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)
{
}
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
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 errors. You just learned how you could both improve the readability of your ...
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');
Videos
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 ...
04:07
Using Null & Undefined | strictNullChecks | TypeScript Tutorial ...
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What you can do is check whether you props is defined initially or not by checking if nextProps.blog.content is undefined or not since your body is nested inside it like
componentWillReceiveProps(nextProps) {
if(nextProps.blog.content !== undefined && nextProps.blog.title !== undefined) {
console.log("new title is", nextProps.blog.title);
console.log("new body content is", nextProps.blog.content["body"]);
this.setState({
title: nextProps.blog.title,
body: nextProps.blog.content["body"]
})
}
}
You need not use type to check for undefined, just the strict operator !== which compares the value by their type as well as value
In order to check for undefined, you can also use the typeof operator like
typeof nextProps.blog.content != "undefined"
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I was face same problem ..... And I got solution by using typeof()
if (typeof(value) !== 'undefined' && value != null) {
console.log('Not Undefined and Not Null')
} else {
console.log('Undefined or Null')
}
You must have to use typeof() to identified undefined
Tim Mousk
timmousk.com › blog › typescript-check-for-undefined
How To Check For Undefined In TypeScript? – Tim Mouskhelichvili
March 8, 2023 - As you can see, checking for undefined in TypeScript is simple. You just need to do a condition check, and voila. Here are other TypeScript tutorials for you to enjoy: ... Hello! I am Tim Mouskhelichvili, a Freelance Developer & Consultant from Montreal, Canada. I specialize in React, Node.js ...
Webtips
webtips.dev › solutions › check-undefined-in-typescript
How to Correctly Check Undefined in TypeScript - Webtips
September 21, 2022 - To check for undefined values in TypeScript, you need to use the strict equal operator against the undefined keyword.
W3Schools
w3schools.com › typescript › typescript_null.php
TypeScript Null & Undefined
Nullish coalescing is another JavaScript feature that also works well with TypeScript's null handling. It allows writing expressions that have a fallback specifically when dealing with null or undefined.
Reddit
reddit.com › r/typescript › null or undefined
r/typescript on Reddit: Null or undefined
December 25, 2022 -
Hello guys, i need your help, i would like to know which you think is a better approach:
1 --------------------------------------------
const {content} = props;
if(content !== "undefined"){
//code
}
2 --------------------------------------------
const {content = null} = props;
if(!content){
//code
}
Top answer 1 of 4
6
I try to use null as an empty value and undefined as an unprovided value. In practice though, most of the code I interact with treats them the same. Ultimately unless you need the distinction between "empty" and "not provided" you can use them interchangeably. One thing to note, the condition !content will evaluate to true for any falsy value (null, undefined, 0, '', NaN, false) so you might get false positives if you just want to check for null.
2 of 4
5
"undefined", though, is without the double quotes.
CodingDeft
codingdeft.com › posts › how-to-solve-typescript-possibly-undefined-value
Object is possibly 'undefined' error in TypeScript [Solved] | CodingDeft.com
Nullish coalescing is similar to logical OR operator except that it evaluates to the right side operand only if the left side operand is null or undefined, whereas logical OR (||) operator does it for other falsy values like an empty string, 0, false, etc. You can use nullish coalescing operator as shown below: ... These are the few ways in which you can fix the error. You can use any one of them based on your use case. If you have liked article, do follow me on twitter to get more real time updates! How to fix React Hook warnings for async functions in useEffect
Bobby Hadz
bobbyhadz.com › blog › react-null-or-undefined-check
Check if a Variable is Null or Undefined in React | bobbyhadz
To check if a variable is null or undefined in React, use the || (or) operator to check if either of the two conditions is met.
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
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What are your type defs? Did you make events optional?
Coding with Rashid
reactnativeforyou.com › home › how to check undefined property in react native
How to Check undefined Property in React Native – REACT NATIVE FOR YOU
May 12, 2019 - The typeof returns string ‘undefined’ when the value is not defined. Hence you can check the value undefined or not with that string. ... const book = { title: 'ReactNativeForYou' }; console.log(typeof book.author); //undefined if (typeof book.author === 'undefined'){ //Do your things }
TypeScript
typescriptlang.org › docs › handbook › release-notes › typescript-2-0.html
TypeScript: Documentation - TypeScript 2.0
The type checker previously considered null and undefined assignable to anything. 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.
2ality
2ality.com › 2013 › 04 › check-undefined
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.
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.
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 - So, I suggest that you always use the typeof operator if you are checking for undefined. Let's move your learning forward together! Follow me on Twitter for your daily dose of developer tips.
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?
GeeksforGeeks
geeksforgeeks.org › typescript › how-to-check-null-and-undefined-in-typescript
How to check null and undefined in TypeScript ? - GeeksforGeeks
July 23, 2025 - Example: Below is an example demonstrating the use of optional chaining and nullish coalescing operators to check for null or undefined values. We declare variables that can be either undefined, null, or assigned a value. let undefinedVar: string | undefined; let nullVar: string | null = null; let assignedVar: string | null | undefined = "Hello, World!"; ... let undefinedVar: string | undefined; let nullVar: string | null = null; let assignedVar: string | null | undefined = "Hello, World!"; const checkUndefinedVar = undefinedVar ??
Coding Beauty
codingbeautydev.com › home › posts › how to check if a variable is null or undefined in react
How to Check if a Variable is Null or Undefined in React
October 4, 2023 - Instead of checking explicitly for null or undefined, you might be better off checking if the variable is falsy. JavaScriptCopied! import { useState } from 'react'; export default function App() { const [variable] = useState(undefined); const [message, setMessage] = useState(undefined); const falsyCheck = () => { // Check if falsy if (!variable) { setMessage('✅ Variable is falsy'); } else { setMessage('❌ Variable is NOT falsy'); } }; return ( <div> <button onClick={() => falsyCheck()}>Check variable</button> <h2>{message}</h2> </div> ); }
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.
Linguine Code
linguinecode.com › home › blog › how to solve typescript possibly undefined value
How to solve TypeScript possibly undefined value
September 26, 2020 - This article goes over 5 methods to solve a problem like TS2532: Object is possibly ‘undefined’. You can use IF conditions, logical operator, non-null assertion and more.