Most Senior React Devs Don’t Know How To Fix This 

been using keys for a long time and this video has actually finally made me understand what they truly are

This is funny; I literally dealt with this recently in a project. Figuring it out was frustrating because like others have mentioned, I had really only been using keys when mapping over arrays. I experienced all the emotions - disbelief, then anger (thinking I had found a bug with React), then frustration, then relief after adding keys, followed by confusion wondering whether I really should use keys - and after much research and digging around and seeing others do this, it made sense. Sometimes I think we miss some fundamentals while we “learn as we go”.

Interestingly, this is usually highlighted in reacts docs: 1. State is tied to a position in the tree 2. Same component at the same position preserves state 3. Different components at the same position reset state

Never used keys outside arrays!! Thanks Kyle! It was quite helpful.

React community is so blessed having you, thank you, Kyle!

As a React learner, this was a very useful video Thank you and keep doing the good work

This short video is actually very useful. I have a scroll position persistence problem in a project and this should solve that

It really helped me a lot to find out the "unique key props" errors in the react projects. Thanks a lot for your efforts as well.

I'm really new to react and watch this yesterday and today I run into just this kind of problem but could now directly solve it. Thank you. 🙏

Ever since I learned about how useful keys are in react it made my life easier, 'cause before I sometimes use useEffect just to get the behavior I wanted, and is prone to unnecessary rerender. I just saw it on TikTok. And yes, this is very useful in so many cases. Thank you Kyle.

I had this exact issue last week with an array of components. I have they keys as the array index so they persisted when I changed the elements of the array. I worked around it a different way but now I know a much cleaner solve.

I don't think i have run into this yet, going to have to keep a lookout for this. Thanks!

This is so helpful! Thank you so much for helping!

I remember encountering this issue 3 years ago when I was building an inventory management system. Took me days to understand why the state was persisting across re-renders.

Thank you for publishing this. This tip has already helped me in multiple projects.

This really helped me improve my mental model around keys in react. Thanks for the video❤

That's wild! I literally experienced this while working with a controlled select element yesterday. Read through a github discussion and learned that keys could reset my select for the next round of my app to do its thing. I didn't fully get it, but I was like "it is what it is". Then YT recommends this video to me a day later. How perfectly timed! I was basically primed to appreciate this video. Thanks Kyle, this was awesome.

Boosting careers out here, keep em coming Kyle we appreciate you very much man!

Nice tutorial, really good explanation of what’s happening under the hood and some applications of how it’s used. Love it

Now I understand the purpose of keys in React. One important point you mentioned is that keys are mostly used in loops when returning JSX elements, and the compiler always warns us to use a key in such scenarios. Thanks to this video, I can now reuse the same component without encountering issues of conflicting distinctions.

That's some gold nugget there! I didn't even know about how react treats the dom like that! Thanks I will definetly get your course now.

I've had a series of sneaky bugs where data was persisting between pages of a dynamic react-router route, and fixed it with some workarounds. Turns out, adding a "key" actually solves all of them! Even better yet, I didnt go searching for this video. It just appeared in my recommended

Best explanation about keys in react I've seen so far

this is actually really important, as knowing reconciler algorithms helps to avoid so much pain one of the reasons why conditional rendering is a thing - it allows us to preserve positions {isTest && } } no matter what value 'isTest' is, Component's element will be on index-1, while index-0 is either false or same thing here - reconciler only cares about position and type. so if type and position is the same - it only updates that's why if you construct component type dynamically, you should memoize it properly (or use another approach tbh), cause if not your component will just keep re-mounting

Loved this, didn't know about it!🙌

We have that "issue" often with our icons. Thanks for your detailed explanation :)

Keys become really important whenever dealing with animations. And there it also becomes super clear how this works. I recently wrote a component that fades in a view and fades out another one at the same time, like a visual animated swap of components. Without keys properly set, animations would never work correctly. Good point again Kyle, thanks for bringing it up!

