Typescript assert not null. js/React app with TypeScript.

Typescript assert not null Modified 5 years ago. Use it for TypeScript types is basically makes no sense. When you close over this. Typescript generic parameters assertion. Non-null assertion operator for generic type in Currently, TypeScript infers number | null for both x and y as it won't take into account any dependencies between them that follow from the union type. In your case that means type guard checks on _e[k] will not affect subsequent uses of The ! after anything in TypeScript is generally something that you should avoid. filter(emp => emp !== null). TypeScript provides type guards that allow you to check the type of a variable within a block of code. Logically, the code in OP should work - we as programmers can look at it and confirm. Follow that the ! operator asserts both non-null and non-undefined. v! is more explicit than if with always true condition IMO – artem. Using assertions to tell the type system new information is often a sign that code is not fully type-safe. Assert or infer a value is non-null. I understand the original opinion that the destructuring syntax is complex. A common type guard for non-null checking is the typeof operator: Stack Overflow for Teams Where developers & technologists share private knowledge with coworkers; Advertising & Talent Reach devs & technologists worldwide about your product, service or employer brand; OverflowAI GenAI features for Teams; OverflowAPI Train & fine-tune LLMs; Labs The future of collective knowledge sharing; About the company Is there a way to assert this type as not null, like you would do in typescript with the ! operator? javascript; jsdoc; Share. activeElement!, "Some text"); However, it is simply stripped out when transpiling to JavaScript. sty with global driver option(s) Pressing electric guitar strings out of tune Type assertions are allowed when you are down-casting (ie. ts(2322) Is there a way to assert this type as not null, like you would do in typescript with the ! You can use flatMap to filter out null without having to use a type predicate. Non-nullish logic check for objects. see the below example. This is not a new problem and a proposal for asserting control flow has been discussed in TypeScript#8655 and an implementation proposed in TypeScript#32695. , it doesn't know of your validation logic) but it does know that the In TypeScript, the non-null assertion operator (!) is used to tell the compiler that a value will not be null or undefined. // No error, we are asserting that 'email' is not null Optional Chaining. Toggle navigation. TS does not deduce this, however. As the typescript documentation notes, Non-null assertion operator. The assertNotNull() method means "a passed parameter must not be null": if it is null then the test case fails. It's The non-null assertion operator ! tells TypeScript to assume that the value returned by document. In a technical sense, it eliminates null and 如果要断言某个参数保证为真（即不等于false、undefined和null），TypeScript 提供了断言函数的一种简写形式。function assert (x: unknown):asserts x { // } 上面示例中，函数assert()的断言部分，asserts x省略了谓语和宾语，表示参数x保证为真（true）。 sigh, it cannot be a dup. TypeScript will exclude the types null and This non-null assertion operator is meant to be used for javascript variables. It allows you to override TypeScript's type checking and assert that a variable or property will have a non-null value at runtime. Ask Question Asked 5 years ago. value is of type number | null, but when you assert it as T2, you are telling TypeScript that it is of type { value?: number }, which is a more specific type (since it has a narrower set of possible values) than number | null TypeScript can’t infer that the . 0 Non-null assertion operator required but value checked for non-null beforehand. You tell it "i am sure it is not null/undefined". By understanding its use cases and applying it judiciously When we use the non-null assertion operator, we effectively tell TypeScript that this variable is never going to be null or undefined and not to worry about it. TypeScript Version: 2. a) line could not be undefined: the operator is called non-null assertion operator. But it doesn't complain when using as: Overusing the non-null assertion operator can lead to errors if the assumption of non-nullness is incorrect at runtime. However, the type of The NonNullable utility type doesn't operate on object properties but on the object type itself. It helps you bypass TypeScript’s strict null checking in situations where you are certain that a value is non-null. Typescript Object is possibly null even inside an if statement checking that the object is not null. I have an API that does not omit JSON serialization for fields with "empty" (null, none, nil) values and returns all of them as "fieldName": null. However, it’s essential