Using a juggling-check, you can test both null and undefined in one hit:
if (x == null) {
If you use a strict-check, it will only be true for values set to null and won't evaluate as true for undefined variables:
if (x === null) {
You can try this with various values using this example:
var a: number;
var b: number = null;
function check(x, name) {
if (x == null) {
console.log(name + ' == null');
}
if (x === null) {
console.log(name + ' === null');
}
if (typeof x === 'undefined') {
console.log(name + ' is undefined');
}
}
check(a, 'a');
check(b, 'b');
Output
Answer from Fenton on Stack Overflow"a == null"
"a is undefined"
"b == null"
"b === null"
Top answer 1 of 16
705
Using a juggling-check, you can test both null and undefined in one hit:
if (x == null) {
If you use a strict-check, it will only be true for values set to null and won't evaluate as true for undefined variables:
if (x === null) {
You can try this with various values using this example:
var a: number;
var b: number = null;
function check(x, name) {
if (x == null) {
console.log(name + ' == null');
}
if (x === null) {
console.log(name + ' === null');
}
if (typeof x === 'undefined') {
console.log(name + ' is undefined');
}
}
check(a, 'a');
check(b, 'b');
Output
"a == null"
"a is undefined"
"b == null"
"b === null"
2 of 16
482
if( value ) {
}
will evaluate to true if value is not:
nullundefinedNaN- empty string
'' 0false
typescript includes javascript rules.
W3Schools
w3schools.com › typescript › typescript_null.php
TypeScript Null & Undefined
TypeScript has a powerful system to deal with null or undefined values. By default null and undefined handling is disabled, and can be enabled by setting strictNullChecks to true. The rest of this page applies for when strictNullChecks is enabled.
Videos
TypeScript Tutorial #12: Handling Null and Undefined in Just ...
04:07
Using Null & Undefined | strictNullChecks | TypeScript Tutorial ...
Best Way to Check 'Null', 'Undefined' or 'Empty' in JavaScript ...
07:25
TypeScript: Undefined vs null - Stop using null (even in ...
05:01
null vs. undefined in JavaScript / TypeScript - YouTube
TypeScript
typescriptlang.org › docs › handbook › release-notes › typescript-2-0.html
TypeScript: Documentation - TypeScript 2.0
Non-null and non-undefined type guards may use the ==, !=, ===, or !== operator to compare to null or undefined, as in x != null or x === undefined. The effects on subject variable types accurately reflect JavaScript semantics (e.g. double-equals operators check for both values no matter which ...
Omarileon
omarileon.me › blog › typescript-null-undefined
mari. | How to Detect Null and Undefined in Your TypeScript Code
February 27, 2024 - Another way to check for null or undefined is to use the nullish coalescing operator (??), which was introduced in TypeScript 3.7. If the left-hand side of the operation is non-null it returns that, otherwise it returns the right-hand side otherwise.
GitBook
basarat.gitbook.io › typescript › recap › null-undefined
Null vs. Undefined | TypeScript Deep Dive
So to check if a variable is defined or not at a global level you normally use typeof: ... if (typeof someglobal !== 'undefined') { // someglobal is now safe to use console.log(someglobal); } Because TypeScript gives you the opportunity to document your structures separately from values instead of stuff like: ... Node style callback functions (e.g. (err,somethingElse)=>{ /* something */ }) are generally called with err set to null if there isn't an error.
GeeksforGeeks
geeksforgeeks.org › typescript › how-to-check-null-and-undefined-in-typescript
How to check null and undefined in TypeScript ? - GeeksforGeeks
July 23, 2025 - We can use typeof or '==' or '===' to check if a variable is null or undefined in typescript.
Rampatra
blog.rampatra.com › null-vs-undefined-in-typescript-or-javascript-how-to-check-for-both-at-once
!== null vs !== undefined in Typescript or Javascript, how to check for both at once?
if (value != null) { // Do something if value is not null or undefined } Prefer strict equality checks (!== or ===) for better type safety and clarity.
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)
Upmostly
upmostly.com › home › typescript › how to detect null and undefined
How to Detect Null and Undefined in Your TypeScript Code - Upmostly
March 28, 2023 - Another way to check for null or undefined is to use the nullish coalescing operator (??), which was introduced in TypeScript 3.7.
