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'
Reddit
reddit.com › r/learnjava › does java have null and undefined like javascript?
r/learnjava on Reddit: Does java have null and undefined like Javascript?
April 10, 2020 -
I know java the most but recently started learning JavaScript, which treats null and undefined as two separate things. From what I remember me and peers used the two terms interchangeably in Java. Is there anything such as undefined in Java, or is it just null value? I might have to be careful about using the term undefined for Java then later.
Beyondjava
beyondjava.net › null-vs-undefined
Null vs. undefined
Beyond Java · Sitemap · Talks & Articles · Projects · Guest Posts · About · Legalese · Statistics · (opt out) · Mastodon
Videos
01:12
Here’s the difference between null and undefined in JavaScript ...
Difference between null and undefined in java script? | #jsshorts ...
00:37
What are Null and Undefined in Javascript? - YouTube
04:21
Are You Using Null And Undefined Wrong? - YouTube
09:39
JavaScript Null & Undefined – Explained Like Never Before - YouTube
05:39
Null vs Undefined - which one should you use in JS? - YouTube
Quora
quora.com › What-is-the-main-difference-between-NULL-and-UNDEFINED-in-programming-Are-they-both-non-existent-values
What is the main difference between 'NULL' and 'UNDEFINED' in programming? Are they both 'non-existent values'? - Quora
Answer (1 of 9): An undefined variable does not exist at all. This error usually occurs when you attempt to access a variable before it has even been created. On the other hand, a NULL variable exists, but holds a value of zero/nothing.
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
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.
Quora
basicprogramming.quora.com › What-is-the-difference-between-NAN-NULL-and-UNDEFINED
What is the difference between NAN, NULL and UNDEFINED? - Basic Programming - Quora
Answer (1 of 6): * Null - Null represents an empty value and carry no meaningful information. * Undefined - Undefined represents any value that has not been defined anywhere in the program.
web.dev
web.dev › learn › javascript › data-types › null-undefined
null and undefined | web.dev
... You might define a variable ... also assign the null value to an existing reference to clear a previous value. undefined is a primitive ......
GeeksforGeeks
geeksforgeeks.org › javascript › undefined-vs-null-in-javascript
Undefined Vs Null in JavaScript - GeeksforGeeks
July 23, 2025 - It means null is equal to undefined but not identical.
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
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.
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.
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)
Hacker News
news.ycombinator.com › item
Ask HN: Should scripting engines distinguish between "null" and "undefined"? | Hacker News
January 28, 2024 - Do scripting languages really need both "null" and "undefined" values · i have long liked that JavaScript provides both "null" and "undefined", and have modeled my scripting languages after that since tinkering with them first became a hobby (right at 20 years ago).
Scaler
scaler.com › topics › javascript › null-and-undefined-in-javascript
Null and Undefined in JavaScript - Scaler Topics
April 21, 2022 - Besides, when a function doesn't return a value, it returns undefined. ... In JavaScript, to check the type of variable, we use the "typeof" operator. ... As discussed above, the type of null is an object.
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.
Coders Campus
coderscampus.com › home › ep37 – null vs undefined in javascript
EP37 - Null vs Undefined in JavaScript - Coders Campus
April 9, 2021 - One thing that threw me off when I started learning JavaScript (with a background in Java) was the fact that JavaScript has both undefined and null as possible values for a variable. In Java, all we have is just null which means that
HowToDoInJava
howtodoinjava.com › home › typescript › difference between undefined and null
JavaScript - Difference between undefined and null
September 22, 2021 - A variable is said to be “undefined” if it has been declared but not initialized. Whereas “null” is assigned to a variable whose value is absent.
TutorialsPoint
tutorialspoint.com › What-is-the-difference-between-null-and-undefined-in-JavaScript
What is the difference between null and undefined in JavaScript?
It means a variable declared, but no value has been assigned to the variable. JavaScript automatically assigns undefined to uninitialized variables. ... null is an intentional absence of a value. It is often used when a developer wants to indicate that a variable or object property has no value.
DEV Community
dev.to › nunocpnp › differences-between-null-and-undefined-keywords-2e2m
Differences between "null" and "undefined" keywords? - DEV Community
September 5, 2019 - They both represent a empty value. Difference nr 1! When you define a variable but not as... Tagged with javascript, webdev, todayilearned, beginners.
Reddit
reddit.com › r/javascript › null vs undefined
r/javascript on Reddit: null vs undefined
June 21, 2017 - Just adding on: Notice how when you initialize a variable, it is always automatically set to undefined at first? It's because it literally does not have a definition. Things that are null do have a definition. They are defined as void. ... Oh man the null thing I didn't know about. Which has led to me go read this whole SO question: https://stackoverflow.com/questions/8511281/check-if-a-value-is-an-object-in-javascript