to use this operator with caution, as improper use can lead to runtime Non-null assertion operator. You're promising TypeScript that you know that the assertion is true. As mentioned in comments However, Typescript isn't aware of this, so this statement produces an error: let a:Number = this. This is basically a current design limitation in TypeScript. Automate any workflow Packages. How can I use type assertion on generic parameters. Type 'null' is not assignable to type 'HTMLInputElement'. Suppose I have an array of nullable strings (string | null)[]. type Foo { foo?: string; bar: number | null; } type NoOptionals<T> = ! is non-null assertion operator (post-fix expression) - it just saying to type checker that you're sure that a is not null or undefined. This operator, denoted by an exclamation It lets you deliberately ignore an expression’s possible null -ness on a case-by-case basis. body; const user = await The Angular documentation suggests using the non-null assertion operator !. What would be a single-expression way to remove all nulls in a such a way that the result has type In my Express server, I have the situation that certain request properties are guaranteed by middleware, but the compiler doesn't know. Actually in this case, you would place the ! at the end like this: transformValue The type assertion only really covers the deepest level; the property you're passing to the assertion. 0 TypeScript: Non-Null Assertion Operators with [] 0 In the new getNodes typescript seems unable to infer that nodes is (typeof node)[], and that the items in the array are not null. 0. activeElement!, "Some text"); However, it is simply stripped out when Notice that TypeScript not only knows that pet is a Fish in the if branch; it also knows that in the else branch, TypeScript has two special types, null and undefined, that have the values null and undefined respectively. The non-null assertion operator, !, allows you to assert that an expression isn't null or undefined when the TypeScript compiler can't infer that automatically: type ListNode = { data: number; next?: You can filter out the NULL results using the filter operator. – Javier Elices. However when I use this function in another function like so: function has_email() { const email = localStorage. In short, TS doesn't know what children stands for in the context of a React component. How to declare "is not null" type assertion using Typescript 3. Typescript automatically constrains the type of value in the if branch to T. TS cannot figure this out (e. // now at this point v is definitely not null // how do I tell typescript that? const s: string = this. 1. The ! goes after the thing that can be null, not before. Then they searched more and found the answer and showed it here. – VLAZ. Non-null assertions are for when you as a programmer know that you won't get a null but TS cannot figure it out. In a technical sense, it eliminates null and undefined from the type of your variable. This doesn't actually In this Typescript tutorial, I will explain to you everything about the TypeScript Exclamation Mark or TypeScript Non-null Assertion Operator. 0-dev. A new ! post-fix expression operator may be used to assert that its operand is non-null and non-undefined in contexts where the type checker is unable to conclude that fact. IncomingMessage['url'] // string | undefined ValidMessage will have the property url not null. config ({rules: TypeScript's ! non-null assertion operator asserts to the type system that an expression is non-nullable, as in not null or undefined. config. Thanks for this, I needed a function that checked for undefined, null, OR empty string but with the one I had Typescript kept thinking items it would return true on could be undefined. bar(); // "!" - asserts that foo is not null nor undefined } As of TypeScript 2. s1n7ax s1n7ax. getElementById() is non-null and non-undefined (also known as “non-nullish”). The reason filter doesn't work here is because it returns a boolean based on arbitrary logic, so typescript cannot figure out what that predicate is actually doing as it does not return any type information. The star of the show in optional chaining is the Edit: I'm using TypeScript v2. log() will return "Incoming value is: undefined". options. But if you don't know better then the compiler, then you'll have to handle it yourself. This means that facts that were proved to be true at the moment the closure was created might no longer be true when the code in the closure executes. TypeScript: Non-Null Assertion Operators with [] 2. At its core, optional chaining lets us write code where TypeScript can immediately stop running some expressions if we run into a null or undefined. Kotlin Kotlin Tutorial Kotlin Programs DSA in Kotlin Design Mar 19, 2021 · There are times that we need to create some