Angular Wiki
angularjswiki.com › angular › how-to-check-both-null-or-undefined-in-typescript-or-angular
How To Check Both null or undefined in TypeScript/Angular | Angular Wiki
different ways we can check both null or undefined in TypeScript or Angular. 1. By using simple if condition 2. By using TypeScript Nullish Coalescing & Optional chaining. 3. By using Array.include() function.
Bobby Hadz
bobbyhadz.com › blog › typescript-check-if-null
How to correctly check for Null in TypeScript | bobbyhadz
The first if statement uses the loose equals (==) operator instead of strict equals (===), and checks if the variable is equal to null or undefined. This checks for both null and undefined because when using loose equals (==), null is equal ...
Tektutorialshub
tektutorialshub.com › home › typescript › null vs undefined in typescript
Null Vs Undefined in TypeScript - Tektutorialshub
March 15, 2023 - But they are neither false nor true. You can use typeof operator to check for undefined but not null as it returns “object”. You can use the == & === operator to check their values · Checking for null.
TutorialsPoint
tutorialspoint.com › javascript-typescript-object-null-check
JavaScript/ Typescript object null check?
In typescript, unassigned values are by default undefined, so in order to make a variable null, we must assign it a null value. To check a variable is null or not in typescript we can use typeof or "===" operator.
Top answer 1 of 14
220
In TypeScript 2, you can use the undefined type to check for undefined values.
If you declare a variable as:
let uemail : string | undefined;
Then you can check if the variable uemail is undefined like this:
if(uemail === undefined)
{
}
2 of 14
88
From Typescript 3.7 on, you can also use nullish coalescing:
let x = foo ?? bar();
Which is the equivalent for checking for null or undefined:
let x = (foo !== null && foo !== undefined) ?
foo :
bar();
https://www.typescriptlang.org/docs/handbook/release-notes/typescript-3-7.html#nullish-coalescing
While not exactly the same, you could write your code as:
var uemail = localStorage.getItem("useremail") ?? alert('Undefined');
Zipy
zipy.ai › blog › debug-typescript-null-or-undefined-value-errors
Solving Typescript Null & Undefined Errors: Expert Tips
February 28, 2024 - Efficiently debug & fix TypeScript null or undefined value errors. Expert tips for handling & resolving errors in TypeScript.
GitHub
yellowduck.be › posts › null-vs-undefined-in-typescript-and-javascript-whats-the-difference
🐥 Null vs. Undefined in TypeScript and JavaScript: what's the difference?
In TypeScript, the handling of null and undefined can be stricter with strictNullChecks. When enabled, variables cannot be null or undefined unless explicitly allowed. ... This condition is true for null, undefined, 0, NaN, "" (empty string), ...
Reddit
reddit.com › r/typescript › null or undefined
r/typescript on Reddit: Null or undefined
December 27, 2022 -
Hello guys, i need your help, i would like to know which you think is a better approach:
1 --------------------------------------------
const {content} = props;
if(content !== "undefined"){
//code
}
2 --------------------------------------------
const {content = null} = props;
if(!content){
//code
}
Top answer 1 of 4
6
I try to use null as an empty value and undefined as an unprovided value. In practice though, most of the code I interact with treats them the same. Ultimately unless you need the distinction between "empty" and "not provided" you can use them interchangeably. One thing to note, the condition !content will evaluate to true for any falsy value (null, undefined, 0, '', NaN, false) so you might get false positives if you just want to check for null.
2 of 4
5
"undefined", though, is without the double quotes.
Honlsoft
honlsoft.com › blog › 2021-07-20-typescript-tips-null-coalescing
Typescript Tips: null(and undefined) Checking | Honlsoft
Thankfully, in Typescript, there are several shorthand ways to deal with nulls. The Elvis operator is a shorthand way to check for null or undefined without requiring a separate if statement.
Webdevtutor
webdevtutor.net › blog › typescript-check-if-value-is-not-undefined-or-null
How to Check if a Value is Not Undefined or Null in TypeScript
You can create a type guard to check if a value is not undefined or null like this: function isDefined<T>(value: T | undefined | null): value is T { return value !== undefined && value !== null; } let exampleValue: string | undefined = "Hello, TypeScript!"; if (isDefined(exampleValue)) { ...
TypeScript
typescriptlang.org › tsconfig › strictNullChecks.html
TypeScript: TSConfig Option: strictNullChecks
When strictNullChecks is true, null and undefined have their own distinct types and you’ll get a type error if you try to use them where a concrete value is expected. For example with this TypeScript code, users.find has no guarantee that it will actually find a user, but you can write code ...