2025) - YouTube
GeeksforGeeks
geeksforgeeks.org › javascript › undefined-vs-null-in-javascript
Undefined Vs Null in JavaScript - GeeksforGeeks
July 23, 2025 - undefined indicates a variable hasn’t been initialized, while null is intentionally assigned to indicate no value. Understanding the distinction helps write cleaner, more predictable code in JavaScript, especially when handling default values or checking for missing data.
web.dev
web.dev › learn › javascript › data-types › null-undefined
null and undefined | web.dev
You can also assign the null value ... value. undefined is a primitive value assigned to variables that have just been declared, or to the resulting value of an operation that doesn't return a meaningful value....
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 - According to the typeof operator, however, typeof null returns 'object'—a quirk dating back to the original JavaScript implementation. If you see null console outputs it indicates a deliberate decision to assign null. This is different from a variable that is simply undefined.
Reddit
reddit.com › r/typescript › undefined vs null
r/typescript on Reddit: Undefined vs null
February 27, 2023 -
Since switching to TypeScript I have been using a lot of optional properties, for example:
type store = {
currentUserId?: string
}
function logout () {
store.currentUserId = undefined
}However my coworkers and I have been discussing whether null is a more appropriate type instead of undefined, like this:
type store = {
currentUserId: string | null
}
function logout () {
store.currentUserId = null
}It seems like the use of undefined in TypeScript differs slightly from in Javascript.
Do you guys/girls use undefined or null more often? And, which of the examples above do you think is better?
Top answer 1 of 5
59
One reason to favor undefined over null is how javascript handle default values: const withDefault = (a = true) => { console.log(a); }; withDefault(); // logs true withDefault(undefined); // logs true withDefault(null); // logs null
2 of 5
44
They have different meanings to be: undefined means that it was never set or defined, nobody touched it. Null means someone explicitly set it to null to signal a non-value. Of course, Typescript is very permissive, you can use undefined for everything and just ditch the null, I've seen this approach used well. Or you can just use null for everything, although you may still find undefined specially if your TS settings aren't strict (they should be as strict as possible imo)
Mozilla
developer.mozilla.org › en-US › docs › Web › JavaScript › Guide › Data_structures
JavaScript data types and data structures - JavaScript | MDN
The Null type is inhabited by exactly one value: null. The Undefined type is inhabited by exactly one value: undefined.
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
Medium
medium.com › @stephenthecurt › a-brief-history-of-null-and-undefined-in-javascript-c283caab662e
A brief history of Null and Undefined in JavaScript | by Steve C | Medium
January 13, 2018 - Just like numbers and characters, ... is `null`. Same with `undefined`. These are used in JavaScript to act as placeholders to let the programmer know when a variable has no value....
Hacker News
news.ycombinator.com › item
The difference between null and undefined in JavaScript is something I wished ha... | Hacker News
October 13, 2021 - I have only seen null vs undefined lead to 2 things in my experience: mistakes and bikeshedding · The "billon dollar mistake" as described by Tony Hoare was not nulls per se
Codedamn
codedamn.com › news › javascript
How to check if value is undefined or null in JavaScript
June 8, 2023 - In this blog post, we have explored various ways to check if a value is undefined or null in JavaScript, as well as gained a deeper understanding of these often misunderstood concepts. By using the methods discussed in this post, you can confidently handle undefined and null values in your JavaScript code.
Greenroots
blog.greenroots.info › javascript-undefined-and-null-lets-talk-about-it-one-last-time
JavaScript undefined and null: Let's talk about it one last time!
November 12, 2020 - undefined and null are primitive values and they represent falsy values. All the similarities between undefined and null end here. undefined value is typically set by the JavaScript engine when a variable is declared but not assigned with any values.
Frank M Taylor
blog.frankmtaylor.com › 2023 › 05 › 25 › why-does-javascript-have-null-and-undefined
Why does JavaScript have null AND undefined? – Frank M Taylor
May 2, 2023 - null is for us to use, in our programs, to tell someone, “The object you wanted isn’t there; here’s an object you don’t want” · undefined is the default state of the JavaScript universe
Syncfusion
syncfusion.com › blogs › post › null-vs-undefined-in-javascript
Null vs. Undefined in JavaScript | Syncfusion Blogs
December 10, 2024 - Some of the significant differences between null and undefined are: JavaScript always assigns undefined to indicate the absence of a value by default, and it never automatically assigns null as a value.
