For example if you wanted to get the position of the element. It will become hidden in your post, but will still be visible via the comment's permalink.

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Type 'undefined' is not assignable to type 'string' (reduced from a null-able function parameter). Have a question about this project? It mostly only infers You are not conforming to these requirements. Discover how to use useRef with TypeScript in React and React native, and how to fix issues with MutableRefObject and object is possibly 'null'. Well occasionally send you account related emails.

It's not clear what the limitations are from the linked discussion. So to solve this, we can either explicitly set the generic type, or be aware of when and how TypeScript is going to infer the generic type. Get my latest articles delivered directly to your inbox. We did not add this feature for a few reasons. Templates let you quickly answer FAQs or store snippets for re-use. if(typeof ez_ad_units!='undefined'){ez_ad_units.push([[300,250],'atomizedobjects_com-banner-1','ezslot_2',164,'0','0'])};if(typeof __ez_fad_position!='undefined'){__ez_fad_position('div-gpt-ad-atomizedobjects_com-banner-1-0')};With that, lets dive straight into how you can use it with TypeScript! privacy statement. The reason why we then need to provide null as the initial argument to useRef is because otherwise it would be initialized to undefined which wont match either null or HTMLParagraphElement which would cause a type error for MutableRefObject which would be something along the lines of "Type 'undefined' is not assignable to type 'HTMLParagraphElement | null'. As you can see, instead of providing the number type, we are now providing the type null | HTMLParagraphElement which just means the ref can be of types null or HTMLParagraphElement. However, in g, the assigned type of y would still allow assignment of undefined. What's going on with index types in TypeScript. So basically I wasnt covering a situation in my reducer where it was possible to return a value of.

I have worked with companies such as Atos, The Telegraph, Capgemini, Babylon Health and more. By clicking Sign up for GitHub, you agree to our terms of service and // Type '"test"' is not assignable to type 'null'. I chose this path because I consider that if my reducer is given a status that is invalid, that status should be rejected. A software development course that doesnt get you stuck with specifics from a tutorial, but challenges you to think like a software architect. Some of the common errors you might see when using useRef with TypeScript include things like object is possibly null, or MutableRefObject. This additional type would let the compiler disallow the call f(undefined) but allow g(undefined). ins.style.display='block';ins.style.minWidth=container.attributes.ezaw.value+'px';ins.style.width='100%';ins.style.height=container.attributes.ezah.value+'px';container.appendChild(ins);(adsbygoogle=window.adsbygoogle||[]).push({});window.ezoSTPixelAdd(slotId,'stat_source_id',44);window.ezoSTPixelAdd(slotId,'adsensetype',1);var lo=new MutationObserver(window.ezaslEvent);lo.observe(document.getElementById(slotId+'-asloaded'),{attributes:true});So by wrapping it in a condition or using the optional chaining operator then you are first checking that the ref exists and that it is not null. Here you will find my personal recomendations to you, for full disclosure I earn a small commission from some of these links, but I only recommend what I trust and personally use.

The Macbook Pro is easily the best laptop for frontend development of any kind (and almost all other types of development) and will boost your productivity like never before. This software engineering book is written by Gayle Laakmann McDowell and is immensely useful if you are looking to learn the skills you need to accelerate your career and land yourself jobs at top tier tech companies such as Google, Facebook, or Microsoft. Thanks for enabling notifications! The fix was simple and I had two options. We might re-visit this in the future, but I don't think our decision will change. This means that the type annotation is not really changing any thing, it is still string | undefined for the caller, and string within the function body. DEV Community 2016 - 2022. It only applies to a couple of cases default initialisers and object literal assignability. Have you been having issues when trying to use useRef with TypeScript in React or React Native? useRef is one of the core React hooks that come shipped directly with React. Some graphics used on this post were made using icons from flaticon. Sign in if(typeof ez_ad_units!='undefined'){ez_ad_units.push([[250,250],'atomizedobjects_com-large-mobile-banner-1','ezslot_6',166,'0','0'])};if(typeof __ez_fad_position!='undefined'){__ez_fad_position('div-gpt-ad-atomizedobjects_com-large-mobile-banner-1-0')};An empty MutableRefObject will be the type for { current: undefined }. this.currentUser = JSON.parse(localStorage.getItem('currentUser') || '{}'); Successfully merging a pull request may close this issue. So basically I wasnt covering a situation in my reducer where it was possible to return a value of undefined. Here is an example of how to use useRef with TypeScript: Firstly, since there are many different uses for refs, we need to look at a few different examples, but they all consist of providing your types via the chevrons to the useRef React hook. How well will you will do? Once unpublished, all posts by dwjohnston will become hidden and only accessible to themselves.