I fixed a bug at work today with this, about a month after watching this video, thanks! 🎉

Good vid! One your most helpful ones (for me, at least)!

I already knew this because of dart/flutter. It works absolutely the same way, with the same syntax too 😂 I think you could start a flutter channel too, cause dart is basically React + static typed variables with null safety.

This is enlightening! I guess this peculiar behavior comes from React's reconciliation mechanism, where React finds the React DOM node that's changed and then updates the changed node and its child nodes, while unchanged nodes and their child nodes remain intact.

Using key was my first idea of how to deal with this issue when I watched the intro :) Thanks for the useful video as always :)

What a piece of work the React, that's why it's happening. Tell us that Snow and Charcoal is exactly the same thing, and go on to tell how to fix this bug in _our_ code. We just need add keys to all of our code, no problem. Otherwise internal state of a component will no any binding to the component itself, and you completely messed up. Oh, we don't need to add keys to all of our code... But you need to guess, do the react knows is this chunk of DOM is the same than the other one or not. So better look better. Peace of beautiful work.

Today this video saved my day (and my mental health). Thank you!

Oh wow. This video is amazing! Thank you :)

The new react doc explained this so well too. Cool video

Keys are fantastic. I learned the power of them a little bit ago from the (back then beta) new react docs, from the 'you might not need an effect' section. Basically immediately went and removed 10 useEffects, each ~15 lines long, just replaced them with initial state setters and keys. It was so nice.

yea cool. Right out of ur awesome react course w arrays. Love it

Thank you so much! Your videos are gems!❤

This video helped a lot in clearing up another React nuance

Very good explanation on how React uses the DOM.

That is great explanation Kyle.

Thank you! this is so clear and helpful.

it was great explanation. thanks!

😮 shocked so many people don’t know this. Well explained though. Always solid content! Knowledge is power 🎉

Nice video. Good to shed some light on the internals of React's virtual DOM. Personally, this makes me so glad I don't use React for my personal projects.

I encountered this bug before and resolved it by utilizing the useEffect hook, with 'name' specified as a dependency. Whenever the name changes, the counter restarts. However, it would have been great if I had known about this earlier. XD

For React "isKyle ? : " is the equivalent of , so it would be mounted only once, and so the state. A stupid solution could have been putting a prop "isKyle" to Counter, and using a useEffect(() => setCounter(0), [isKyle]), so everytime Kyle state changes, the useEffect will be triggered, resetting the counter. The best solution can be using "key" as he said during the video. The solutions, sometimes, can be a lot, especially when the code becomes more complex as you go. But the "best" (better) solution can be known only when you deep understand what you are using (React framework, in this case). That's why you should always deepen your knowledge about a new framework/library/sdk/language, when start learning it, otherwise you will always be a "half developer", that will cause spaghetti code, bad maintained code, no extendibility, etc. I worked with people that didn't know about useMemo, useCallback and memo(), Suspense, Lazy, etc. and they didn't know how to use them. In big projects you risk to destroy the application performance, because you tend to use the solution that can work in that very moment. Instead, the best practice is always trying to create/find the best solution, thinking that can be reusable for other components, thinking that in the future the code can be changed, thinking that that code can be read by someone else. When you always try to do better, it will be easier for your brain to find the best solution for that problem, because you are training your brain to think Out of the box. Instead, if you find the cheap solution, your brain will always stay at the same level. Hope it helps to new programmers, or new React developers.

I always come here when I need to get smth quick fast 👍great work

Neat. Well explained as always.

Informative, Thanks !

good topic, perfect explanation

well done! Great Job bro!

You are really incredible man awesome keep going👌👌👍

Nice deep dive, thanks

Very informative kyle. You are awesome

I understood the problem before the reveal. I feel special. :D

wow this seems super useful!!👍

Very usefull explaination, keep going man.

Happy to know I knew the answer to this

wow it's really helpful video. thank you

I'm glad to know I actually learnt this like 5 years ago... I been doing react since early 2015, but great tip tho!

