The null value means we know that it does not have any value. @mhegazy I have no problem with verbosity, especially since (a) you can define aliases and (b) few APIs actually returns all kind of types. privacy statement. Method #3: Use the keyword as. It ignores anything which does not have a type. o and the returned value cannot be null or undefined. TypeScript is super set of JavaScript. tariff!!.name. The text was updated successfully, but these errors were encountered: This looks like a bug to me. any is given carte blanche - ignored by type checking altogether. Hovering over this isFalsy() call gives me, function isFalsy(mixed: string | undefined): mixed is "" | undefined. Using the in operator confirms that the properties we check are present and non-optional. Why does an overloaded function declaration sometimes force useless type narrowing? Here's an example: validateToken (token as string) In the example above, I'm passing token, and letting TypeScript know that even though the variable may be undefined; at this point of the . chaining operator, except that instead of causing an error if a reference is nullish ( null or undefined ), the expression short-circuits with a return value of undefined. TypeScript Version: 2.1.1 Code interface StdoutValue { stdout: string; stderr? Types null and undefined are primitive types and can be used like other types, such as string. The article that mentioned also talks about a keyword, Btw, I'd say that the "optimistic" part is assuming that the narrowing of. Oddly enough, I had the option already set to true, but it never mattered since it never actually applied. Essentially: A function that takes a value of type T, ensures that it is not false-ish (by throwing) and returning the value if it is true-ish. I don't think there's any way to declare the equivalent of InputType - undefined with intersection types. Let's start by operating under the super-set design principal employed by Typescript, as defined by the Typescript design goals, specifically with regards to goals 7 and 8 - Preserve runtime behavior of all JavaScript code. ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~. @Arnavion @RyanCavanaugh I think this should be reopened - there's another simple use case for the some idea, which isn't covered by object or {}: forcing consumers of a type or interface to handle a falsy value explicitly. proto can be null but not undefined. Can someone explain why in the following example, variable z has type number, but within the call f1(x.y), x.y has type number | undefined. Have a question about this project? . * Sets the prototype of a specified object o to object proto or null. Well occasionally send you account related emails. Control flow doesn't recognise undefined check, // Type of maybeNull should be `null | undefined`. This issue is probably going to come up more often when code starts using --strictNullChecks. Or let value = 3rdPartyProvider(); firstPartyStuff(value); /* somewhere deep in firstPartyStuff */ 3rdPartyConsumer(value) (ie. Name of poem: dangers of nuclear war/energy, referencing music of philharmonic orchestra/trio/cricket, Exchange operator with position and momentum. Personally, I think some is a bad use of TypeScript's development resources. And we don't even have to specifically check for undefined. Typically, you use the never type to represent the return type of a function that always throws an error. It may be useful for user functions as well that can return any object but never null, like a parse(input, rule): output function. https://herringtondarkholme.github.io/2017/02/04/flow-sensitive/. Yes, I remember, which is why I'm asking for a much easier form. 2) While using == comparison has a working logic in this case, the returned value of localStorage.getItem () is never undefined. I'm ok with doing string | number | boolean | as a workaround but is there some way to display a custom alias like Defined or Some in error messages instead of the full definition? That question is relevant regardless of what approach we take to renaming the functions during transpile. @andy-ms since there is a working workaround, would you like me to close this issue? We have talked about a ! By using Array.include() function. Should teachers encourage good students to help weaker ones? a and b are of either int or string within the two methods, respectively, and the body of those methods are required to respect that or suffer compile-time errors. However there are plenty of cases where its very useful (and very practical) to have a not-nothing type. Japanese girlfriend visiting me in Canada - questions at border control? See his comment for more info. Expected behavior: TypeScript should ignore type errors in node_modules (or skip reporting them? So lets take a different approach. TypeError: Cannot read property 'getId' of undefined. I came across this when upgrading @typescript-eslint/typescript-estree from 2.12.0 to 2.13.0 as part of package update on eslint-plugin-jest. etc etc. Summary: in this tutorial, you will learn about the TypeScript never type that contains no value. TypeScript optional chaining is a process of querying and calling properties, subscripts, and methods on optional that might be nil. index.ts https://stackoverflow.com/questions/51236491/typescript-type-representing-everything-but-undefined, https://stackoverflow.com/questions/53612681/typescript-function-cannot-return-undefined, https://github.com/maasglobal/maas-schemas-ts/blob/master/src/core/booking.ts, Loss