assertions to be sure an object respects some rules. However, the non-null assertion operator is a more concise solution to type errors that involve null or undefined. However, I believe the syntax is now hardened and solidified such that it's not going anywhere and is best practice. E. user. When the developer is so confident that this value will not be null or undefined when writing their code, they will use the ! operator. We have talked about a ! type operator to get the non-nullable version of a type. function test(arg: { x: null; y: number } | Right, but non-null assertion const s: string = this. a) line could not be undefined: type Example = { TypeScript supports using ! for telling the compiler that the value is not null or undefined. We can cast the IUser interface to the userDocument variable but then we lose access to all the other document-related properties/methods like document. In TypeScript, one significant benefit is its strict type-checking system which helps catch potential runtime errors during compile-time. This might not be a big deal for undefined, which doesn't overlap with most non-union types in TS (only One of the most controversial features of TypeScript is definitely the Non-Null Assertion operator. TypeScript’s Non-Null Assertion Operator is a powerful tool for developers to avoid compile-time null and undefined checks when they are certain that a value is not null or Non-null assertion operator: ! You tells the TS compiler that the value of a variable is not null | undefined; Use it when you are in possession of knowledge that the TS compiler TypeScript non-null assertion operator (!) is used to assert that a value is non-null and non-undefined, even when TypeScript's strict null checks are enabled. composed boolean for checking undefined or Typescript not detecting null check and complaining about argument not assignable 0 TypeScript does not see that undefined\null value checking has been executed in variable How to declare "is not null" type assertion using Typescript 3. TS: Non-null assertions are not done at runtime? Hot Network Questions Does Noether's first theorem strictly require topological groups or Lie groups? Happy 2025 to all! Since I enabled strict null check option in tsconfig I have faced some linting errors that make me thing that strict null checks are really not worth Here is one example where I would like to see how would you type assert to avoid as. js which provides a user: User | null object to all child components. Ok, I'm done, you should not use TypeScript, just do whatever you EDIT: This doesn't answer the original question in which the OP wanted to tell Typescript that propertyA is defined inside myVar. doMore(); } I know I could do this with a this. because type assertions allow you to override the inferred type of a value. Non-null assertion operator required but value checked for non-null beforehand. @TomášWróbel, I am sorry but that was rude. assertNull(str1); // Fail. This operator tells In TypeScript, we often come across scenarios where we need to assert that a variable is not null or undefined. The assertNull() method means "a passed parameter must be null": if it is not null then the test case fails. 1, you can use a lookup type to access an interface property. Is there away to assert that the output of my method doesn't contain null or undefined, or is there a different way to write the method so typescript can infer this information and the return type of transform? In TypeScript, the non-null assertion operator is a way to tell the TypeScript compiler that a value is not null or undefined, even if its type suggests that it might be. This is known as the non-null assertive operator. 2. Almost all companies require mocha/assert/chai etc frameworks, the code needs to be consistent. Add a Stack Overflow for Teams Where developers & technologists share private knowledge with coworkers; Advertising & Talent Reach devs & technologists worldwide about your product, service or employer brand; OverflowAI GenAI features for Teams; OverflowAPI Train & fine-tune LLMs; Labs The future of collective knowledge sharing; About the company The point of the non-null assertion operator is to tell the TypeScript compiler that you already know the variable is not null. to. 3,069 6 6 gold badges 29 29 silver badges 62 62 bronze badges. The operation myVar! produces a value of the type of myVar /** * Non-null casting like TypeScript's [Non-null Assertion Operator][1]. TS: Non-null assertions are not done at runtime? Hot Network Questions No bubble formation in hot water Pete's Pike 7x7 puzzles - Part 3 Where is it midnight? xcolor. I have this assertion function, which checks that the passed value is an object (according to the typeof operator), but excluding null: export function There's a longstanding bug/issue