JavaScript is very flexible with regards to checking for "null" values. I'm guessing you're actually looking for empty strings, in which case this simpler code will work:
if(!pass || !cpass || !email || !cemail || !user){
Which will check for empty strings (""), null, undefined, false and the numbers 0 and NaN.
Please note that if you are specifically checking for numbers, it is a common mistake to miss 0 with this method, and num !== 0 is preferred (or num !== -1 or ~num (hacky code that also checks against -1)) for functions that return -1, e.g. indexOf).
Top answer 1 of 16
1094
JavaScript is very flexible with regards to checking for "null" values. I'm guessing you're actually looking for empty strings, in which case this simpler code will work:
if(!pass || !cpass || !email || !cemail || !user){
Which will check for empty strings (""), null, undefined, false and the numbers 0 and NaN.
Please note that if you are specifically checking for numbers, it is a common mistake to miss 0 with this method, and num !== 0 is preferred (or num !== -1 or ~num (hacky code that also checks against -1)) for functions that return -1, e.g. indexOf).
2 of 16
729
To check for null SPECIFICALLY you would use this:
if (variable === null)
This test will ONLY pass for null and will not pass for "", undefined, false, 0, or NaN.
Additionally, I've provided absolute checks for each "false-like" value (one that would return true for !variable).
Note, for some of the absolute checks, you will need to implement use of the absolutely equals: === and typeof.
I've created a JSFiddle here to show all of the individual tests working
Here is the output of each check:
Null Test:
if (variable === null)
- variable = ""; (false) typeof variable = string
- variable = null; (true) typeof variable = object
- variable = undefined; (false) typeof variable = undefined
- variable = false; (false) typeof variable = boolean
- variable = 0; (false) typeof variable = number
- variable = NaN; (false) typeof variable = number
Empty String Test:
if (variable === '')
- variable = ''; (true) typeof variable = string
- variable = null; (false) typeof variable = object
- variable = undefined; (false) typeof variable = undefined
- variable = false; (false) typeof variable = boolean
- variable = 0; (false) typeof variable = number
- variable = NaN; (false) typeof variable = number
Undefined Test:
if (typeof variable == "undefined")
-- or --
if (variable === undefined)
- variable = ''; (false) typeof variable = string
- variable = null; (false) typeof variable = object
- variable = undefined; (true) typeof variable = undefined
- variable = false; (false) typeof variable = boolean
- variable = 0; (false) typeof variable = number
- variable = NaN; (false) typeof variable = number
False Test:
if (variable === false)
- variable = ''; (false) typeof variable = string
- variable = null; (false) typeof variable = object
- variable = undefined; (false) typeof variable = undefined
- variable = false; (true) typeof variable = boolean
- variable = 0; (false) typeof variable = number
- variable = NaN; (false) typeof variable = number
Zero Test:
if (variable === 0)
- variable = ''; (false) typeof variable = string
- variable = null; (false) typeof variable = object
- variable = undefined; (false) typeof variable = undefined
- variable = false; (false) typeof variable = boolean
- variable = 0; (true) typeof variable = number
- variable = NaN; (false) typeof variable = number
NaN Test:
if (typeof variable == 'number' && !parseFloat(variable) && variable !== 0)
-- or --
if (isNaN(variable))
- variable = ''; (false) typeof variable = string
- variable = null; (false) typeof variable = object
- variable = undefined; (false) typeof variable = undefined
- variable = false; (false) typeof variable = boolean
- variable = 0; (false) typeof variable = number
- variable = NaN; (true) typeof variable = number
As you can see, it's a little more difficult to test against NaN;
MDN Web Docs
developer.mozilla.org › en-US › docs › Web › JavaScript › Reference › Operators › null
null - JavaScript | MDN
JavaScript is unique to have two nullish values: null and undefined. Semantically, their difference is very minor: undefined represents the absence of a value, while null represents the absence of an object.
Videos
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How to Check for Undefined or Null in JavaScript - YouTube
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JavaScript Null & Undefined – Explained Like Never Before - YouTube
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Null in JavaScript: Primitive or Object? 🤔🔎 - YouTube
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What are null, undefined and NaN values in javascript | Explained ...
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Null vs Undefined - which one should you use in JS? - YouTube
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Javascript null data type explained | JS for Beginners Ep. 14 - ...
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.
Programiz
programiz.com › javascript › null-undefined
JavaScript null and undefined
When comparing null and undefined with equal to operator ==, they are considered equal. For example, ... In JavaScript, == compares values by performing type conversion. Both null and undefined return false.
GeeksforGeeks
geeksforgeeks.org › javascript › null-in-javascript
Null in JavaScript - GeeksforGeeks
June 5, 2024 - In JavaScript, `null` indicates the deliberate absence of any object value. It's a primitive value that denotes the absence of a value or serves as a placeholder for an object that isn't present.