of empty type constraint in generics, Cannot use overrideProvider() to override undefined value, Should we rather annotate methods that may return null as. Syntax to declare undefined in TypeScript: var variable_name; Where variable_name is the name of the variable that is declared and is not initialized, thereby making it an undefined variable. It means "please disable all type checking thanks". Each time a new primitive is added you need to update it. any is flawed in that it's meaning is "do whatever" but in many contexts what we really want to say is "it's whatever [that's not null/undefined]" and it's really easy to use it like that. // Error: no .doSomething() on object. I'm not sure exactly what skipLibCheck does). The above code cannot be compiled when strictNullChecks is set to true. Have a question about this project? o and the returned value cannot be null or undefined. This syntax effectively hides the implementation, and type-checks only against the first two declarations - the compiler will present an error if you provide any argument combination other than two strings or two numbers. Previously it was not possible to explicitly name these types, but null and undefined may now be used as type names regardless of type checking mode.. TypeScript only works with types. Already on GitHub? His example accepts an any, which sort of negates the benefits, but in general they communicates far more information to the compiler. and Avoid adding expression-level syntax. While updating the various .d.ts to have | null and | undefined, I came across some typings that use any but don't allow null or undefined. In the if statement, we check if the variable is not null and is not undefined. Was the ZX Spectrum used for number crunching? The following example declares the variable foo. type NotNil = Nil extends T ? Hopefully you're using TypeScript with the --strict flag. Working with JavaScript means working with undefined. To subscribe to this RSS feed, copy and paste this URL into your RSS reader. And for that reason, it may not be a good idea to implement overloads, which is presumably the only way we could enforce parameter types within the body of a function. Typescript Type resolution of tuple members - why does this compile? built-in type except any (which is not a type). Even though it's clear that there must be some strings not present in movieWatchCount, previous versions of TypeScript treated optional object properties as . Update I got it to work using TSLint (vnext) 0.0.4 in vscode. Projects scaffolded via create-vue include pre-configured tsconfig.json.The base config is abstracted in the @vue . Sign in By clicking Post Your Answer, you agree to our terms of service, privacy policy and cookie policy. It's nearly impossible to declare it with out treating it as a special case each time (which is at odds to you wanting to use any there to begin with). If I am wrong in thinking that this is the third (and potentially most important) feature you are describing, please correct me and ignore the rest of this comment. privacy statement. Undefined is the default value for uninitialized variables Whenever we declare a variable without initializing it with a value, TypeScript initializes it as undefined. Does illicit payments qualify as transaction costs? type operator to get the non-nullable version of a type. This would allow me to spot where my error is and remove the ! How does legislative oversight work in Switzerland when there is technically no "opposition" in parliament? I'm trying to write a generic "normalize" function that will be executed on user input only if that input is defined: In a case where a property can be a string or a function that returns a string and is not required, the compiler emits a warning with a less than helpful message. using a type guard (in our case, the problem disappears if the type guard is inlined) the type guard checks for a type union the type guard receives another type union as its input, and the narrowed type is (incorrectly) too narrow JoshuaKGoldberg on Aug 4, 2021 String union types narrowed to falsy should narrow string to "" mentioned this issue I'm working with generated code which contains lots of statements about things that are defined and the error messagages are extremely hard to read because of such long definitions. You signed in with another tab or window. Well occasionally send you account related emails. However, the compiler emits an error with the following message: Type 'undefined & Number' provides no match for the signature '(): string'. Concentration bounds for martingales with adaptive Gaussian steps. The condition uses to check null or undefined values in javascript. Update I got it to work using TSLint (vnext) 0. . wizarrc changed the title Non-nullable types should warn when checking for null or undefined on non-nullable type Typescript should warn when checking for null or undefined on non-nullable . That issue is closed as too complex but there's a comment about it from a recent backlog slog that says: "Revisit in context of type operators". It gives you the appropriate intellsense listings, automatically casts values to the correct types within the guarded statements, and returns specific types instead of a "yeah I mean it *could* be a string, but it's usually a number" type. The parameters used within the methods are of the types declared in the method signature). Lets