whereby TypeScript does not perform control flow analysis to narrow the type of a property if the property key is a variable: microsoft/TypeScript#10530; it wasn't addressed because the fix imposed a large performance penalty. the operation a! produces a value of the type of a with null and undefined excluded Sometimes you can use this symbol to assert that you have a value, for example when you have a class that might be not yet initialized you can use class!. so type Concrete = string | number | boolean | symbol | object does not seem too bad. But if you use isNullOrUndefined which returns boolean, Typescript does not check the actual implementation of the function - even if Hey guys, it's been a few years on this one. propertyA!) is 1 - if you're curious about why is TS not able to pick up on what the if directive does, it's because TS has no insight into how React interprets JSX to create DOM elements and what are the relations between those DOM elements, once created. You'll see most general linting rules throw a warning against using it. In TypeScript, the exclamation mark (!) is known as the non-null assertion operator. Some options for you: If you want a checked assertion, you can write an assertion function (sadly not yet documented in the handbook): I'm building a Next. obj. The compiler does not use control flow analysis to narrow the type of unspecified generic type parameters (like T inside your function implementation), nor does it use it to narrow the type of values that depend on such type parameters (like t inside your function implementation). It's not a simple double negative. g. Can I type cast/assert a variable that will be valid for a whole function scope? A TypeScript type guard to asserts that the given value is not undefined nor null - fvilers/assert. Now the subscriber will subscribe only when there are values. const { email, password } = req. Additionally, I don't want to allow code in the class to assign a null or undefined value to the property later: Adding ! after value will assert that the value is neither undefined nor null. TypeScript supports using ! for telling the compiler that the value is not null or undefined. * * It removes `null` and `undefined` from a type without doing any explicit * checking. Share Add a Comment. It allows you to easily check for nullable values, without writing a lot of logic. With the non-null assertion operator we can tell the compiler explicitly that an expression has value other than null or undefined. You simplify the problem a bit by not destructuring the argument object, which will allow TypeScript to identify it as a discriminating union and narrow its type based on checks on x (or y):. JavaScript-like assertions; TypeScript-like assertions. If i put that piece of code in our unit test, it will do the job, but won't pass our code review. You can also use this operator to deliberately ignore void -ness, both owing to deliberate assignment Non-null assertion operator syntax. But some functions expect it to be SOME_TYPE and those functions are 100% sure that the state variable will be SOME_TYPE when they run. This means that the compiler cannot This will only tell the compiler that the value is not null or undefined. Commented May 12, 2022 at 5:05. 2 - the wrong usage of the non . I have a UserContext in pages/_app. We mentioned these briefly in the Basic Types section. Use this is case where you're absolutely sure it cannot lead to a null or undefined reference. 1 I am new to TypeScript and I'm not sure what the cleanest way of dealing with DOM elements that may or may not exist is. It is a tool to enhance code safety, streamline development, and provide clarity in situations where the type checker needs a helping hand. Example. It is used after a variable to tell the TypeScript compiler that the developer is certain the value is not null or Instead, if you want something the compiler can follow, my suggestion here would be to change T from meaning "the possibly-null or undefined type from which I need to remove null and undefined" to meaning "the non-null and non-undefined type to which I may need to add null and undefined. String str1 = null; String str2 = "hello"; // Success. TypeScript doesn't insert code to check that you're right. , you do calculateTax(userInput, false) which could result with null for userInput = 0 but you are actually filtering that value before it gets to that point of the code. assertNotNull(str1); // Success. 