In this guide you will discover everything you need to know about React Keys to be able to use them properly in any React app along with the ins and outs. @mhegazy But can't we just do something similar with something like: I understand that sometimes if we write (p = '') in parameter list we would want the type of p to be string, but maybe we should treat p: string | undefined = '' differently as it explicitly specifies the type of p. Yes, I think there is a difference between "I said this could be undefined for callers" vs "this can always be undefined". To fix this issue we can either ensure we initialize the ref to null, or we could add undefined to the type. Given that, heres the final reducer. I hope this post has helped, but before you go, make sure you try my fun interactive quiz to see if you are a novice, intermediate, or expert React engineer. There werent any blatant syntax errors or type mismatches. You signed in with another tab or window.

Option #2 was to halt the execution of the function and therefore not even allowing a return value to happen. This will make sense when you realize that in Javascripta default value will be returnedif you dont add a return statement to a function.

To use useRef with TypeScript, it is actually pretty easy, all you need to do is provide the type that you want to the function via open and closed chevrons like you would with other React hooks like so: const myRef = useRef(0). This will give you hints as to why TypeScript is complaining. Sometimes the error doesnt say youre missing X. So I dug a little further.

The type passed in for A will be used for the function argument action.. This post will cover how to wait for all promises to resolve in JavaScript rather than having to take them one by one when you have multiple promises that need resolving. All we are doing here is providing the correct types to the ref so it can use it as a ref for our paragraph element. For example if you pass in the type string, the type would be returned as MutableRefObject which will look like { current: string }. For this reason, you have to actually use your analytic skills to figure out the problem root problem. That is, leaving the generic parameter off is equivalent of doing: The generic parameter determines the type of the value and setValue variables. // ok, undefined was removed by narrowing, // missing other1: error before and after, // missing other3: error before and after. My reducer didnt specify a return type. These cases are both of limited scope; common usage won't encounter the distinction very often. For example this is what your code might look like to get the object is possibly null error: And to fix it all you need to do is either add a condition or use the optional chaining operator like so: To read more about the optional chaining operator and to see how it can benefit you, I highly recommend having a read of my post about nullish coalesing and optional chaining . It is complex: the compiler would need yet another null-like type and control flow would have to correctly narrow it when used in parameters and object literals. To summarise how to use useRef with TypeScript in React or React native, the main thing to remember is to make sure that you add in your types using the chevrons, and check to make sure the ref is not null before using it. This leaves TypeScript to infer the generic type from the arguments given. if(typeof ez_ad_units!='undefined'){ez_ad_units.push([[250,250],'atomizedobjects_com-leader-1','ezslot_5',165,'0','0'])};if(typeof __ez_fad_position!='undefined'){__ez_fad_position('div-gpt-ad-atomizedobjects_com-leader-1-0')};Because of that we need to give it another value to start with temporarily, this can be anything you like but generally it would be null for an element. They can still re-publish the post if they are not suspended.

I looked into the definition of the Reducer generic.