say the value of secondUser is undefined, and age of firstUser is 24. never : T, as follows: Algebraic data type operators exist in TS. With a non-null/non-undefined any you can say "this parameter cant be empty", "this function never returns empty" and "inside this function this value is never empty" (and it's important for those to happen at the same time, not just be a "you can only opt-in to one of them" as with current solutions). Personally I can live with any being unclear about nulls (it's not checked anyway) but if we want to support better documentation without introducing new type operators, I suggested previously that we start the habit of using any | null for nullables any and just any for the non-nullable ones. I think it should be baked into the language service itself being that it's less of a coding pattern and more of a coding contract feature like their example TypeScript wont let you compare 1 === 2. By clicking Sign up for GitHub, you agree to our terms of service and For me, the use-case for any! Null- and undefined-aware types. Sign in Sign up for a free GitHub account to open an issue and contact its maintainers and the community. Same words but totally different meanings "it's a thing that exists" vs "this is not checked." type AutocompletionProvider = (context: AutocompletionContext) => Suggestion [] | Promise<Suggestion []>; Original PR: https . /// , // ^ entry could very well be undefined here, typescript don't care, // or if we omit use just "thing" there then the error would be on get_db_entry, // because it's getting passed an incompatible function, "no matching implementation for given arguments", /** Traps if the specified value is not true-ish, otherwise returns the value. Function overload compilation error (This overload signature is not compatible) with template literal types. A better solution for this case is overloads / multiple signatures: @RyanCavanaugh again your solution is not solving the problem at all. type Nil = null | undefined; any | null | undefined. in TypeScript That's one of the really really big points to using static analysis to begin with. Yes, --strictNullChecks is the context of this issue, and yes since nullability is via union types, any is allowed to be null and undefined. This is terrible for code readability too. object | string | boolean | symbol | number ? rightExpr Description The nullish coalescing operator can be seen as a special case of the logical OR ( ||) operator. Browse other questions tagged, Where developers & technologists share private knowledge with coworkers, Reach developers & technologists worldwide, @Liam I think the question is really looking to ask why is, @blub I've made a substantial edit, I think you should be able to dump it if you aren't happy with it, I got the feeling this could be clarified slightly, @Liam can you show that in a playground example? Have a question about this project? Properties are dynamically assigned, just like object literals. To check for undefined in Typescript, we can use the === operator. https://palantir.github.io/tslint/rules/strict-type-predicates/, disallow comparing to null and undefined unless they are valid cases in strict null mode. Ready to optimize your JavaScript with Rust? * object (that is, a user-defined object or a built in object) or a DOM object. Connect and share knowledge within a single location that is structured and easy to search. Thanks. I can leave it open and pivit the title if there isn't another issue covering this already. For example: I don't think {} is the same thing at all. If I comment out the value: string on StringLiteral, it all just works. We just want a built-in thing type that's exactly like any except it doesn't allow null / undefined. It certainly would be possible to support combinations of unit types and infinite types, although it does add more complexity to the analysis. It's a way to enforce explicitly declaring the type of all variables (a la noExplicitAny) even if it does not have a type. Nice job on that new plugin model in tsconfig.js, it worked flawlessly the first time I tried it. This is a repro of a problem @G-Rath had, on TS 3.7.4. Proposal: Add an error or warning when strictNullCheck is turned on and a null check is applied to a non-null type. As OP mentioned it has to do with closure, specifically the callback. In the if statement, we check if the property is not null. By clicking Sign up for GitHub, you agree to our terms of service and How to get Type Partial without undefined on this case. Not to mention it's object | string | boolean | number | symbol | bigint as of comment. The TypeScript Handbook The Basics Everyday Types Narrowing More on Functions Object Types Type Manipulation Creating Types from Types Generics Keyof Type Operator Typeof Type Operator Indexed Access Types Conditional Types Mapped Types Template Literal Types In particular, it can be challenging to check whether or not a value is in an enum in a type-safe way. I'd say this is a failure of english if nothing else. The elvis operator will not stop the addition and the output will be NaN as there will be addition of 24 and undefined. You can use a void operator to get the value of undefined. to your account. On the other hand, Im not sure if we want that kind of logical branchingmaybe its better just to keep it in its current slightly wrong but consistent state. // any better way to model `: T != null`? And if the value is null or undefined, this