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');
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 - The logical AND operator (&&) evaluates to false if the left operand is false or undefined. You can use this in conjunction with a check for another value to ensure both conditions are met.
(!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
29
0
September 25, 2024Variable undefined check should goes through functions in front of them.
Search Terms variable, check, undefined Suggestion While checking whether a variable is undefined or null, checker should go through all the code including the function runned before the variable i... More on github.com
4
How do you check if a variable is undefined in TypeScript? I am looking for ways to resolve typescript check if undefined - LambdaTest Community
How do you check if a variable is undefined in TypeScript? I am looking for ways to resolve typescript check if undefined. More on community.lambdatest.com
0
August 2, 2024Why does TypeScript still think this number could be undefined?
Note: Be careful with “!someNumber”. Zero is also falsy. Try changing your check to “a === undefined && b === undefined”. The problem here with the type narrowing is that, even after the exception isn't thrown, TypeScript can't know which numbers are undefined and which are not. Try this: function myFunc(a?: number, b?: number): number { if (a !== undefined) { return a; } else if (b !== undefined) { return b; } else { throw new Error('No value provided') } } This appears to appease the type checker . I'm still not a huge fan of this function though, and that's because it makes a promise it can't keep from the signature alone. You're passing in two possible undefined values and it's saying it's going to always give you a number. The caller doesn't know the function can throw unless they go in and read the code. Functions should be black boxes, and until TypeScript adds checked exceptions, we unfortunately can't use exceptions to do things like this in a type-safe way. I'd try to think some more about what you're doing here and maybe refactor a bit. More on reddit.com
38
9
August 23, 2020Videos
TypeScript Tutorial #12: Handling Null and Undefined in Just ...
04:07
Using Null & Undefined | strictNullChecks | TypeScript Tutorial ...
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 ...
Codedamn
codedamn.com › news › javascript
How to check if value is undefined or null in JavaScript
June 8, 2023 - The easiest way to check if a value is either undefined or null is by using the equality operator (==). The equality operator performs type coercion, which means it converts the operands to the same type before making the comparison.
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 ==
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.
MDN Web Docs
developer.mozilla.org › en-US › docs › Web › JavaScript › Reference › Global_Objects › undefined
undefined - JavaScript | MDN
// x has not been declared before // evaluates to true without errors if (typeof x === "undefined") { // these statements execute } // Throws a ReferenceError if (x === undefined) { } However, there is another alternative. JavaScript is a statically scoped language, so knowing if a variable ...
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.
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).
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.
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> ...
GitHub
github.com › microsoft › TypeScript › issues › 25856
Variable undefined check should goes through functions in front of them. · Issue #25856 · microsoft/TypeScript
July 21, 2018 - I have a function that required a non-undefined argument. I do the required field check in a separate function and if the required variable is undefined or null, it should throw an error and the variable is not really used.
Published Jul 21, 2018
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 - Learn how to check if a variable is undefined in JavaScript with simple and effective methods to avoid errors in your code.
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
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.
ui.dev
ui.dev › check-for-undefined-javascript
How to check for undefined in JavaScript
The way I recommend to check for undefined in JavaScript is using the strict equality operator, ===, and comparing it to the primitive undefined.
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.
TypeScript
typescriptlang.org › docs › handbook › 2 › everyday-types.html
TypeScript: Documentation - Everyday Types
TypeScript has two corresponding types by the same names. How these types behave depends on whether you have the strictNullChecks option on. With strictNullChecks off, values that might be null or undefined can still be accessed normally, and the values null and undefined can be assigned to a property of any type. This is similar to how languages without null checks (e.g.
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 - In this article, you will learn the various methods and approaches you can use to know if a variable is undefined in JavaScript. This is necessary if you want to avoid your code throwing errors when performing an operation with an undefined variable. In case you are in a rush, here are the three standard methods that can help you check if a variable is undefined in JavaScript:
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.
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.