typescript undefined check - Brave Search

How to check undefined in TypeScript

stackoverflow.com › questions › 43716263 › how-to-check-undefined-in-typescript

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)
{

}

Answer from ashish on Stack Overflow

stackoverflow.com › questions › 43716263 › how-to-check-undefined-in-typescript

How to check undefined in TypeScript - Stack Overflow

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)
{

}

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.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

Using Null & Undefined | strictNullChecks | TypeScript Tutorial ...

TypeScript Tutorial #12: Handling Null and Undefined in Just 2 ...

November 4, 2024

Null vs Undefined - which one should you use in JS? - YouTube

October 30, 2023

Solving your “undefined” problem with well-placed errors ...

February 13, 2023

null vs. undefined in JavaScript / TypeScript - YouTube

TypeScript Tutorial #12: Handling Null and Undefined in Just ...

typescriptlang.org › docs › handbook › release-notes › typescript-2-0.html

TypeScript: Documentation - TypeScript 2.0

TypeScript has two special types, Null and Undefined, that have the values null and undefined respectively. Previously it was not possible to explicitly name these types, but null and undefined may now be used as type names regardless of type checking mode.

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!

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.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.

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.

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.

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.

Find elsewhere

Google Bing Mojeek

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.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.

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?

try undefined extends T

Wrap it in a tuple: [undefined] extends [T] ? true : false this will check for never types cases. Also to be more robust you can check if T is 'any' before check for undefined.

index.dev › blog › check-undefined-variable-javascript

How to Check if a Variable is Undefined in JavaScript

TypeScript, a superset of JavaScript, provides static type checking at compile time, which eliminates many runtime errors and ensures robust code. // 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> => (value: unknown): value is T => check(value); // Implementation examples const isNonEmptyString = createTypeGuard<string>( (value): value is string => typeof value === 'string' && value.trim().length > 0 ); const isValidDate = createTypeGuard<Date>( (value): value is Date => value instanceof Date && !isNaN(value.getTime()) );

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?

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

What are your type defs? Did you make events optional?

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.

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:

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.

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.

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.

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.

stackoverflow.com › questions › 28975896 › is-there-a-way-to-check-for-both-null-and-undefined

typescript - Is there a way to check for both `null` and `undefined`? - Stack Overflow

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"

if( value ) {
}

will evaluate to true if value is not:

null
undefined
NaN
empty string ''
0
false

typescript includes javascript rules.