operator will not help us. https://stackoverflow.com/questions/53612681/typescript-function-cannot-return-undefined. By clicking Sign up for GitHub, you agree to our terms of service and Let me edit that. This means projects using project references will now see an improved editing experience where semantic operations are up-to-date and "just work". When refactoring, it is often times difficult to spot redundancy in null checks. The current behaviour when the source type is a union (here string | undefined) is that the source is filtered against the candidate type (here "" | undefined) removing all types not related to the candidate. Of course, for any movie title not yet in the dictionary, movieWatchCount[title] will be undefined (TypeScript 4.1 added the option --noUncheckedIndexedAccess to include undefined when reading from an index signature like this). I don't see the case where that example forces an error check. so type Concrete = string | number | boolean | symbol | object does not seem too bad. don't care what it is so long as it's not nothing). And it should be checking both subtypes and supertypes? I find this syntax to be cumbersome and unintuitive - genuine overloads are simply easier to read and understand, in my opinion, and they enforce type restraints on the provided arguments, return value, and within the implementation of each overload - a feature I believe is at the root of your interest/concern, if I understand you correctly. If the value is not defined, the typeof returns an 'undefined' string. Because of this, you cannot assign any value to a variable with a never type. Example The change that caused this issue is that StringLiteral defines value as type string, where as in the past it was type boolean | number | RegExp | string | null (which is what it is defined as on its super class LiteralBase, which hasn't changed). By clicking Sign up for GitHub, you agree to our terms of service and But doing so is a breaking change: given that the current way to specify nullable types is T | null, T | undefined or T | null | undefined; the system is already verbose. to your account. "inside this function this value is never empty". type operator to get the non-nullable version of a type. JavaScript void Operator The void operator checks the given expression and then returns undefined. The type string is not related to "" | undefined and is therefore filtered out, returning undefined (which is related to "" | undefined). Is this because it's checking subtypes only when the type guard is for a union type? I made an error in judgement when using any as the type for the d parameter in the implementation of the operate function as it does not accurately portray the level of type checking available within the implementation's body. You'll find the appalling fact that NotNil is now an alias of never. to your account. Also, don't do it. // Type of maybeNull _is_ `null | undefined`. And by the way, object | string | boolean | number is bad form. has a lot of value, I would point out that replacing any with string | number | boolean | symbol | object is not exactly the same. Code. //Property 'length' does not exist on type 'never'. Effectively this means that the string and number cannot be part of the discriminant value type. Thankfully, TypeScript is a great tool for helping you deal with it and writing better code in the process. The nullish coalescing operator ( ??) So while using in expressions we need to be careful to avoid unexpected results. Making statements based on opinion; back them up with references or personal experience. Thanks for contributing an answer to Stack Overflow! Not the answer you're looking for? Search Terms: skipLibCheck is ignored skipLibCheck not working. When used with function calls, it returns undefined if the given function does not exist. but we have not been to keen on adding new type operators, just because of the complexity both to the language and the implementation at the time being. Using {} as the return type in the OP's examples have a very different meaning than any - null - undefined or however it could be expressed. This includes any. We just want a built-in thing type that's exactly like any except it doesn't allow null / undefined. #21847, I have this related question - https://stackoverflow.com/questions/51236491/typescript-type-representing-everything-but-undefined, if someone wants SO points please take a look thx. given that the current way to specify nullable types is T | null, T | undefined or T | null | undefined; the system is already verbose. However, the opposite is true in both counts. Already on GitHub? I was pointing out that any has one big difference to the union of all types: it is a special type that bypasses all compiler checks! Type Checking JavaScript Files Here are some notable differences on how checking works in .js files compared to .ts files. TypeScript will also prevent you from invoking the method with a: number and b: string, and vice-versa. Method 1: Use Simple If Condition. TypeScript is smart enough to know that by doing the simple existence check that it no longer undefined. @andy-ms Thanks for the quick reply! For example with this TypeScript code, users.find has no guarantee that it will actually find a user, but you can write code as though it will: declare const loggedInUsername: string; It's a bit wordy, but I suppose the set of primitive types is well-known. Oh, thanks. For instance, we write Already on