typescript when to use null vs undefined
This is how it was intended to be used in the spec, I don't read it that way. Like you say, many libraries have their own philosophies but you include many libraries for 1 project. Many also don't work that way. It wouldn't bother me. when you have Vim mapped to always print two? On the other hand, use null if and only if you intentionally want a variable to have "no value". Set a default parameter value for a JavaScript function. The global undefined property represents the primitive value undefined. as immutable outside the component that created them. In more complicated cases other developers have to read the whole components code to understand how it works and pass data accordingly. Which seems to be a bit strict, since it would limit the use of undefined to properties/ variables which have not been set. value, writability, enumerability and configurability). Use optional parameters only if its really optional and component will work as expected if user does not pass it. When do I initialize variables in JavaScript with null or not at all? Is there a legal reason that organizations often refuse to comment on an issue citing "ongoing litigation"? Of course, we all know that accidentally defining a global variable is totally impossible in javascript Just wanna add that with usage of certain javascript libraries, null and undefined can have unintended consequences. Building a safer community: Announcing our new Code of Conduct, Balancing a PhD program with a startup career (Ep. Not even to mention that using them in typescript means two different things. The nextSibling of the last child of an element is null. I will argue there is no best way, and even standard functions sometimes choose one or the other. Mozilla Docs Didn't answer my question google didn't either :\. What should I return when object is not found in TypeScript using strict mode? How can I correctly use LazySubsets from Wolfram's Lazy package? By clicking Accept all cookies, you agree Stack Exchange can store cookies on your device and disclose information in accordance with our Cookie Policy. I'm adding my 2 cents. And you've provided a good one. Undefined typically refers to something which has not yet been assigned a value (yet). Which is why in my code I never use null unless something I don't control returns null (regex matching for example). In this episode on the series to learn about TypeScript I will explain a couple of reasons.Chapters0:00 Introduction0:40 Undefined better than null1:00 TypeScript Optional Parameter1:45 TypeScript Triple Equals3:05 Null and undefined typeof3:30 TypeScript Desctruction Difference with null and undefined4:30 State Tracking Alternative6:15 ConclusionSource Codehttps://github.com/MrDesjardins/010-TypeScript-Null-vs-UndefinedSocial Networks Follow me on Twitter: https://twitter.com/mrdesjardins Follow me on LinkedIn: http://linkedin.com/in/patrickdesjardins My blog: http://patrickdesjardins.com/blogMy Books \u0026 Courses My TypeScript Book: https://typescriptbook.com/ My TypeScript Course: https://www.educative.io/courses/learn-typescript-complete-course undefined implies no variable or object exists in the compilation at all. operator treat both undefined and null as equivalent. The term "object" value is misleading, since in JS even the primitives behave like objects for the most part due to autoboxing, Yeah, that makes sense. Not ideal and can be improved but at least component behaves sort of as expected. 19 Answers Sorted by: 110 I don't really have an answer, but according to Nicholas C. Zakas, page 30 of his book "Professional JavaScript for Web Developers": When defining a variable that is meant to later hold an object, it is advisable to initialize the variable to null as opposed to anything else. That is why its always better to check if the variable exists and has been assigned a variable using undefined before checking if the value is null or empty. That is why I would not rely on "truthy" checks for true/false with null and undefined, even though they will both return a false response, as undefined implies an additional step for missing feature, object, or variable, not just a true/false check. And besides, !undefined and !null are both true so most programmers treat them as equivalent. undefined is used to describe variables that do not point to a reference. Like null, undefined is also a primitive value type. If you want to write javascript that isn't awful, always use triple equals === and never use null (use undefined instead). getNextCard however seems to be able to compute the next card (if it exists) and also be able to compute if there is no next card. In your example the concept of a next card exists but the card itself may not exist. This same issue forces you to use typeof var === 'undefined' when getting function results. Feb 16, 2021 1 The most important thing to understand is that "?" optional property and same property declared as required but possibly undefined mean absolutely different things. TypeScript has two special values for Null and Undefined. If you have a missing undeclared variable, truthy statements will trigger an ERROR! Instead, use solely undefined. Whole meaning of using null is just bind variable or property with object which is singleton and have meaning of emptiness,and also null usage have performance purposes. When a variable or a property on an object does not have a value it is undefined , and for a value to be null you need to assign it a null value. At compile time the JS engine will set the value of all hoisted variables to undefined. It is not used consistently enough to be meaningful. I literally never need to or want to use falsy checks in my code and always use, How do you know if an attempt was made to assign a value, but it failed and. Other languages tend to use null widely, a lot of them don't even have undefined (php for example). typeof null returns an object. It really depends on what you're trying to check. General Assumptions Consider objects like Nodes, Symbols, etc. (When) do filtered colimits exist in the effective topos? Connect and share knowledge within a single location that is structured and easy to search. Is there a way to make Mathematica support Chemmacros of LaTeX? If another developer sees "undefined" they're not going to intuitively assume it's you who made it undefined, they're going to assume it's not been initialized when in fact it was. How can I determine if a variable is 'undefined' or 'null'? A document.getElementById reference is null if the element does not exist in the document. So in this context, I would initialize these types of things with 'undefined' to be consistent with JavaScript (for variables, you can do var myVar; instead of var myVar = undefined;). difference between null and undefined in JavaScript? What are some practical reasons to use null instead of undefined? In