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'
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.
web.dev
web.dev › learn › javascript › data-types › null-undefined
null and undefined | web.dev
You might define a variable as null with the expectation that it reflects either a value assigned to it at some point in a script or an explicitly absent value. You can also assign the null value to an existing reference to clear a previous 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.
Videos
05:39
Null vs Undefined - which one should you use in JS? - YouTube
01:12
Here’s the difference between null and undefined in JavaScript ...
09:39
JavaScript Null & Undefined – Explained Like Never Before - YouTube
05:53
How to Check for Undefined or Null in JavaScript (2025) - YouTube
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)
Medium
medium.com › @o_o_o › null-vs-undefined-can-i-use-only-one-a3b7db5468f2
null vs. undefined: Can I use only one? | by OOO | Medium
March 29, 2022 - null vs. undefined: Can I use only one? I tried it on my React app Why do you wanna use only one in the first place? null and undefined are used interchangeably to represent non-value. Why can’t I …
MDN Web Docs
developer.mozilla.org › en-US › docs › Web › JavaScript › Reference › Operators › null
null - JavaScript | MDN
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.
GeeksforGeeks
geeksforgeeks.org › javascript › undefined-vs-null-in-javascript
Undefined Vs Null in JavaScript - GeeksforGeeks
July 23, 2025 - When we define a variable to undefined then we are trying to convey that the variable does not exist . When we define a variable to null then we are trying to convey that the variable is empty.
SheCodes
shecodes.io › athena › 2227-what-is-the-difference-between-null-and-undefined-in-javascript
[JavaScript] - What is the Difference Between Null and Undefined in JavaScript?
Learn what is the difference between null and undefined in JavaScript and how to distinguish between them. Check how to use a typeof operator.
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
Sentry
sentry.io › sentry answers › javascript › undefined versus null in javascript
Undefined versus null in JavaScript | Sentry
null and undefined are considered the same when using loose equality (==) but not when using strict equality (===).
Quora
quora.com › Why-is-null-undefined-true-in-JavaScript
Why is (null==undefined) true in JavaScript? - Quora
Answer (1 of 7): Actually, there is no satisfactory reason for that. I am too confused with this. Some say both are actually falsy values, so it evaluates to true, but i don’t get this logic. I mean i want to know what thing is being converted to what if type coercion is happening.
Web Dev Simplified
blog.webdevsimplified.com › 2021-01 › null-vs-undefined
Null Vs Undefined
If no entry exists it makes the most sense to return null since you are stating that there is no value found. On the other hand undefined means that there is no value because no value has been set yet.
CodeBurst
codeburst.io › javascript-null-vs-undefined-20f955215a2
JavaScript — Null vs. Undefined
January 16, 2018 - Here’s an example. We assign the value of null to a: ... Undefined most typically means a variable has been declared, but not defined.
Flexiple
flexiple.com › javascript › undefined-vs-null-javascript
Undefined vs Null - Javascript - Flexiple
Many times we often get confused on what the difference between UNDEFINED and NULL is. Simply put, undefined means a variable has been declared but has not yet been assigned a value. undefined is a type by itself (undefined).
Javatpoint
javatpoint.com › null-vs-undefined
Difference Between Null and Undefined - javatpoint
Difference Between Null and Undefined with typescript tutorial, typescript introduction, versions, typescript and javascript, features, components, installation, typescript types, etc.
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 - When you assign null, you are specifying that no meaningful value exists at the moment, but the variable has been declared so it can hold future data. Also, note that unlike null, undefined usually arises without explicit action from the developer.
TutorialsPoint
tutorialspoint.com › What-is-the-difference-between-null-and-undefined-in-JavaScript
What is the difference between null and undefined in JavaScript?
null represents intentional absence, while undefined signals an expected missing value.
CodeBurst
codeburst.io › javascript-whats-the-difference-between-null-undefined-37793b5bfce6
JavaScript — What’s the difference between Null & Undefined? | by Brandon Morelli | codeburst
July 5, 2017 - null is also an object. Interestingly, this was actually an error in the original JavaScript implementation: ... Undefined means a variable has been declared, but the value of that variable has not yet been defined.
Programiz
programiz.com › javascript › null-undefined
JavaScript null and undefined
When comparing null and undefined with equal to operator ==, they are considered equal.
Scaler
scaler.com › topics › javascript › null-and-undefined-in-javascript
Null and Undefined in JavaScript - Scaler Topics
April 21, 2022 - As the name suggests, undefined means "not defined". So we declare a variable but do not assign a value to it, the variable becomes undefined. Unlike null, the value of an undefined variable is set by JavaScript as undefined. The variable gets created at the run-time.
GitBook
basarat.gitbook.io › typescript › recap › null-undefined
Null vs. Undefined | TypeScript Deep Dive
Something hasn't been initialized : undefined. Something is currently unavailable: null.