null undefined - Brave Search

What is the difference between null and undefined in JavaScript?

stackoverflow.com › questions › 5076944 › what-is-the-difference-between-null-and-undefined-in-javascript

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 undefined

null 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 object

From 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'

Answer from sebastian on Stack Overflow

stackoverflow.com › questions › 5076944 › what-is-the-difference-between-null-and-undefined-in-javascript

What is the difference between null and undefined in JavaScript? - Stack Overflow

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 undefined

null 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 object

From 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'

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.

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

Discussions

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

r/typescript

51

46

February 27, 2023

Undefined vs null

Hello. What is the difference between undefined and null? More on forum.freecodecamp.org

forum.freecodecamp.org

0

November 19, 2019

Falsy values vs null, undefined, or empty string

I've worked with jQuery over the years. However, recently, I've found myself getting deeper into the JavaScript language. Recently, I've heard about "truthy" and falsey values. However, I don't fully More on softwareengineering.stackexchange.com

softwareengineering.stackexchange.com

arithmetic on NULL pointer -is it undefined behaviour?

~~The arithmetic itself is not undefined here. Look up how NULL is defined.~~

Edit: I stand corrected (see below)

More on reddit.com

r/C_Programming

64

19

August 7, 2019

Videos

Here’s the difference between null and undefined in JavaScript ...

January 23, 2024

Null vs Undefined - which one should you use in JS? - YouTube

October 30, 2023

JavaScript Null & Undefined – Explained Like Never Before - YouTube

How to Check for Undefined or Null in JavaScript (2025) - YouTube

February 12, 2024

Are You Using Null And Undefined Wrong? - YouTube

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

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.

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?

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

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)

syncfusion.com › blogs › post › null-vs-undefined-in-javascript

Null vs. Undefined in JavaScript | Syncfusion Blogs

December 10, 2024 - Since undefined is the default value assigned by JavaScript to uninitialized variables, if you want to indicate the absence of a deal explicitly, always use null instead of undefined to avoid confusion. To check if a variable has any value before proceeding further in a program, you can use the loose equality ==null to check for either null or undefined.For example, in the following program, the function assignVal() checks whether the num is undefined or null and assigns the value given by the user only if the variable num is not initialized to any value.

Find elsewhere

Google Bing Mojeek

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.

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.

w3schools.com › typescript › typescript_null.php

TypeScript Null & Undefined

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.

JavaScript.info

javascript.info › tutorial › the javascript language › javascript fundamentals

July 9, 2024 - In JavaScript, null is not a “reference to a non-existing object” or a “null pointer” like in some other languages. It’s just a special value which represents “nothing”, “empty” or “value unknown”. The code above states that age is unknown. The special value undefined also stands apart.

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.

sentry.io › sentry answers › javascript › undefined versus null in javascript

Undefined versus null in JavaScript | Sentry

October 21, 2022 - null and undefined are considered the same when using loose equality (==) but not when using strict equality (===).

typescriptlang.org › docs › handbook › 2 › everyday-types.html

TypeScript: Documentation - Everyday Types

JavaScript has two primitive values used to signal absent or uninitialized value: null and undefined.

forum.freecodecamp.org › javascript

Undefined vs null

November 19, 2019 - Hello. What is the difference between undefined and null?

codeburst.io › understanding-null-undefined-and-nan-b603cb74b44c

Understanding null, undefined and NaN. | by Kuba Michalski | codeburst

November 12, 2018 - In basic maths operations, null value is converted to 0. The global undefined property represents the primitive value undefined. It is one of JavaScript's primitive types (source: MDN). It basically tells us that something isn’t defined. You get this e.g.

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 …

dev.to › icncsx › js-explain-undefined-null-and-nan-2lka

JS / explain undefined, null, and NaN - DEV Community

May 28, 2020 - Unlike undefined, null is never assumed by default. Null is typically used when we want to reset a variable; this is typically more clear than reassigning a variable to an empty string, 0, or undefined.

softwareengineering.stackexchange.com › questions › 253721 › falsy-values-vs-null-undefined-or-empty-string

Falsy values vs null, undefined, or empty string

In programming, truthiness or falsiness is that quality of those boolean expressions which don't resolve to an actual boolean value, but which nevertheless get interpreted as a boolean result.

In the case of C, any expression that evaluates to zero is interpreted to be false. In Javascript, the expression value in

if(value) {
}

will evaluate to true if value is not:

null
undefined
NaN
empty string ("")
0
false

See Also
Is there a standard function to check for null, undefined, or blank variables in JavaScript?

The set of "truthy" and "falsey" values in JavaScript comes from the ToBoolean abstract operation defined in the ECMAScript spec, which is used when coercing a value to a boolean:

+--------------------------------------------------------------------------+
| Argument Type | Result                                                   |
|---------------+----------------------------------------------------------|
| Undefined     | false                                                    |
|---------------+----------------------------------------------------------|
| Null          | false                                                    |
|---------------+----------------------------------------------------------|
| Boolean       | The result equals the input argument (no conversion).    |
|---------------+----------------------------------------------------------|
| Number        | The result is false if the argument is +0, −0, or NaN;   |
|               | otherwise the result is true.                            |
|---------------+----------------------------------------------------------|
| String        | The result is false if the argument is the empty String  |
|               | (its length is zero); otherwise the result is true.      |
|---------------+----------------------------------------------------------|
| Object        | true                                                     |
+--------------------------------------------------------------------------+

From this table, we can see that null and undefined are both coerced to false in a boolean context. However, your fields.length === 0 does not map generally onto a false value. If fields.length is a string, then it will be treated as false (because a zero-length string is false), but if it is an object (including an array) it will coerce to true.

If fields should be a string, then !fields is a sufficient predicate. If fields is an array, your best check might be:

if (!fields || fields.length === 0)

vuejs.org › guide › components › props

defineProps({ // Basic type check // (`null` and `undefined` values will allow any type) propA: Number, // Multiple possible types propB: [String, Number], // Required string propC: { type: String, required: true }, // Required but nullable string propD: { type: [String, null], required: true }, // Number with a default value propE: { type: Number, default: 100 }, // Object with a default value propF: { type: Object, // Object or array defaults must be returned from // a factory function.

typescriptlang.org › docs › handbook › advanced-types.html

TypeScript: Documentation - Advanced Types

By default, the type checker considers null and undefined assignable to anything. Effectively, null and undefined are valid values of every type. That means it’s not possible to stop them from being assigned to any type, even when you would like to prevent it.