why null == undefined is true - Brave Search

why null==undefined is true in javascript

stackoverflow.com › questions › 16607761 › why-null-undefined-is-true-in-javascript

The language specification explicitly says:

If x is null and y is undefined, return true

I'm not aware of any records of the language design process that explain the reasoning for that decision, but == has rules for handling different types, and "null" and "undefined" are both things that mean "nothing", so having them be equal makes intuitive sense.

(If you don't want type fiddling, use === instead).

Answer from Quentin on Stack Overflow

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.

stackoverflow.com › questions › 16607761 › why-null-undefined-is-true-in-javascript

why null==undefined is true in javascript - Stack Overflow

The language specification explicitly says:

If x is null and y is undefined, return true

I'm not aware of any records of the language design process that explain the reasoning for that decision, but == has rules for handling different types, and "null" and "undefined" are both things that mean "nothing", so having them be equal makes intuitive sense.

(If you don't want type fiddling, use === instead).

Using the double-equal operator forces Javascript to do type coercion.

In other words, when you do x == y, if x and y are not of the same type, JavaScript will cast one value to another before comparing, like if string and number are compared, the string is always cast into a number and then compared

For this reason, many comparisons of mixed types in JavaScript can result in results that may be unexpected or counter-intuitive.

If you want to do comparisons in JavaScript, it is usually a better idea to use the triple-equal operator === rather than double-equal. This does not do a type coercion; instead if the types are different, it returns false. This is more usually what you need.

You should only use double-equal if you are absolutely certain that you need it.

Videos

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

October 30, 2023

Are You Using Null And Undefined Wrong? - YouTube

JavaScript Fundamentals: Checking for null and undefined - YouTube

November 14, 2019

JavaScript null vs. undefined - YouTube

September 10, 2017

Null vs Undefined - Beau teaches JavaScript - YouTube

February 14, 2017

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 - A falsy (sometimes written falsey) value is a value that is considered false when encountered in a Boolean context. In JavaScript, there are 6 falsy values including undefined and null, ... Apart from the similarities mentioned above, undefined and null are way apart from each other. They are strictly not equal, (undefined === null) // returns, false (undefined !== null) // returns true...

codeburst.io › javascript-null-vs-undefined-20f955215a2

JavaScript — Null vs. Undefined

January 16, 2018 - The code above creates a function named logHi. This function requires one parameter and sets the default of that parameter to hi if it isn’t supplied. Here’s what that looks like: ... With default parameters, undefined will use the default while null does not.

Net Informations

net-informations.com › js › iq › nuvsun.htm

Null and Undefined in JavaScript

Unlike undefined, null is explicitly assigned. var z = null; console.log(z); // Output: null var user = null; console.log(user); // Output: null var arr = [1, 2, 3]; arr = null; console.log(arr); // Output: null · console.log(undefined == null); // Output: true console.log(undefined === null); // Output: false

Web Dev Simplified

blog.webdevsimplified.com › 2021-01 › null-vs-undefined

Null Vs Undefined

January 11, 2021 - The reason you would want to do this is to essentially reset a variable. By setting a variable to undefined you are conveying the message that the variable no longer contains any useful information, while if the value is null then you are ...

flexiple.com › javascript › undefined-vs-null-javascript

Undefined vs Null - Javascript - Flexiple

March 10, 2022 - As you can see, when the equality operator is used it compares only the values. Both undefined and null are falsy by default. So == returns true.

Find elsewhere

Google Bing Mojeek

naukri.com › code360 › library › difference-between-null-and-undefined-in-javascript

Difference Between Null And Undefined In Javascript

January 7, 2025 - Almost there... just a few more seconds

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.

forum.freecodecamp.org › javascript

Why the heck is null != undefined evaluating to true in my code?

It is a string? undefined != null // false undefined !== null // true 'undefined' != null // true

levelup.gitconnected.com › javascript-null-vs-undefined-2acda986f79f

JavaScript Null vs Undefined. Understanding the difference between… | by Aiman Rahmat | Level Up Coding

January 30, 2020 - So by not declaring a value to a variable, JavaScript automatically assigns the value to undefined. However, when you assign null to a variable, you are declaring that this value is explicitly empty.

geeksforgeeks.org › javascript › undefined-vs-null-in-javascript

Undefined Vs Null in JavaScript

July 23, 2025 - null == undefined // true null === undefined // false · It means null is equal to undefined but not identical.

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

Undefined versus null in JavaScript | Sentry

October 21, 2022 - console.log(undeclaredVar); // will throw a ReferenceError console.log(typeof undeclaredVar); // will print "undefined" In contrast, null is a value that represents nothing. Think of null as an empty container and undefined as the absence of ...

moviecultists.com › why-undefined-null-is-true

Why undefined == null is true?

The == comparison operator doesn't check the types. null and undefined both return false . That's why your code is actually checking if false is equal to false . However their types are not equal. However, if you coerce null to a number it is coerced to +0 so +null == false actually returns true .

Tutorial Republic

tutorialrepublic.com › faq › how-to-determine-if-variable-is-undefined-or-null-in-javascript.php

How to Determine If Variable is Undefined or NULL in JavaScript

In JavaScript if a variable has been declared, but has not been assigned a value, is automatically assigned the value undefined. Therefore, if you try to display the value of such variable, the word "undefined" will be displayed.

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 - Despite both being falsy in a Boolean context, null or undefined is not loosely equal (==) to any other falsy value such as zero (0), an empty string (” “, ‘ ‘), or Not-a-Number (NaN). The only values null is loosely equal to are undefined and itself.

developer.mozilla.org › en-US › docs › Web › JavaScript › Reference › Global_Objects › null

null - JavaScript | MDN

May 23, 2022 - 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

null == undefined > true null === undefined > false · Unlike the reserved keyword null, undefined is a property of the global object. This was a design decision made early in JavaScript's development, and it let legacy browsers overwrite undefined completely.

freecodecamp.org › news › javascript-nullable-how-to-check-for-null-in-js

JavaScript Nullable – How to Check for Null in JS

July 7, 2022 - null == undefined evaluates as true because they are loosely equal. null === undefined evaluates as false because they are not, in fact, equal. <null_variable> === null is the best way to strictly check for null.