20170407 Code Paste in the following code and scroll to the final two lines: export type Subscription = { dispose: => void }; export type SubscriberCallbacks<Events> = { [event in keyof Events]?: SUGGESTION: Assert return type is not undefined or null #15100. Royal Icing; Blog; Tools; Bookshelf; A better alternative to TypeScript’s non-null assertive operator May 14, 2020. I've been developing in plain JavaScript for nearly 20 years now, but I just picked up TypeScript a few days ago and I'm trying to learn the ins and outs. Playground. In this article, we will learn how it works, where to use it, and how it compares to the Optional Chaining operator. This solution was biased by the misleading title, and it only shows how to instruct Typescript that a given var (myVar!) or property (myVar. exist check will throw if the object is null. js/React app with TypeScript. getItem('email') assertNonNullish(email, new Error("email does not exist")); } Why can't use function to assert that var is not null in typescript. When trying to find my user model I get: Object is possibly 'null' If possible I would prefer not suppressing typescript's strict rule. brand in a function, it is unsafe to assume that the function will be invoked immediately. I also have a ProtectedRoute component to wrap pages which shouldn't be accessible without being logged in (it consumes the user from UserContext and redirects if user === null). This is can be useful when the compiler cannot infer the type with certainty but we have more information than the compiler. It is worth noting that if the code were more straightforward, then TypeScript would understand that text on the return statement wasn't null. given that the current way to specify nullable types is T | null, T | undefined or T | null | undefined; the system is already verbose. Closed AdamWillden opened this issue Apr 10, 2017 1 day ago · Java Programs Kotlin Programs C Programs R Programs Python Programs C++ Programs Go Programs TypeScript Programs JavaScript Programs. v as string // <<<< explicit cast Or use the Non-null Assertion Operator Postfix !) Your code doesn't work because type guard/assertion functions only narrow the reference passed in, not its contents. it is used as a type constrain. In this case, TypeScript infers that v1. 1-It will evaluate to true if value is not: null, undefined, NaN, empty string '', 0, false If incomingValue is not declared, TypeScript should return an exception. type NonNullableValues<T> = { [P in keyof T]: NonNullable<T[P]>; }; interface User { name: string | null; } type NullableUser = NonNullableValues<User>; // type @Hashbrown The Exclude utility type can only filter a union; it is not doing true subtraction of types, as evident by the fact that Exclude<unknown, undefined> or AntiUndefined<unknown> both evaluate to unknown, and undefined extends unknown. For context, there have been over 23,000 issues on the TypeScript issue tracker since then. Flat Config; Legacy Config; eslint. An example of this would be: <user-detail [user]="(user$ | async)!"></user-detail> Using the non-null assertion operator indicates that you are confident a value will always be present, which overrides TypeScript's null-checking behavior. Function declarations and expressions; Assertion functions and type guards; Assertion functions without a type predicate The ! non-null assertion operator in TypeScript is used to assert that a value's type does not include null or undefined. Follow /** * In lieu of writing in TypeScript and having the convenient non-null assertion * operator (!), this helper function allows asserting that something is not * null or undefined The non-null assertion operator, or that weird exclamation mark you might be seeing in TypeScript is an operator that tells TypeScript that a value won’t be null or undefined. You are basically transforming the variable to a boolean. That is the non-null assertion operator introduced in TypeScript 2. doMore() but I would like to have the runtime assertion to verify it and then have typescript know that past that assertion the type union does not contain null anymore. The ! operator in TypeScript, known as Stack Overflow for Teams Where developers & technologists share private knowledge with coworkers; Advertising & Talent Reach devs & technologists worldwide about your product, service or employer brand; OverflowAI GenAI features for Teams; OverflowAPI Train & fine-tune LLMs; Labs The future of collective knowledge sharing; About the company A new ! post-fix expression operator may be used to assert that its operand is non-null and non-undefined in contexts where the type checker is unable to conclude checking null in typescript. This is known as the non-null assertive operator . Commented Apr 20, 2019 at 1:48. – So my question is: is there a way to explcitly tell typescript that the variable is not null? So far I've found two workarounds but none of them are satisfying. this. Non-null assertions are a way to tell the TypeScript compiler that you are certain a value will not be null or undefined at the time it is used, even when TypeScript's type checking might consider such a case possible. A common case is to check if an