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. The same is true for the relationship of T to ...

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.

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").

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> ...

TypeScript has a powerful system to deal with null or undefined values. By default null and undefined handling is disabled, and can be enabled by setting strictNullChecks to true. The rest of this page applies for when strictNullChecks is enabled.

November 14, 2024 - if (value != null) { // Do something if value is not null or undefined } Prefer strict equality checks (!== or ===) for better type safety and clarity.

March 15, 2023 - But using the strict equality checker (===) returns false because the data type of null is different from the data type of undefined. The boolean value of undefined is considered as falsy. i.e. TypeScript implicitly converts the value undefined ...

February 18, 2025 - Loose equality can be less precise than strict equality because it also returns true for null. The void operator evaluates an expression and returns undefined. It is typically used to explicitly return undefined from a function.

March 18, 2025 - In TypeScript, the handling of null and undefined can be stricter with strictNullChecks. When enabled, variables cannot be null or undefined unless explicitly allowed. ... This condition is true for null, undefined, 0, NaN, "" (empty string), ...

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.

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.

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. C#, Java) behave. The lack of checking for these values tends to be a major source of bugs; we always recommend people turn strictNullChecks on if it’s practical to do so in their codebase.

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.

February 28, 2024 - Efficiently debug & fix TypeScript null or undefined value errors. Expert tips for handling & resolving errors in TypeScript.

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

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.

March 8, 2023 - In TypeScript, you can use the optional chaining operator to check if an object is undefined before calling a function. ... In this example, if the myEmail constant is undefined, the indexOf function is never called (so the code doesn't break).

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. For further actions, you may consider blocking this person and/or reporting abuse