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!');

July 23, 2025 - We can use typeof or '==' or '===' to check if a variable is null or undefined in typescript.

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.

The compiler checks that variables are definitely assigned by performing control flow based type analysis. See later for further details on this topic. Optional parameters and properties automatically have undefined added to their types, even when their type annotations don’t specifically include undefined. For example, the following two types are identical: ... A property access or a function call produces a compile-time error if the object or function is of a type that includes null or undefined.

January 2, 2021 - If you want to check if the property is undefined like in my example - a simple if-statement with typeof is the way to go.

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 ...

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. This operator returns the left-hand side of the expression if it’s not null or undefined, and the right-hand ...

June 8, 2023 - If you want to check if a value is specifically undefined or null without type coercion, you can use the identity operator (===). The identity operator checks for both value and type equality, which means it does not perform type coercion.

August 18, 2020 - So to check if a variable is defined or not at a global level you normally use typeof: Copy · 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: Copy ·

November 7, 2024 - With this we can now use the datatype to check undefined for all types of data as we saw above. Here is what the check will look like for all three scenarios we have considered: if(typeof user.hobby === "undefined"){} if(typeof scores[10] === ...

Admittedly, this is a rare occurrence which is why I recommend the solution above. In cases where you're not sure if a variable has been declared, you can use the typeof operator and compare it to the string of 'undefined'.

March 8, 2023 - Here is how to check if a constant is falsy in TypeScript: typescriptconst myEmail: string | undefined; if (!myEmail) { console.log('email is undefined'); } This code not only checks for undefined but also for other falsy values.

September 21, 2022 - type AuthorHandle = string | undefined | null const twitter: AuthorHandle // ❌ This will throw an error (Cannot read properties of undefined) const formattedHandle: string = twitter.toLowerCase() // ❌ This will throw an error for null (Cannot read properties of null) if (twitter !== undefined) { const formattedHandle: string = twitter.toLowerCase() } // ✔️ Check if variable is not undefined OR null if (twitter !== undefined && twitter !== null) { const formattedHandle: string = twitter.toLowerCase() } // Note that null and undefined are not the same thing undefined === null -> false

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.

July 24, 2022 - const isNotNullable = <T>(element: T | null | undefined): element is T => typeof element !== "undefined" && element !== null; const func = (item: { name: string } | null | undefined) => { if (isNotNullable(item)) { // Here `item` is of type `{ name: string }`, guaranteed! return item.name; } return "Nullable!"; } console.log(func({ name: 'Typescript' })); // Typescript console.log(func(null)); // Nullable!

July 8, 2025 - let x; if (x === undefined) { // these statements execute } else { // these statements do not execute } 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.

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> ...