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"
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.
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.
Videos
03:18
💥 Typescript Strict Null Checks and the strictNullChecks Compiler ...
04:07
Using Null & Undefined | strictNullChecks | TypeScript Tutorial ...
TypeScript: The Null and Undefined Types
Best Way to Check 'Null', 'Undefined' or 'Empty' in JavaScript ...
Best Way to Check 'Null', 'Undefined' or 'Empty' in JavaScript
TypeScript
typescriptlang.org › docs › handbook › release-notes › typescript-2-0.html
TypeScript: Documentation - TypeScript 2.0
A new ! post-fix expression operator may be used to assert that its operand is non-null and non-undefined in contexts where the type checker is unable to conclude that fact. Specifically, the operation x! produces a value of the type of x with null and undefined excluded. Similar to type assertions of the forms <T>x and x as T, the ! non-null assertion operator is simply removed in the emitted JavaScript code.
TutorialsPoint
tutorialspoint.com › javascript-typescript-object-null-check
JavaScript/ Typescript object null check?
To check a variable is null or not in typescript we can use typeof or "===" operator. The typeof operator in JavaScript is used to find the datatype of the variable.
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. By using typescript compiler tcs we transpile typescript code to javascript and then run the javascript file.
Bobby Hadz
bobbyhadz.com › blog › typescript-check-if-null
How to correctly check for Null in TypeScript | bobbyhadz
The type of null is "object" in JavaScript, so you shouldn't use the typeof operator to check for null. ... Copied!type Person = string | null; const person: Person = null; console.log(typeof person); // 👉️ "object" ...
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.
GitBook
basarat.gitbook.io › typescript › recap › null-undefined
Null vs. Undefined | TypeScript Deep Dive
Interestingly in JavaScript with ==, null and undefined are only equal to each other: ... // Both null and undefined are only `==` to themselves and each other: console.log(null == null); // true (of course) console.log(undefined == undefined); // true (of course) console.log(null == undefined); // true // You don't have to worry about falsy values making through this check console.log(0 == undefined); // false console.log('' == undefined); // false console.log(false == undefined); // false
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.
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?
Use != null only when you explicitly want to check for both null and undefined. ... If offerPrice might explicitly be null, you should use offerPrice != null to handle both cases. Take your presentation to the next level. ... Put your face and name on your screen. ... Show keypresses on your screen. ... Your to-dos on your menu bar. ... Fill forms using your right-click menu. ... Preview your Mac app icons. ... question typescript javascript ...
Reddit
reddit.com › r/reactjs › is null checking necessary in typescript for runtime safety?
r/reactjs on Reddit: Is Null Checking Necessary in TypeScript for Runtime Safety?
August 12, 2024 -
TypeScript types are only enforced at compile time and do not exist at runtime. Given this, if I define a TypeScript function that accepts a string parameter, is it necessary to perform null checks? This question arises because if the API returns null, it could lead to runtime errors.
Consider the following code snippet:
const capitalise = (val: string) => {
// Question: Is this check redundant due to TypeScript's type checking?
if (!val) {
return '';
}
return val.toUpperCase();
};
// Assume 'name' is fetched from an API or another source that could return null
const name: string = /* fetch from API or another source */;
capitalise(name);What are the best practices in such scenarios to ensure runtime safety in TypeScript applications?
Top answer 1 of 9
35
Is your program going to work with a null value in that spot? <- focus on this
2 of 9
3
I would say that the type-checking isn't necessary in the capitalise function. It might in fact make it harder to discover the error later. I would suggest that the best course of action would be to perform a null check on all the values that can't be null on the data received after doing the API-call. Assuming the API-call is its own function (which is should be), as a part of that function, check each field in the data received for null or undefined values where you don't want a null or undefined value. If one is found, either throw an error or revert to default values ('' for string, -1 for int, and so on)
Scaler
scaler.com › topics › typescript › strictnullchecks
StrictNullChecks in TypeScript - Scaler Topics
February 8, 2023 - The easiest approach to check for ... distinguish null from the other false values. In JavaScript, use the equality operators == or === to check for a null value....
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
There are different ways we can check both null or undefined in TypeScript or Angular. By using simple if condition. By using TypeScript Nullish Coalescing & Optional chaining. By using Array.include() function. TypeScript is super set of JavaScript.