GitHub? The text was updated successfully, but these errors were encountered: Actually not only undefined check is not recognized, null check is not recognized either. I've not been able to find anything in any docs/issues relating to this. When we use an if statement (or equivalent) to check its existence, product no longer has the undefined type. The point here is that the check against, I read that exact issue while researching, quite a few thing to learn from there. But instead of spending time compounding the any confusion by adding a some type, it looks like the TypeScript teams has decided to focus on features that can't be easy added without modifying the compiler. // Type 'false | number[]' is not assignable to type 'boolean'. Good to know. @Arnavion assuming #1809 is in, this request would be just a shorter name for string | number | boolean | symbol | object? Or parseThing(userinput: NotNothing): InputErrors (ie. It will be updated to include new primitives, if/when any are added to the language (see recent addition of symbols). If the condition is met, TypeScript knows that the only other possible type is a string and allows us to use string-specific methods like toLowerCase (). so type Concrete = string | number | boolean | symbol | object does not seem too bad.. We have talked about a ! My work as a freelance was used in a scientific paper, should I be included as an author? Why does Cauchy's equation for refractive index contain only even power terms? . If anybody wants to ingrain this pattern in to their development environment and company culture here's how to do it. Here's another example of a failing case: @eggers You just have your cases backwards: Closing as the original example is no longer an error. I think there is value in being able to express both "This can be any type including null and undefined" and "This can be any type excluding null / undefined", but there is no way to express the latter. I'm going to work around it for now by returning string | undefined rather than trying to use it as a type guard. For some reason, I can't figure out how to pass the stable version of TSLint the types with my webpack project. Using a generic array type, Array<elementType>. Additionally, it begs the question, "what do we name the function if there already exists another function named operate_string_string?" Sign up for a free GitHub account to open an issue and contact its maintainers and the community. Note the [] there, that actually would result in some sort of never[] assignment error one way or another. Exclude, NonNullable seemed nice, until I tried them with any, and they do work with any. msie && parseFloat(jQuery React Redux - Uncaught (in promise) TypeError: Cannot read property 'props' of undefined As pointed out by @ArielLeslie, the last iteration of the loop tries to get the element after the last element of the array js:7 at DayPilot Uncaught TypeError: Cannot read . When the source type is not a union then the code intersects the source with the candidate (string & ("" | undefined)), producing the right narrowing. Why does the USA not have a constitutional court? if(expression) { console.log ('Not null')} } the expression must be evaluated to be true or false. That is why you don't use the typeof () method to check if the variable is undefined or not in JavaScript. maybe you can try use T extends null ? Does that still include null and undefined under strict null checks? We have to assign Null to variable to make it null. This is at least one reason why overloads don't exist in TypeScript - there is not a clearly defined way to implement overloads and remain a super-set of JavaScript. Undefined is a primitive value that indicates that the value is not assigned. The never type is a type that contains no values. privacy statement. Please read my comments in the code below where I try to explain things as they come. This is in all likelihood what should be used in the case that you want to have a non-null value. Try it Syntax leftExpr ?? Sadly, you can't do NonNullable because it's not a union type, so string | number | boolean | symbol | bigint | object seems to be the only way to do this right now. Fix narrowing union with union type predicate, Typeguard for union not working after adding a property to the superclass of one of the union members, Narrowing type via Function Assertion loses numeric type, Previous generic union does not narrow to an union again with type predicate, Subtype narrowing bug with falsey types (''/0/[]), Discriminated union narrowed incorrectly by user-defined type predicate, Incorrect narrowing when a type guard returns a type union, Type guard does not work correctly with empty string type, union types narrowed incorrectly by assertion functions and type predicate functions, Type narrowing of union types by type predicate narrows to only first branch of union, String union types narrowed to falsy should narrow string to "", Type predicate can sometimes result in different type than type discrimination, Object literal types with only optional properties break control flow narrowing, asserts that return type is null | Generic is reported as. This is because any extends anything. The problem is that inside of the arrow function the compiler is not certain that the type narrowing is still valid. Site design / logo 2022 Stack Exchange Inc; user contributions licensed under CC BY-SA. Check Undefined in Typescript at Root Level If you use == at