other words, the function is total since it terminates for every input. This is useful when working with data that may not always be present, such as optional function parameters or properties on an object that may or may not exist. ts(2532) We can use the non-null assertion operator to tell the compiler explicitly that this variable has a value and is not null or undefined. Why is my JS only working if I put it after the HTML, even though I have a window.onload? Why is there a `null` value in JavaScript? Good point, I guess this is similar to this article. Is Spider-Man the only Marvel character that has been represented as multiple non-human characters? @Oriol, I think what @choz was trying to say (as a few other also mentioned on this question) that if I want to return. This 2 code have difference execution time. Is there any function difference between using != null and != undefined in javascript? Undefined means that parameter must be passed in but its value may be undefined. If you want it null you have to explicitly use: And even then you'll be forced to initialize it at least with "null". Also, I rarely use null. In this way code can communicate to you whether something was caused by uninitialized stuff or null values. I see what you're doing there - and I can't say you're wrong - because in a sense I think you could do this here and it would be fine. This is as opposed to undefined meaning "this thing isn't there at all". that object has a value of undefined. Rationale for sending manned mission to another star? In that case, I would recommend returning a null. It implies something more. 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.. Then I'm (pretty) sure that. Note that a function with no specified return value implicitly returns undefined. Does the policy change for AI-generated content affect users who (want to) What is the difference between null and undefined in JavaScript? What do the characters on this CCTV lens mean? Further, if I am doing, This should be the accepted answer. Please post an answer to this effect and I will accept it. Only use null if you explicitly want to denote the value of a variable as having "no value". From the ECMAScript2015 spec. However, the property may not exist. undefined is meant to say that the reference is not existing. On the other hand, you can also initialize the variable with, The only problem I have (since I started with JavaScript 15 years ago) is that you too often have to test, This should have been the selected answer. Is there any philosophical theory behind the concept of object in computer science? The variable has either not been declared or declared with a missing value so not initialized. Or using things like null in props for something like a classname will end up with 'null' in the .html code and break your css classes. By clicking Post Your Answer, you agree to our terms of service and acknowledge that you have read and understand our privacy policy and code of conduct. For example, lodash's get function, which accepts a default value as a 3rd argument: Another example: If you use defaultProps in React, if a property is passed null, default props are not used because null is interpreted as a defined value. And if you're in the habit of using == or simply stuff like if(x) , stop it. What is the purpose of it? That said, there is one major difference - variables that aren't initialized (including function parameters where no argument was passed, among other things) are always undefined. Of course, there must be a way to say that an object does not inherit from any other one. The code that calls this function knows how to deal with undefined (it actually won't ever really happen unless something goes horribly wrong). We dont want to pass isLarge as false all the time when we need the most common use case a small button: In your application you will have both cases of optional ? parameter and undefined type and its absolutely normal and depends on the design of your component. Where an object is always returned, needed or wanted by design, use null for falsy results (eg, +1 yes, I agree and I always try to remember to initialize vars to null. (39 answers) Closed 5 years ago. undefined is a primitive value automatically assigned to variables. Connect and share knowledge within a single location that is structured and easy to search. */. So at the end of the day, the convention is practically useless in anything other than very small programs in simple environments. lib.d.ts contains 473 times of | null . undefined implicitly represents the absence of meaning of that value in your application's context. Yah @Oriol, this is why I actually enjoy debates, even on Q/A sites. If the answer is yes then use null else use undefined. For example, if you want to see whether a global variable named 'myVar' exists, then. While this is a good stance, Id like to add this answer: @FrederikKrautwald thanks for the link -- that is a very clear delineation. Many javascript libraries work like this way. the placeholder for an undefined point x of a partial function f which is often written f(x) = ). If I would make that property intentionally empty, then it must be null so you know that it's on purpose. The reason being is that null is of type of object, where as undefined is of type undefined. /* If isSubmit is not passed then component will not break and will show Cancel button text. Typescript. Just understand they represent different states of variables and objects. How do I check for an empty/undefined/null string in JavaScript? @Pete - but it tells you nothing about the "disaster", so what's the point? It would not be the last time that you concat a null object to a string and get a "null" as a string. I know in javascript it is possible to use both of them in order to check a variable has no value. So, the analogy to undefined is a bit tenous. There are some general rules about usage of optional parameters in the code: 3. Should convert 'k' and 't' sounds to 'g' and 'd' sounds when they follow 's' in a word for pronunciation? The most important thing to understand is that ? optional property and same property declared as required but possibly undefined mean absolutely different things. Also notice a variable assigned a null value is actually an object type unlike undefined types. Notice declared variables that are not initialized are assigned a value of undefined, the primitive value type but variables that do not exist are undefined types. Update the question so it can be answered with facts and citations by editing this post. or to initialize some. It seems that undefined is always preferable and serves the same purpose programmatically. What reason is there to use null instead of undefined in JavaScript? I just wanted to point out that destructuring argument defaults don't work with null. getUserData({ name: 'Mark', age: undefined }); const MyButton: React.FC