TypeScript
typescriptlang.org › docs › handbook › release-notes › typescript-3-7.html
TypeScript: Documentation - TypeScript 3.7
For more details, check out their pull request and the nullish coalescing proposal repository. ... There’s a specific set of functions that throw an error if something unexpected happened. They’re called “assertion” functions. As an example, Node.js has a dedicated function for this called assert. ... In this example if someValue isn’t equal to 42, then assert will throw an AssertionError. Assertions in JavaScript ...
Honlsoft
honlsoft.com › blog › 2021-07-20-typescript-tips-null-coalescing
Typescript Tips: null(and undefined) Checking | Honlsoft
The Elvis operator is a shorthand way to check for null or undefined without requiring a separate if statement. It looks something like this in practice. console.log(obj?.hello) // logs null if the obj is null, and does not reference hello · Well this is a simple case, some more uncommon ones I don't often see samples for are arrays and function calls. What if I want to access an object as an array, but it could be null or undefined?
C# Corner
c-sharpcorner.com › article › explain-null-handling-in-typescript
Explain Null Handling in TypeScript
March 21, 2024 - Null handling in TypeScript involves managing null and undefined values effectively to prevent runtime errors. Techniques like nullable types, optional chaining, type guards, and non-null assertion operators help ensure code reliability.
Tektutorialshub
tektutorialshub.com › home › typescript › null vs undefined in typescript
Null Vs Undefined in TypeScript - Tektutorialshub
March 15, 2023 - This is a very old bug in JavaScript · The only value that you can assign to an undefined variable is undefined. You can assign null only if StrictNullCheck is disabled). Any other values like string, object, numbers, etc are not allowed. The only value that you can assign to it is null. You can also assign undefined only if StrictNullCheck is disabled. Both null & undefined is falsy value in TypeScript.
Reddit
reddit.com › r/typescript › how to reason about possibly null values during development?
r/typescript on Reddit: How to reason about possibly null values during development?
January 12, 2024 -
I'm coming at this as a typescript beginner and haven't had a deep dive into the docs. So apologies if I'm missing something very obvious.
During development specifically, I feel like I'm adding null checks to my code too early to make TS happy, and this makes it harder to reason about my code. It'll go like this:
-
I perform an operation on a variable that can be null sometimes
-
TS alerts me to this, so I add a null check with an elvis operator/ternary/if
-
The variable is in fact null in an unexpected scenario, so I have a functional error in my app
-
Because I'm null-checking to shush the compiler, I don't immediately know where the bad value originates from and have to dig around for it
I can imagine this getting very annoying once I'm several layers of calls deep. So, how do I think about this? Should I be checking for null values differently? Should I always be doing something in the control flow to indicate an unexpected null was passed? Should I be throwing more?
Advice welcome. Thanks!
Top answer 1 of 10
61
If null isn’t truly valid for a variable, the typedef shouldn’t allow null in the first place. When you’re receiving input from somewhere (i.e a db query, or from client input), do your validation there and throw an error if the data is null when it shouldn’t be. From that point on, all other components in your application can type that data as non-nullable, and you don’t have to worry about null checks anymore! In general, design your types to represent the data as you want to deal with it within your application, and sanitize or validate on the boundaries coming into your system.
2 of 10
28
It sounds like TS is doing what its supposed to be doing The variable is in fact null in an unexpected scenario, so I have a functional error in my app Find out where and either make it not null or throw some kind of error. Because I'm null-checking to shush the compiler, I don't immediately know where the bad value originates from and have to dig around for it this is what I find most people starting with typescript struggle with. They just do whatever they can to shut the compiler up instead of realizing that the compiler is trying to warn you that there is maybe something you haven't thought of. Rather than endlessly add null checks and if statements refactor the code to not make it an issue anymore https://wiki.tcl-lang.org/page/Define+Errors+Out+of+Existence
Gitbooks
hamednourhani.gitbooks.io › typescript-book › content › docs › tips › null.html
null is bad · typescript-book
If your code base interacts with other APIs that might give you a null you check with == undefined (instead of ===). Using this is safe even for other potentially falsy values. /// Image you are doing `foo == undefined` where foo can be one of: console.log(undefined == undefined); // true ...
Shinesolutions
shinesolutions.com › home › writing safer code with typescript strict null checks & type guards
Writing safer code with TypeScript strict null checks & type guards - Shine Solutions Group
January 6, 2017 - In this article I will show you how to write safer TypeScript code by leveraging a feature called strictNullChecks. It’s quite easy to miss because it’s turned off by default, but can help a lot to produce more robust code. I’ll also briefly introduce another lesser-known feature of the language called type guards. Some Java full-stack developers (like me) always wanted to have statically typed JavaScript.