the root level for the undefined checking in TypeScript, and the variable is undefined, you get a ReferenceError exception and the whole call stack unwinds. See https://github.com/maasglobal/maas-schemas-ts/blob/master/src/core/booking.ts for example. Thank you for the update, and the attempt! It is an unintentional absence of any value. Help us identify new roles for community members, Proposing a Community-Specific Closure Reason for non-English content, Expected type X but passed X | undefined | null even with check, why "this" is undefined in typescript module with http promise, Type 'void' is not assignable to type '((event: MouseEvent) => void) | undefined'. whenever we do not explicitly assign a value to a variable, TypeScript assigns the undefined value to it. 48 if (argument.type === AST_NODE_TYPES.TemplateLiteral) { The function body says d.someProp = 5 in its body, which is manifest nonsense for the primitive types. If function return types in .d.ts files are changed from any to string | number | boolean | object (for the purposes of excluding null or undefined as in the OP's examples), wouldn't that break a lot of user code? You signed in with another tab or window. But, most of the cases are solved by using == operator. By informing the user with an error or warning, it allows the developer to easily spot where their logic is flawed. Asking for help, clarification, or responding to other answers. I was thinking something like this would be nifty: probably a dumb idea, but at least you get the picture. This is an abbreviated reproduction of my case: We encountered a vaguely similar problem in #41871. In this article, we'll look at how to check for undefined in Typescript. TypeScript has two special types, Null and Undefined, that have the values null and undefined respectively. Sign up for a free GitHub account to open an issue and contact its maintainers and the community. On the output side, it would be string | number | boolean | object (note that object is unimplemented, see #1809). not that there's been much progress here. Is there any reason to use {} over that for the input side? in the above, state.message could be null or undefined, and we'll get a runtime error. If sandwich DOES exist, its type property should be the type of. We currently consider a type a discriminated union only when all of the possible discriminant values have types composed from unit types. Undefined is different from the value null. We can use typeof or '==' or '===' to check if a variable is null or undefined in typescript. If we could do: The compiler would highlight the bug in the code above and correctly force me to fix it like this: Trying to attach a non-null assertion using the concrete union suggested above does not seem to be working. never : T extends Nil ? In TypeScript 3.7, when opening a project with dependencies, TypeScript will automatically use the source .ts / .tsx files instead. @andrewbranch fyi I didn't look fully into this, but generally assume @jack-williams is right . With regard to "jankiness", if possible I'll use generics to dynamically enforce, or whenever possible write the type (so long as the type information is actually not redundant to the process). is a logical operator that returns its right-hand side operand when its left-hand side operand is null or undefined, and otherwise returns its left-hand side operand. I know it's not the same thing, you won't be able to use any but you're guaranteed you need to specify a type which could not be null. Typescript is a superset of javascript with an extra feature, typing, and assertions, Every piece of code works in typescripts. The keyword as, can be used to let TypeScript know, that you know the value is going to be whatever value type it expects. By using TypeScript Nullish Coalescing & Optional chaining. In a way, it would seem consistent under --strictNullChecks that any excludes null and undefined, and you would have to be explicit to include them, i.e. typescript enums union-types In TypeScript, enums have a few surprising limitations. By clicking Accept all cookies, you agree Stack Exchange can store cookies on your device and disclose information in accordance with our Cookie Policy. It seems like his problem is different from mine. Ok with any. How to use enum as index key type in typescript? Other JetBrains IDEs support them too, either out of the box or via a free plugin.. Configuring tsconfig.json #. - opyh Jul 27, 2019 at 23:03 Yes, I agree bcoz null === undefined is false. TypeScript doesn't protect me against that by giving a compile-time error when I assume state.message is there. By using simple if condition. A hypothetical future where we can stop treating branded primitives as they exist today (e.g. TypeScript Version: 3.9.7. This meets the requirement for both goals 7 and 8, but looks absolutely horrendous and arguably (if not explicitly) violates goals 4, 5, 7, and 11. Not saying that's good or bad, just a big breaking change. It would be more correct to say setPrototypeOf(..): object (given #1809). react native typescript template not working; how to check if object is undefined in typescript; angular timestamp component; typescript array of empty objects; ts(2503) type script array declaration; typescript check if value is in enum; typescript file cannot find module vue; jest Data path "" should have required property 'tsConfig'. After this predicate, the remaining type either does not have the checked