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)
{
}
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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');
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?
(!data) vs. ( data == undefined)
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. More on reddit.com
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September 25, 2024Checking if field in a union type is defined raises TS error
The short answer is that that’s just how TypeScript works. You can’t reference properties that don’t definitively exist. In general a type guard like you wrote in the way to get around this, but luckily in your specific case, there is a better way. Just check in your test function if typeof val === ‘string’ Wrap that check in a condition, and anywhere that condition is evaluated as falsy, val will be statically assumed to be ComplexType (by process of elimination). More on reddit.com
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December 19, 2023How to check variable for 0 or higher number?
Why not just do something like: if (typeof myval === 'number' && myval >= 0) { // do something } typeof myval === 'number' will make sure myval is a number (not null nor undefined), and myval >= 0 makes sure it is not NaN and a value greater or equal than 0 (no need to check if myval is NaN separately) More on reddit.com
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May 23, 2022Basic JS question: when to check for undefined, null, etc
TL;DR: Use value != null. It checks for both null and undefined in one step.
In my mind, there are different levels of checking whether something exists:
0) 'property' in object - Returns true if the property exists at all, even if it's undefined or null.
-
object.property !== undefined- Returns true if the property exists and is not undefined. Null values still pass. -
object.property != null- Return true if the property exists and is not undefined or null. Empty strings and 0's still pass. -
!!object.property- Returns true if the property exists and is "truthy", so even 0 and empty strings are considered false.
From my experience, level 2 is usually the sweet spot. Oftentimes, things like empty strings or 0 will be valid values, so level 3 is too strict. On the other hand, levels 0 and 1 are usually too loose (you don't want nulls or undefineds in your program). Notice that level 1 uses strict equality (!==), while level 2 uses loose equality (!=).
More on reddit.com 15
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September 10, 2016Videos
04:07
Using Null & Undefined | strictNullChecks | TypeScript Tutorial ...
05:01
null vs. undefined in JavaScript / TypeScript - YouTube
08:03
Javascript "undefined" - the best way to check this for every ...
TypeScript Tutorial #12: Handling Null and Undefined in Just ...
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
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!
W3Schools
w3schools.com › typescript › typescript_null.php
TypeScript Null & Undefined
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.
Omarileon
omarileon.me › blog › typescript-null-undefined
mari. | How to Detect Null and Undefined in Your TypeScript Code
February 27, 2024 - The most straightforward way of checking is of course the equality operator. const myValue = maybeNullOrUndefined(); if (myValue === null) console.log('Null!'); if (myValue === undefined) console.log('Undefined!');
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.
Akos Komuves
akoskm.com › how-to-solve-variable-is-possibly-undefined-in-typescript-even-when-its-defined
How to solve ”variable is possibly undefined” in TypeScript - even when it’s defined | Akos Komuves
function PlayerWelcome({ players ... safely handled. The solution is to either explicitly check for the value before usage or pass the value as a required argument to functions, ensuring type safety and preventing runtime ...
Index.dev
index.dev › blog › check-undefined-variable-javascript
How to Check if a Variable is Undefined in JavaScript
January 21, 2025 - // 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> ...
TypeScript
typescriptlang.org › docs › handbook › 2 › everyday-types.html
TypeScript: Documentation - Everyday Types
The lack of checking for these values tends to be a major source of bugs; we always recommend people turn strictNullChecks on if it’s practical to do so in their codebase. With strictNullChecks on, when a value is null or undefined, you will need to test for those values before using methods or properties on that value. Just like checking for undefined before using an optional property, we can use narrowing to check for values that might be null: ... TypeScript also has a special syntax for removing null and undefined from a type without doing any explicit checking.
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?
November 14, 2024 - You want to check for both null and undefined together, since null and undefined are considered equal in loose equality (==). Commonly used when you don’t care about the specific type of “empty.” · if (value != null) { // Do something if value is not null or undefined }
BrowserStack
browserstack.com › home › guide › how to check if a variable is undefined in javascript
How to Check if a Variable is Undefined in JavaScript | BrowserStack
February 18, 2025 - The typeof operator can be used to check if a variable is undefined by comparing its type with the string ‘undefined’. Here is an example of the code.
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.
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I never use ==
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 - Null might mean the search has been completed, but the data wasn’t found, whereas undefined might indicate that the fetch was never completed successfully. Another way to check for null or undefined is to use the nullish coalescing operator (??), ...
freeCodeCamp
freecodecamp.org › news › javascript-check-if-undefined-how-to-test-for-undefined-in-js
JavaScript Check if Undefined – How to Test for Undefined in JS
November 7, 2024 - But we would replace undefined with void(0) or void 0 as seen below: if(typeof user.hobby === void 0){} if(typeof scores[10] === void 0){} if(typeof name === void 0){} ... if(typeof user.hobby === void(0)){} if(typeof scores[10] === void(0)){} ...
Tim Mousk
timmousk.com › blog › typescript-check-for-undefined
How To Check For Undefined In TypeScript? – Tim Mouskhelichvili
March 8, 2023 - typescriptconst myName: string | undefined; if (myName === undefined) { console.log('name is undefined'); } This article will explain checking for undefined in TypeScript and answer some of the most common questions.
SPGuides
spguides.com › typescript-check-if-undefined
TypeScript Check If Undefined [With Examples]
July 9, 2025 - TypeScript understands that after this check, the value is not undefined. Helps with strict type safety. From my experience, I am sharing with you some common problems and best practices. TypeScript treats undefined and null as separate types. If you want to check for both, use:
Medium
medium.com › deno-the-complete-reference › five-ways-to-check-for-undefined-in-javascript-b5568090df77
Five ways to check for undefined in JavaScript | Tech Tonic
March 10, 2024 - Use strict equality to compare the result with the string "undefined". This can be useful in cases where you need to handle other data types that might evaluate to false in an if statement (e.g., 0, empty string).
MDN Web Docs
developer.mozilla.org › en-US › docs › Web › JavaScript › Reference › Global_Objects › undefined
undefined - JavaScript | MDN
The undefined global property represents the primitive value undefined. It is one of JavaScript's primitive types. function test(t) { if (t === undefined) { return "Undefined value!"; } return t; } let x; console.log(test(x)); // Expected output: "Undefined value!"
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.
ui.dev
ui.dev › check-for-undefined-javascript
How to check for undefined in JavaScript
Checking for `undefined`` this way will work in every use case except for one, if the variable hasn't been declared yet.