typeof undefined typescript - 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');

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!

Videos

stackoverflow.com

variable === undefined vs. typeof variable === "undefined"

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Historical mistake in JavaScript | typeof null is 'object'? ...

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October 26, 2023

JavaScript’s “typeof” Quirks Explained (typeof null) - YouTube

14 typeof, undefined and null Part 1 | - YouTube

geeksforgeeks.org › typescript › how-to-check-null-and-undefined-in-typescript

How to check null and undefined in TypeScript ? - GeeksforGeeks

July 23, 2025 - To make a variable null we must assign null value to it as by default in typescript unassigned values are termed undefined. We can use typeof or '==' or '===' to check if a variable is null or undefined in typescript.

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

TypeScript: Documentation - TypeScript 2.0

So, whereas T and T | undefined are considered synonymous in regular type checking mode (because undefined is considered a subtype of any T), they are different types in strict type checking mode, and only T | undefined permits undefined values.

developer.mozilla.org › en-US › docs › Web › JavaScript › Reference › Operators › typeof

typeof - JavaScript | MDN

typeof is generally always guaranteed to return a string for any operand it is supplied with. Even with undeclared identifiers, typeof will return "undefined" instead of throwing an error.

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.

github.com › microsoft › TypeScript › issues › 50548

TypeScript fails to narrow out `undefined` via `typeof` check on generic indexed access · Issue #50548 · microsoft/TypeScript

August 30, 2022 - Inside a conditional which can only be entered if a constant type is not undefined, TypeScript excludes | undefined from the possible types of the constant.

Published Aug 30, 2022

typescriptlang.org › docs › handbook › advanced-types.html

TypeScript: Documentation - Advanced Types

Luckily, you don’t need to abstract typeof x === "number" into its own function because TypeScript will recognize it as a type guard on its own. That means we could just write these checks inline. ... These typeof type guards are recognized in two different forms: typeof v === "typename" and typeof v !== "typename", where "typename" can be one of typeof operator’s return values ("undefined", "number", "string", "boolean", "bigint", "symbol", "object", or "function").

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The Valley of Code

thevalleyofcode.com › how-to-check-undefined-property-javascript

How to check if a JavaScript object property is undefined

if (typeof car.color === 'undefined') { // color is undefined } Want to master TypeScript?

thisthat.dev › variable-undefined-vs-typeof-variable-undefined

variable === undefined vs typeof variable === "undefined" - this vs that

It's not the case in modern browsers nowadays. From ES5, undefined can't be changed because its Writable property is set to false. Always use typeof.

reddit.com › r/typescript › undefined vs null

r/typescript on Reddit: Undefined vs null

February 27, 2023 -

Since switching to TypeScript I have been using a lot of optional properties, for example:

type store = {
  currentUserId?: string
}

function logout () {
  store.currentUserId = undefined
}

However my coworkers and I have been discussing whether null is a more appropriate type instead of undefined, like this:

type store = {
  currentUserId: string | null
}

function logout () {
  store.currentUserId = null
}

It seems like the use of undefined in TypeScript differs slightly from in Javascript.

Do you guys/girls use undefined or null more often? And, which of the examples above do you think is better?

One reason to favor undefined over null is how javascript handle default values: const withDefault = (a = true) => { console.log(a); }; withDefault(); // logs true withDefault(undefined); // logs true withDefault(null); // logs null

They have different meanings to be: undefined means that it was never set or defined, nobody touched it. Null means someone explicitly set it to null to signal a non-value. Of course, Typescript is very permissive, you can use undefined for everything and just ditch the null, I've seen this approach used well. Or you can just use null for everything, although you may still find undefined specially if your TS settings aren't strict (they should be as strict as possible imo)

geeksforgeeks.org › javascript › variable-undefined-vs-typeof-variable-undefined-in-javascript

variable === undefined vs. typeof variable === “undefined” ...

July 12, 2025 - Check by Value (Strict equality Operator): Here you will get whether the variable is assigned a value or not if the variable is not assigned a value it will display undefined. Check the type (Typeof operator): Here you will get what type of variable was that if there is no variable assigned then it will display "undefined".