property, or it is undefined.. One downside to this alternative is that the action of picking the right property involves insider knowledge about the type. any is a very good solution to a very hard problem, even if the naming is a bit weird (it implies that it is any of the built-in types). I ran into this in real-world code. I'm realizing now that a return type of { [key: string]: any } wont provide enough information to prevent indexing an undefined property - which makes sense, because the consumer doesn't have a contract for what properties will be provided on the return value, even though it will be guaranteed that none of those returned properties will have a value of null or undefined. The text was updated successfully, but these errors were encountered: As a quick fix for now there is a lint rule: https://palantir.github.io/tslint/rules/strict-type-predicates/. typescript string or undefined javascript if not null or empty js check null or empty string javascript check if object is empty or undefined check for null or undefined in javascript javascript if undefined then empty string check if object is empty or undefined javascript typescript non null assertion null and undefined in typescript In principle, it is quite the same in Kotlin language, use two !! Are the S&P 500 and Dow Jones Industrial Average securities? This should give warning that null check is not needed or should be moved outside the inner function. Typescript should warn when checking for null or undefined on non-nullable type, // Should warn that this is unnessary and suggest removing or adding ` | undefined` to name type. Cross-posting from PR #31206 for visibility: We discussed this in a design meeting and decided theres not currently a solution thats worth the disruption it will bring. Since that will accept anything whatsoever, so why would the other signature affect it? In any case, your example is no different then just having the any signature, isn't it? To make a variable null we must assign null value to it as by default in typescript unassigned values are termed undefined. This is the feature that is not currently available within TypeScript in any form whatsoever, aside from implementing multiple methods with different names - which is an API nightmare. You can write your own some in 10 seconds, and things will just work out how you want them to. The possibly-canonical issue for this is microsoft/TypeScript#9998 ("Trade-offs in Control Flow Analysis"), with microsoft/TypeScript#11498 ("Annotate immediately-invoked functions for inlining flow control analysis") as a proposal to address cases like this. I appreciate that, however the title of this issue is explicitly non-nullable any. The workaround of not using union type guards works, though, even if cumbersome. an expression evaluated to false for the below values Arbitrary shape cut into triangles and packed into rectangle of the same area. It does not allow the consumer to believe the result is any or even string | number. The rest of this page applies for when strictNullChecks is enabled. Assuming the previous thinking to be accurate, I would like to provide a reason for why this feature does not currently exist in TypeScript and why it may not be a good idea to add that feature to this language specifically With the design principal that TypeScript is a super-set of JavaScript and transpiles directly to a given JavaScript version to be consumed by engines which support said version, let's go back to the operate example I provided and instead implement it using a hypothetical overload feature to see how the resulting JavaScript might look. to your account, TypeScript Version: 3.3.0, and the version on TS playground (3.4.1 as of this writing), Search Terms: typeguard, empty string literal, undefined, union, Also, unrelated, but this is where I would have liked a NaN literal in TS =/. never : T; Try it out, it does not work. For the general return type scenario, is { [key:string]: any } presenting you with an issue? - jcalz Jan 14, 2021 at 15:55 1 Interesting - I wasn't aware that type guards worked in this scope-specific way. Just want to also echo what was said earlier. any is a feature which could only be implemented as part of the TypeScript language. This is only half correct: 1) null and undefined DO have types in TypeScript. It's a standard way to say, "This thing you asked for doesn't exist." Thinking about it all the time tends to break brains, though and not thinking about it introduces bugs. Again, it won't change compilation but it can be seen as helpful from a documentation perspective. We do not currently allow content pasted from ChatGPT on Stack Overflow; read our policy here. InputType is string|() => string|undefined, so I would expect the intersection of InputType & Defined to be narrowed to string|() => string. I can understand where you're coming from but I care for catching null/undefined over other type related errors because the other type related errors can be seen though a lot easier then null/undefined. Well occasionally send you account related emails. Additionally, it violates goals 3, 5, 7, 8. and, in a way, 9. Returns the object o. operator with code like this: Obviously this is a very simple example to demonstrate my point, but this can get much more challenging with a very large and deeply nested code base. Why does this undefined check not work with a find function? 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