Very useful content! thanks

im currently working on a project and i dealt with this and i just gave up on solving it really thank u u helped me alot and more ppl like me thanks again ♥

I was running multiple times on this issue but it was ok because React notify you on your browser elements should have keys haha except for the case early he was explaining, this can be tricky! well done :)

Great content, as usual, Kyle

Nice tip. Thanks.

0:21 i had something similar in flutter, where i was changing the rendering order of a few stacked widgets and they were behaving weirdly until i added keys

I don't think it's senior-level stuff, but I understand that the clickbaity video title is good for the channel in general.

Awesome explaination

after 10sec the answer is key :) The learn react tutorial was really amazing and was covering this example. I suggest all new react learners to work trough the official react tutorials they are very good.

This is helpful 👏

I watch your videos for such a long time that I said "..you have to use key" with you. Greeting from UHI in inverness.

I can't think of a situation where I have used this setup and used the same element for both true and false so I'd argue it still works how you expect it to just in this specific case it has unwanted side effects

Hey @WebDevSimplified, when is the course expected to come out?

Thank you for this video. It makes me happy I skipped React and use Svelte 😂. Of course, sometimes we have to do assignments to force reactivity, so pick your poison. function addNumber() { numbers.push(numbers.length + 1); numbers = numbers; }

That totally saved me!!!

I've learned this the hard way. We've had a weird bug in our cart system whenever you edit the quantity of an item in a cart the item above it lose its quantity, this time we actually had keys which was relying on an id, the problem is the dev before me was extracting id from an object instead of _id so all items had undefined as a key which led to the bug..

Thanks!

If you have read the docs on React's reconciliation algorithm this is explicitly pointed out. Keys are the things that help React bring down a o(n³) diffing algorithm down to o(n) To quote React docs "The state of the art algorithms have a complexity in the order of O(n3) where n is the number of elements in the tree. If we used this in React, displaying 1000 elements would require in the order of one billion comparisons. This is far too expensive. Instead, React implements a heuristic O(n) algorithm based on two assumptions: 1. Two elements of different types will produce different trees. 2. The developer can hint at which child elements may be stable across different renders with a key prop."

Got similar behavior on nextjs page change. where i was charging the order of list using msth random , but the ui wasn't updating because there were only image renders. it was working fine when i added some text in the item component

If you can’t fix that, you’re not a senior in React. It’s covered in the introductory documentation, and if you’ve built anything nontrivial you have come across a problem like this in one form or another.

Great video!

I spent the whole day racking my brain over it and couldn't figure it out. Thank you! By the way, are you a wizard?

informative as usual 👌

Thank you, Kyle

Thanks 👍

Nice video!

You could also have an useEffect set the state back to 0 in the Counter component with the prop name as dependency

I'd say that this approach with using key is debatable. Counter could be reset by using useEffect, and overall this "bug" would not float up if code would be structured in different way. Theres no point to confuse junior developers with less known react features, when code can be simply restructured.

Such kind of use cases are rare, but they exist. I used manual keys once in 2017 and recently this year in April.

you could just have a useEffect within the counter which resets the count state on name change, this is expected behaviour afterall and would avoid a entirely new instance of the component because it just updates the state, also there no need to tenary the entire comp can just do that on the prop that makes it a bit less ambiguous. Using "useLocalStorage"(which btw is also a terrible idea because its a sync main thread operation which can be slow for big data blobs) it probs doing the above mentioned under the hood so you would not need any keys.

Hats off ❤

Well, as a kinda senior Vue dev I knew what the probelm is - it's quite common in Vue that when I render someting in similar way I need to key the components.

Very helpful

Greaaat video, what a teacher

Huh, can't say in 5 years I've ever encountered this but interesting nonetheless. Would've been nice to see you write out `` to drive the point home that all React sees is that Props changed, not the whole component. I absolutely would've expected React to figure this out on its own, but good to be aware of.

I also was interested in how you made your useLocalStorage hook but sadly you didn't toggle the function body which I was waiting for.

Awesome!!

very cool use case