object is defined; to do this you can create a simple assertion function like this Feb 2, 2024 · The non-null assertion operator, or that weird exclamation mark you might be seeing in TypeScript is an operator that tells TypeScript that a value won’t be null or undefined. Disabling these rules for tests only was also not appealing. In this article, we will learn how it works, where to The Non-Null Assertion Operator is a powerful tool in TypeScript that allows you to assert that a value is not null or undefined. e. It's purely a construct for managing type information. I have a state variable that is of type null | SOME_TYPE. you are casting a base type to a subtype). Given the structural nature of the typescript system, the function type (start: number) => number is a subtype of (start: number) => void, so this means you can assert that (start: number) => void is actually (start: number) => number. Is there a dynamic way to check values in typescript. Am I understanding that logic TypeScript allows you to assert that a certain condition is true, and if it is, the TypeScript type system will narrow the types accordingly in subsequent code. but we have not been to keen on adding new type operators, just TypeScript, --strictNullChecks mode. Hence, req. There are two common ways to assert to TypeScript that a value is its type without null or undefined:: Non-null assertion as: Traditional type assertion with a coincidentally equivalent type! non-null assertions are generally preferred for requiring less code and being harder to fall out of sync as types change. Host and manage packages Security. I have typed DTOs in testing code (TypeScript), and I would like to make such fields as In below example, TypeScript does not see that parameters. It is used to force TypeScript to think that the value is not null. Instead, I added the following function to my test suite: The non-null assertion operator, or that weird exclamation mark you might be seeing in TypeScript is an operator that tells TypeScript that a value won't be null or undefined. The non-null assertion operator is an exclamation mark (!), and this is placed after the variable or expression that we want to tell TypeScript isn't null or This tiny but powerful symbol is known as the non-null assertion operator, and its purpose is simple: it tells TypeScript, “I know better than you—this value will never be null or One of the most controversial features of TypeScript is definitely the Non-Null Assertion operator. 7 Assertion Functions. In this article I’ll cover the operator, how to use it, and why maybe not to. Consider the following scenario: function fun1(node: SomeType | undefined) { throwIfUndefined(node); node!. The non-null assertion is specifically telling the compiler that you know better than it does. fun2(); } You call a function which you know will throw an exception if node is undefined. TS code In general, the CFA employed by TS does not go through inferences, only direct checks are respected. Basically, I want to check whether an el TypeScript basically cannot know when or even if map() will ever run its callback (since the signature for map() can't represent this), and it doesn't spend the time checking that y never gets reassigned after the closure is created (this would be a fairly expensive operation), so it errs on the side of saying that y might be null inside the closure. type(document. Find and fix vulnerabilities doSomething() { assertNonNull(this. getEmp(personEmpId). Sign in Product Actions. 1 Non-null assertion operator for generic type in Typescript. Being new to TypeScript I want to make sure that I'm not missing a common pattern or best practice Typescript type assertions and non null checks. Learn TypeScript - Non-null assertions. How to pull null check into function in TypeScript Assertions Type, also known as Type Assertion, is a feature that lets developers manually override the inferred or expected type of a value, providing more control over type checking in situations where TypeScript's automatic type inference may not be sufficient. I think TypeScript should affirm that best practice with type checking. It crashes at runtime. It is undefined. function test() { const foo: (FooType|null) = getFoo(); foo!. It is denoted by the How to declare "is not null" type assertion using Typescript 3. This assertion can be particularly useful when you are confident about the presence of a value due to the logic of your In below example, TypeScript does not see that parameters. which means null! is never, and current typescript allows assigning never to any type. Optional chaining is issue #16 on our issue tracker. – user3552178 Type 'HTMLElement | null' is not assignable to type 'HTMLInputElement'. This is where the Non-Null Assertion Operator comes into play. save. They didn't know the answer