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
January 21, 2026 - JavaScript is an unusual programming language in that it has two null-like values: undefined and null. These are distinct values (null !== undefined).
Built In
builtin.com › software-engineering-perspectives › javascript-null-check
How to Check for Null in JavaScript | Built In
Summary: JavaScript offers several ways to check for null, including strict (===) and loose (==) equality, Object.is() and boolean coercion. Developers often use typeof and optional chaining (?.) to safely identify null, undefined or undeclared ...
Dmitri Pavlutin
dmitripavlutin.com › javascript-null
Everything about null in JavaScript
Do not use typeof operator to detect a null value. As mentioned previously, use the strict equality operator myVar === null. If you'd like to check whether a variable contains an object using typeof operator, you have to check againts null too: ... null might appear, often unexpectedly, in situations when you expect an object. Then if you try to extract a property from null, JavaScript throws an error.
JavaScript Tutorial
javascripttutorial.net › home › an essential guide to javascript null
An Essential Guide to JavaScript null
September 29, 2020 - JavaScript uses the null value to represent the intentional absence of any object value.
GeeksforGeeks
geeksforgeeks.org › javascript › how-to-check-for-null-values-in-javascript
How to check for null values in JavaScript ? - GeeksforGeeks
July 23, 2025 - The null values show the non-appearance of any object value. It is usually set on purpose to indicate that a variable has been declared but not yet assigned any value. This contrasts null from the similar primitive value undefined, which is ...
CSS { In Real Life }
css-irl.info › handling-null-undefined-and-zero-values-in-javascript
CSS { In Real Life } | Handling Null, Undefined and Zero Values in JavaScript
Using the || (or) operator, the message text will read “Unknown days since last post” if the daysSinceLastPost value doesn’t exist. const message = `${daysSinceLastPost || 'Unknown'} days since last post` The issue here is that if daysSinceLastPost is 0, the text will read “Unknown days since last post” too! Instead, we could use the ?? operator, otherwise known as the nullish coalescing operator.
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.
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
MDN Web Docs
developer.mozilla.org › en-US › docs › Web › JavaScript › Reference › Operators › Nullish_coalescing
Nullish coalescing operator (??) - JavaScript | MDN
null || undefined ?? "foo"; // raises a SyntaxError true && undefined ?? "foo"; // raises a SyntaxError · Instead, provide parenthesis to explicitly indicate precedence: ... In this example, we will provide default values but keep values other than null or undefined.
freeCodeCamp
freecodecamp.org › news › how-to-check-for-null-in-javascript
JS Check for Null – Null Checking in JavaScript Explained
November 7, 2024 - Null is a primitive type in JavaScript. This means you are supposed to be able to check if a variable is null with the typeof() method. But unfortunately, this returns “object” because of an historical bug that cannot be fixed. let userName ...
The Trevor Harmon
thetrevorharmon.com › blog › loose-null-checks-in-javascript
Loose null checks in JavaScript | The Trevor Harmon
I have observed that JavaScript ... this, like in the exec() method of the regular expression object). null indicates the intentional absence of a value....
SheCodes
shecodes.io › athena › 142857-understanding-the-values-null-and-undefined-in-javascript
[JavaScript] - Understanding the values null and undefined in JavaScript
Learn the meaning of null and undefined in JavaScript and how they are used to represent the absence of a value.
TutorialsPoint
tutorialspoint.com › how-do-i-check-for-null-values-in-javascript
How do I check for null values in JavaScript?
In this article, we will learn how to check the null values in JavaScript. The null value indicates the intentional absence of any object value. It is a JavaScript primitive value that is false when used in Boolean operations. This distinguishes null from the related primitive value undefined, ...
Favtutor
favtutor.com › articles › null-javascript
Check for Null in JavaScript | 3 Easy Methods (with code)
January 5, 2024 - If the variable holds a null value, it returns true else returns false. The typeof operator in JavaScript determines the data type of a variable. We can combine the typeof operator and a conditional statement to check if a variable holds a null ...
Top answer 1 of 4
13
You don't have to do that:
var n=null;
if(n)alert('Not null.'); // not shown
if(!n)alert('Is null.'); // popup is shown
Your error implies otherwise:
var n=null;
alert(n.something); // Error: n is null or not an object.
In the case above, something like this should be used:
if(n)alert(n.something);
2 of 4
3
The !== operator returns true when two variables are not the same object. It doesn't look at the values of the objects at all
To test if something is null:
myVar == null
Your code was testing to see if the variable 'pNonUserId' referred to the same object as "", which can never be true as "" will always be a new instance of the empty string.
As an aside, a test such as:
var n = something();
// do stuff
if (n)
doSomethingElse();
Is a bad idea. If n was a boolean and false, but you were expecting the if block to test nullify you'll be in for a shock.