DEV Community A constructive and inclusive social network for software developers. It will only take about 1 - 3 minutes, so you have nothing to lose, test your abilities now. For the most part if you have the object is possibly null error message from trying to use useRef in TypeScript, it probably means you are referencing a ref object that could be null because it might not yet have been initialized with an element at the place you are using it. At this point I had an error that ACTUALLY MADE SENSE. Notice the error I throw and where I throw it: If youre wondering how Im typing the rest of my reducer here are those pieces. Without going into too much detail, what this means is that useRef will enable you to be able to store data inside of a ref that wont trigger any re-renders and will keep its value between re-renders as well so you wont ever lose what is inside of it.

Remove undefined from the type of default initialised parameters when narrowing. Typing everything in TypeScript comes with a lot of surprises because oftentimes the error youre given is cryptic. Are you a novice, intermediate or expert react engineer? The mixture of complexity, limited applicability and possibility of unwanted breakage sank this feature. One way to think of it is kind of like a state that you can get and set directly without causing re-renders. The type passed in as S will be used for the prevStatefunction argument and the return type of the function. This code produces the following TypeScript error: Or set the initial state to an empty string. The default that is returned is, . In this post we will cover everything you need in order to know how to use useRef with TypeScript as well as fixing the object is possibly null error as well as any MutableRefObject errors you might be having. Dont worry, I hate spam too and I wont ever disclose your contact information to 3rd parties. This post will cover everything you need to know about React Fragments in a clear and concise way and will be the ultimate resource for any React Fragment related query. In this case, the setValue function is going to be the equivalent of: The input component doesn't like this, the event is creating has a target.value of type string. The useState function has a generic parameter which we are not setting/declaring in the original case.

One thing that does help to debug a TypeScript problem is to understand what youre expecting and what youre actually encountering. Tricky little TypeScript bits (2 Part Series).

Now my reducer signature looked like the following: Now I get a new TypeScript error on my useReducer function. should add missing instead of undefined as well: As the error says, localStorage.getItem() can return either a string or null. I am also an open source contributor to projects such as GatsbyJS. 2022 Jorge Colon Consulting. if(typeof ez_ad_units!='undefined'){ez_ad_units.push([[300,250],'atomizedobjects_com-box-4','ezslot_3',154,'0','0'])};if(typeof __ez_fad_position!='undefined'){__ez_fad_position('div-gpt-ad-atomizedobjects_com-box-4-0')};A common use is paring it with components/elements so you can access them and adjust them with vanilla JS. The default that is returned isundefined. It lets you use a mutable value that will be persisted across re-renders. Become an expert in ReactJS, TypeScript, and JavaScript. If you want to learn more about useRef and get a better understanding of it, you should check out this post I have written all about how to use useRef. This is because when you first call useRef it does not start with an elements ref, and it will only be able to access it after the first render. That's the missing piece here. Are you sure you want to hide this comment? would add it; I'm not sure if there would be syntax in the language for it. Schedule a free consultation with me today, if you dont add a return statement to a function.

I am a senior software engineer specializing in React, JavaScript and TypeScript with over 8 years of professional experience. Now lets take a look at how this would change if we wanted to use an element. JSON.parse() requires a string, so you should test the result of localStorage.getItem() before you try to use it. See #12033 for more details on why and how it was implemented. Once suspended, dwjohnston will not be able to comment or publish posts until their suspension is removed. Made with love and Ruby on Rails. I always recommend looking at the actual implementations of the type definitions to see how the generics are used. It would allow the compiler to distinguish these cases: By letting the assigned type of x: string | missing and y: string | undefined | missing. and only accessible to David Johnston.

This means whatever type you pass in will be used to create the final type that is returned. this post I have written all about how to use useRef, Generic types which you can read more about here.

It is also worth noting that these fixes not only apply to React, but also apply when using useRef with TypeScript in React Native as well.

For further actions, you may consider blocking this person and/or reporting abuse. For example if you have a ref that only accepts null but you are trying to define it as something else then you might see this error: However if you then change this to allow strings as well we no longer will get that error: To fix this you just need to make sure you wrap your ref in a condition or use the optional chaining operator to safely check if you can use the ref. Join the platform that top tier companies are using.Master the skills you need to succeed as a software engineer and take your career to the next level with Pluralsight. Thats possible by throwing an Error object. One thing I realized here for the Reducer type is that a return type is part of the requirement of making that type valid. Already on GitHub?

