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)
{
}
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');
W3Schools
w3schools.com › typescript › typescript_null.php
TypeScript Null & Undefined
Even with strictNullChecks enabled, by default TypeScript assumes array access will never return undefined (unless undefined is part of the array type).
Videos
04:07
Using Null & Undefined | strictNullChecks | TypeScript Tutorial ...
TypeScript Tutorial #12: Handling Null and Undefined in Just ...
05:01
null vs. undefined in JavaScript / TypeScript - YouTube
02:40
TypeScript Tutorial #12: Handling Null and Undefined in Just 2 ...
05:39
Null vs Undefined - which one should you use in JS? - YouTube
04:52
Solving your “undefined” problem with well-placed errors ...
TypeScript
typescriptlang.org › docs › handbook › release-notes › typescript-2-0.html
TypeScript: Documentation - TypeScript 2.0
Effectively, null and undefined were valid values of every type and it wasn’t possible to specifically exclude them (and therefore not possible to detect erroneous use of them). strictNullChecks switches to a new strict null checking mode.
DEV Community
dev.to › kais_blog › how-to-check-for-undefined-in-javascript-typescript-3men
How to Check For Undefined in JavaScript / TypeScript - DEV Community
January 2, 2021 - The correct way to check if something is undefined in JavaScript / TypeScript: Use the typeof operator!
Reddit
reddit.com › r/typescript › (!data) vs. ( data == undefined)
r/typescript on Reddit: (!data) vs. ( data == undefined)
September 25, 2024 -
What do you prefer to check for null and undefined?
Feel free to comment on why which one is better.
Top answer 1 of 5
24
If you want to check only for null and undefined, I'd prefer data == null, which checks both explicitly but ignores other falsy values like 0 or empty strings. If you are okay with all falsy values (including 0, false, empty strings, etc.), then !data works well for simplicity.
2 of 5
18
I never use ==
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.
Reddit
reddit.com › r/typescript › how check for undefined in conditional type
r/typescript on Reddit: How check for undefined in conditional type
November 14, 2023 -
This code do not work at all, but not sure why.
type IsUndefined<T> = T extends undefined ? 1 : 0;
For example:
type IsTypeUndefined = IsUndefined<number | undefined>; // This returns: 1 | 0
Is there a way for check if a type is undefined in a conditional?
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.
Palantir
palantir.com › docs › foundry › functions › undefined-values
Functions • TypeScript v1 • Handle undefined values • Palantir
1 2 3 4 5 6 7 @Function() public getFullName(employee: Employee): string { if (!(employee.firstName && employee.lastName)) { throw new UserFacingError("Cannot derive full name because either first or last name is undefined."); } return employee.firstName + " " + employee.lastName; } By checking that both the firstName and lastName fields are defined, the TypeScript compiler knows that the final line with the return statement can compile correctly.
Index.dev
index.dev › blog › check-undefined-variable-javascript
How to Check if a Variable is Undefined in JavaScript
// Advanced type checking patterns type TypePredicate<T> = (value: unknown) => value is T; type Nullable<T> = T | null | undefined; // Custom type guard implementation const createTypeGuard = <T>(check: (value: unknown) => boolean): TypePredicate<T> ...
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.
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 › typescript not acknowledging 'not undefined' assertion
r/typescript on Reddit: Typescript not acknowledging 'not undefined' assertion
November 29, 2023 -
Update: For simplicity, I replicated the original error with simpler code.
Update 2: I got around this by using a type guard for T. Thank u/ritajalilip for the suggestion. I still think Typescript should handle this better though.
As you can see on the screenshot, inside the if statement I'm asserting that x is not undefined. However Typescript is failing to acknowledge and still thinks that x can be undefined.
Any idea of what's going on?
Top answer 1 of 5
4
Typescript won't automatically narrow down the type of a variable if you narrow down it's property. You can: Make a type guard for Required Return the narrowed down events explicitly: return { ...receipt, events: receipt.events } Type cast return receipt as Required
2 of 5
3
What are your type defs? Did you make events optional?
Tim Mousk
timmousk.com › blog › typescript-check-for-undefined
How To Check For Undefined In TypeScript? – Tim Mouskhelichvili
March 8, 2023 - Other JavaScript falsy values include: false, 0, -0, 0n, '', null, NaN. In TypeScript, you can use the optional chaining operator to check if an object is undefined before calling a function.
LambdaTest Community
community.lambdatest.com › general discussions
How do you check if a variable is undefined in TypeScript? I am looking for ways to resolve typescript check if undefined - LambdaTest Community
August 2, 2024 - How do you check if a variable is undefined in TypeScript? I am looking for ways to resolve typescript check if undefined.
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:
Delft Stack
delftstack.com › home › howto › typescript › undefined checking in typescript
How to Check for Undefined in TypeScript | Delft Stack
February 2, 2024 - The first line sets the data type of variable userEmail as either string or undefined. After setting the datatype, it validates the variable in the if condition. === in TypeScript will allow checking both the variable type as well as its value and performs the desired operation after the validation.
Atomic Spin
spin.atomicobject.com › optional-undefined-typescript
How to Deal with “Optional” and “Undefined” in TypeScript
November 25, 2024 - TypeScript understands a number of these kinds of checks and can use them to narrow the picture of what types can possibly make it through to a specific bit of code. We can use a typeof check against the bar property to see if it is undefined.
2ality
2ality.com › 2013 › 04 › check-undefined.html
Checking for undefined: === versus typeof versus falsiness
Recommendation: check for undefined either via === undefined or via falsiness. It is normally more important for code to be easy to understand than to be completely safe.
Zipy
zipy.ai › blog › debug-typescript-null-or-undefined-value-errors
Solving Typescript Null & Undefined Errors: Expert Tips
February 28, 2024 - This design choice is inherited from JavaScript, providing flexibility in indicating the absence of a value (null) versus an uninitialized variable (undefined). ... Zipy offers proactive error monitoring and session replay capabilities, allowing developers to see exactly what led to an error, including the user's actions and the application state at the time of the error. Understand TypeScript's Type System: Leveraging TypeScript's strict typing can prevent many Null or Undefined Value Errors.