2ality.com › 2013 › 04 › check-undefined.html

Checking for undefined: === versus typeof versus falsiness

It has two advantages: First, it is safe with regard to a changed undefined (not that important under ECMAScript 5). Second, it also works for unknown variables: > typeof iDontKnowThisVariable === 'undefined' true > iDontKnowThisVariable === undefined ReferenceError: iDontKnowThisVariable is not defined

stackoverflow.com › questions › 2703102 › typeof-undefined-vs-null

The difference between `typeof x !== "undefined"` and `x != null`

typeof is safer as it allows the identifier to never have been declared before:

if(typeof neverDeclared === "undefined") // no errors

if(neverDeclared === null) // throws ReferenceError: neverDeclared is not defined

If the variable is declared (either with the var keyword, as a function argument, or as a global variable), I think the best way to do it is:

if (my_variable === undefined)

jQuery does it, so it's good enough for me :-)

Otherwise, you'll have to use typeof to avoid a ReferenceError.

If you expect undefined to be redefined, you could wrap your code like this:

(function(undefined){
    // undefined is now what it's supposed to be
})();

Or obtain it via the void operator:

const undefined = void 0;
// also safe

Tektutorialshub

tektutorialshub.com › home › typescript › null vs undefined in typescript

Null Vs Undefined in TypeScript - Tektutorialshub

March 15, 2023 - Both null & undefined is falsy ... false. 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 ......

reddit.com › r/javascript › [askjs] why does typeof undefined return "undefined" — and is there any actual use case where this is helpful?

r/javascript on Reddit: [AskJS] Why does typeof undefined return "undefined" — and is there any actual use case where this is helpful?

April 16, 2025 -

I’ve seen this behavior for years, but I’m trying to understand if there’s a real-world use case where typeof undefined === "undefined" is practically useful, versus just a quirky historical thing.

For example, in older codebases, I see checks like if (typeof myVar === "undefined"), but nowadays with let, const, and even nullish coalescing, this feels outdated.

So — is there a valid modern use case for typeof undefined comparisons, or is it mostly just something legacy that we put up with?

typeof still needs to return something for undefined, so what would you propose it returns? It might've been historically useful when undefined could be rebound to something else, but in modern code a normal value === undefined is enough (or == if you want to check for null as well). You can also use the nullish coalescing operator, but sometimes you just simply want to know if a value is undefined and that's all.

You can't really check for undefined through nullish coalescing since it doesn't differentiate between null and undefined, which are different things in JS. I have no idea what you mean with let and const, something like const c = undefined; is valid code. The reason why older codebases would check for undefined via typeofis that it will never fail. Back then, you could (and still can, outside of strict mode) assign whatever value to undefined, so e.g undefined = 42 would be valid code and would mess up any future attempt to compare a variable or property to undefined. This did not influence typeof and it would work regardless. Nowadays, strict mode won't allow you to override undefined, but honestly if you're looking for undefined specifically you should still be using typeof for consistency with other types. On the other hand, typeof null === "object", so maybe we're cooked either way.

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

How to Check if a Variable is Undefined in JavaScript

This article delves into how to accurately check for undefined variables and explores why this knowledge is vital in modern web and application development. The typeof operator is a reliable method to check if a variable is undefined.

Tutorial Republic

tutorialrepublic.com › faq › how-to-determine-if-variable-is-undefined-or-null-in-javascript.php

How to Determine If Variable is Undefined or NULL in JavaScript

So the correct way to test undefined variable or property is using the typeof operator, like this: if(typeof myVar === 'undefined').

developer.mozilla.org › en-US › docs › Web › JavaScript › Reference › Global_Objects › undefined

undefined - JavaScript | MDN

Note: The strict equality operator (as opposed to the standard equality operator) must be used here, because x == undefined also checks whether x is null, while strict equality doesn't. This is because null is not equivalent to undefined. See Equality comparison and sameness for details. ... One reason to use typeof is that it does not throw an error if the variable has not been declared.

delftstack.com › home › howto › typescript › undefined checking in typescript

How to Check for Undefined in TypeScript | Delft Stack

February 2, 2024 - If you use == at the root level ... and the whole call stack unwinds. So for the checking, if the variable is undefined or not at the root level, typeof is suggested....