Harness
harness.io › blog › feature management & experimentation › why you should use undefined instead of null in javascript
Why You Should Use Undefined Instead of Null in Javascript | Blog
1 month ago - The blog outlines best practices and edge cases, concluding that `undefined` provides cleaner and more predictable results. JavaScript is an unusual programming language in that it has two null-like values: undefined and null. These are distinct values (null !== undefined).
ServiceNow Community
servicenow.com › community › in-other-news › undefined-null-and-oh-my › ba-p › 2291977
undefined, null, ==, !=, ===, and !== - Oh My! - ServiceNow Community
June 3, 2024 - The undefined value is actually implemented in JavaScript by a special Undefined object, which only has one instance (it's a singleton). Horace has an SSN property, because he's a U.S. citizen, but he forgot what it was — so we assigned a null for him (Horace is a null kinda guy, actually).
Web Dev Simplified
blog.webdevsimplified.com › 2021-01 › null-vs-undefined
Null Vs Undefined
January 11, 2021 - Both null and undefined mean that there is no value. If a variable is set to null or undefined it has no value and if a function returns null or undefined then it is saying it has no value to return.
W3Schools
w3schools.com › typescript › typescript_null.php
TypeScript Null & Undefined
let value: string | undefined | null = null; value = 'hello'; value = undefined; Try it Yourself » · When strictNullChecks is enabled, TypeScript requires values to be set unless undefined is explicitly added to the type. Optional chaining is a JavaScript feature that works well with TypeScript's null handling.
Reddit
reddit.com › r/learnjavascript › 'null' or 'undefined': what should i use if i want to clear the variable from the memory?
r/learnjavascript on Reddit: 'null' or 'undefined': What should I use if I want to clear the variable from the memory?
June 7, 2023 -
Please consider the following:
var myFruits = ['Banana', 'Apple', 'Strawberry'];// SOME CODING// SOME CODINGmyFruits = undefined; // Is this better?myFruits = null; // or is this better?
Further question, what is the distinction between the two? Is there any cases where only null is used or undefined is used? Thanks.
Top answer 1 of 3
5
As NateDzMtz says, the memory considerations are the same. null and undefined are unique values and don't involve any references into the heap. In this regard, false would have the same effect. As far as which is convenient for programming, since indexing an object with a key that is not found in the object returns undefined, storing undefined as the value almost simulates absence of the key. Of course, a query can be made to distinguish the case that foo has no key bar from the case where it has the key bar but undefined is stored as the value at that key. But if your design is such that those cases don't have different meanings, it's convenient to stifle slots by putting undefined in them. Note that delete can be inefficient in some engines and they are not required by the standard to make it efficient. I think that the conventional meanings of the special values are, more or less: undefined -- maybe was never initialized; isn't associated to any particular data type. null -- no object, where an object might be expected. NaN -- no number, where a number might be expected. false -- just not true, no other meaning. Note that typeof null is "object", even though you can't index null. typeof undefined is "undefined". typeof NaN is "number", even though NaN explicitly and exactly means "Not a Number"!
2 of 3
5
In my opinion I typically would use null to denote the absence of the variable for purposes of debugging. It helps with identifying that the variable was intentionally set to a null value as to not be confused with the variable not being defined in the first place. Additionally, using null can be useful when you want to explicitly assign a "no value" state to a variable. This can be helpful in scenarios where you want to differentiate between an intentional absence of a value and a variable that has not been assigned any value yet. On the other hand, undefined is often used by JavaScript itself to indicate that a variable has been declared but has not been assigned any value. It is the default value for uninitialized variables. In most cases, you don't need to explicitly set a variable to undefined because JavaScript does it automatically. However, it's worth noting that both null and undefined have similar behaviors when it comes to memory management. Assigning either of them to a variable will release the memory occupied by the previous value and make the variable eligible for garbage collection. In conclusion, while both null and undefined can be used to clear a variable from memory, null is typically preferred when you want to denote an intentional absence of value, while undefined is automatically assigned by JavaScript when a variable is declared but not assigned a value.
CodeBurst
codeburst.io › understanding-null-undefined-and-nan-b603cb74b44c
Understanding null, undefined and NaN. | by Kuba Michalski | codeburst
November 12, 2018 - Even though it points to something non existing, nothing, it’s a global object (and one of JavaScript’s primitive values). Negating null value returns true , but comparing it to false (or either true) gives false. In basic maths operations, null value is converted to 0. The global undefined property represents the primitive value undefined.