Once unpublished, this post will become invisible to the public Sign up for a free GitHub account to open an issue and contact its maintainers and the community. If we added missingType, default initialisers and ? With you every step of your journey. Because we have not added undefined as part of are types we cant then assign undefined to the ref we have created.

Follow me on Twitter to stay up to date and learn frontend, React, JavaScript, and TypeScript tips and tricks! The text was updated successfully, but these errors were encountered: you can work around it by doing function foo(bar = '') { @krisselden Nice try but that will make the variable string | undefined when entering the function. ins.dataset.adChannel=cid;if(ffid==2){ins.dataset.fullWidthResponsive='true';} In this post we cover the differences between, React.Fragment vs div, the benefits of a fragment and when you should use one over the other.

If you are seeing an error message including "MutableRefObject" it probably just means that you havent provided the correct type to the useRef hook compared to how you are using your ref. Option #1 was to just add a return value passing back the previous state.

The crux here is outside the function the type is string | undefined. (This may not be appropriate for all cases, ie maybe you really do want to leave that value undefined). var asau='1244499959';var cid='7539148863';var pid='ca-pub-8554082412373655';var slotId='div-gpt-ad-atomizedobjects_com-leader-2-0';var ffid=1;var alS=1702%1000;var container=document.getElementById(slotId);container.style.width='100%';var ins=document.createElement('ins');ins.id=slotId+'-asloaded';ins.className='adsbygoogle ezasloaded';ins.dataset.adClient=pid;if(typeof window.adsenseNoUnit=='undefined'){ins.dataset.adSlot=asau;} We have talked in the past about distinguishing yet another "empty" value called missing, which would allow us to distinguish between things like f() and f(undefined) or {} and { x: undefined }.

in side the function, since there is a default, the undefined is stripped out of the type leaving you with only string. That was the biggest clue as to what was going on. And inside the function the default initialiser would remove both undefined and missing, so the type of both would be string. Built on Forem the open source software that powers DEV and other inclusive communities. I hope that helps you figuring out TypeScript errors, especially ones related to generics.

This means that before the first render the ref could be null which is where the object is possibly null error comes from. I was missing a return statement for a default condition! This is the code that I had for my reducer: This didnt make any sense to me since my code looked right. @sandersn. This is how you would use Generic types which you can read more about here. We need to provide a second type of null because the ref wont start initialized to the paragraph ref because it only gets initialized to that after the first render. In my case I was seeing an error of undefined coming from a type definition where I never specified anything about undefined. [1] To be consistent object literal ? to your account. All rights reserved. For example: You don't know JS yet is one of the best, most complete books in the industry if you want to get to know JavaScript in depth. Because we dont need to use undefined, the best option here is to initialize the useRef hook with null instead. ts(2322)".

This means that your ref could equal null instead of what you want it to equal. An Introduction to Algorithms is pretty popular amongst software engineers because it is one of the best and most complete books you will find on computer algorithms. // ^ Type 'undefined' is not assignable to type 'string'. Once unsuspended, dwjohnston will be able to comment and publish posts again. A MutableRefObject is the type of a ref that is returned from useRef in React. We're a place where coders share, stay up-to-date and grow their careers.

This is really a design limitation in the existing system. The implementation for that looks like this: So this reads as, The type Reducer takes two generic type parameters, S and A. If dwjohnston is not suspended, they can still re-publish their posts from their dashboard. TypeScript doesn't know that you later want to set a string into state here. As you can see from the above example, we are allowing the ref to only be of type number. In this case useState() with no arguments, TypeScript infers the type as undefined. So thats what I did next.

There probably wouldn't be any values at all of type missing because it signifies the absence of values.