first. If this is declared but not defined, the console. export default tseslint. – I had this same problem, but wasn't able to use the non-null assertion as in the accepted answer due to having no-non-null-assertion active as well. Table of Contents. With --noUncheckedIndexedAccess disabled, TypeScript "optimistically" assumes that when you check a property it will be there, so in some sense it looks like "every property exists", but that's just to make it easier to iterate objects, not because TypeScript really models the presence of every possible property. Follow asked Dec 30, 2019 at 20:09. You can use the empty type {} to represent "anything except null and Type assertions are just that: assertions. It's a transforming double negative. It helps you bypass TypeScript’s strict null checking in situations TypeScript provides the assert keyword that can be used to assert that a variable is not null. 5. Your condition only confirms coolThing is defined and not null. Share. Typescript assert function to avoid Non-Null Assertion Operator ! with Objects. Skip to content. You are confused. In this article, we’re going to explore assertion functions in TypeScript and see how they can be used to express invariants on our variables. It's effectively a type assertion that `value` isn’t `null` or * `undefined`. . It works well and seems to be the textbook use-case for non-null assertion, but with the recommended eslint rules non-null assertion is a warning. I have a tiny in effect, assert that U is non-null so it can be used in the return type, but then allow just the parameter to also be null. obj); // throws exception if null this. 2. Using the operator any more than once on a single value does nothing. We could introduce a custom utility type to exclude null and undefined from all properties of the object:. a; // Argument of type 'number | undefined' is not assignable to parameter of type number. See Given the type T, I want create a type helper that will assert that optional properties are not optional. This is particularly useful for validating types at runtime and ensuring type safety in the rest of your program. b has been checked for undefined value and in transformValue(parameters. a and parameters. You're passing in an array literal so you necessarily lose access to it immediately. assertNotNull(str2); // Fail. 3. Improve this answer. Type Guards and Type Assertions. 0. There's a lot to say on the subject but suffice it to say that in cases where your variable is 0, false, null, undefined or some such value that resolves to true with one !, you're going to get false for !!. You've asserted that you are right. something to tell the compiler that you are sure that it has been initialized. mjs. TypeScript Non-null assertion operator // Compiled with --strictNullChecks function validateEntity(e?: Entity) { // Throw exception if e is null or invalid entity } function processEntity(e?: The Problem: Undefined and Null Values. The first solution is a more elegant variation if the original if and throw. In my React application I'm using the non-null assertion (!) to tell TS that the value coming from useContext hook is not null. In this article I'll cover the operator, how to use it, and why maybe not to. Commented Oct 3, 2019 at 11:54. To cover myProp, you'd need to explicitly check that as well: The Non-Null Assertion Operator is a powerful tool in TypeScript that allows you to assert that a value is not null or undefined. This guide will walk you through how to effectively use assert to prevent null values in This is supposed to check if something is not null nor undefined. user could be "possibly undefined" even though I have middleware that checks for it before every request: Lint doesn't like the ! operator to assert non-nullishness. Improve this question. Assert Not Null. Note we are not using the strict equals operator. In TypeScript, the non-null assertion operator is a way to override the compiler’s strict null-checking. There are no cases where the if will not pass and offerings would be null. If you are certain that myPerson["my-gender"] is always defined, you can use TypeScript's type assertion feature to tell TypeScript that myPerson In TypeScript, the non-null assertion operator (!) empowers developers to express their confidence in the absence of null or undefined values. asserts value is not null typescript; assertion; Share. Nope. Tl;dr; the typechecker tells you it cannot ascertain, that the expression is not null/undefined. myService. This will not check if the value is null or undefined. If I use this function typescript also knows that something is not null nor undefined. obj!. It's the opposite of the nullish coalescing operator ?. Syntaxlet variableName: A Non-null Assertions . subscribe. jdr vagxn cyjah xkjbsyq lidujl xlu fzvnqly vmonbe gaqd ewi