I am wondering if there is any benefit of using React Query instead of the traditional following approach in cases where the response does not target the output of the component.
useEffect( () => {
const callback = async () => {
const env = await axios.get(`/api/environment`).then(res=>res.data);
window.localUrl = env.url;
};
callback();
}, []);
The above code assigns an API response to a global variable. Would it be beneficial to use React Query in that case and why?
P.S. Please do not make any comments about the usage of global variables. Take is as an example.
There's no significant benefit to using React Query instead of the traditional approach for this particular case, as the response does not directly target the output of the component. However, if you plan on using the API response in multiple places or if you need to handle API errors and loading states, React Query can provide a convenient and centralized way to manage API requests in your React components. Additionally, React Query has features for caching and optimizing API requests, which can improve the performance and UX of your application.
I am learning Redux Toolkit. From a React POV it seems very intuitive to access whatever part of the state you need from within useSelector using an inline arrow function, and then conduct any calculations. As an example consider a cart item with its data (like item count) in redux store.
function CartItemCounter({ itemId }){
const cart = useSelector(state => state.cart);
const itemInCart = cart.items[itemId];
const count = itemInCart?.count || 0;
return <div>{itemId} - {count} nos</div>
}
But I'm seeing all this information saying you should define your selectors beside your slice, use createSelector and all. What is the right way, and why is it better?
The information that is out there is essentially talking about different levels of optimization while using useSelector.
What you need to understand before anything else, is how useSelector works internally.
How does useSelector work?
When you pass a function to useSelector (obviously inside a react component), it essentially hooks on to the global redux state. Whenever any change happens in any part of the global state (i.e. when dispatch() is called from any part of the app), redux will run all the functions you passed to useSelector in your app, and perform certain checks.
Redux will take the result from each function, and compare it to the value it got the last time it ran the same function.
How does it make this comparison?
It uses a reference equality for this comparison. So if redux has to think that the result of the function hasn't changed, either the value returned from the function has to be a primitive and equal.
4 === 4 // true
'itemA' === 'itemA // true
Or, the value returned must be a derived data type (arrays, objects), with the same reference. So essentially the same object.
const x = { name: 'Shashi' }
const fn1 = () => x;
const fn2 = () => x;
const fn3 = () => { name: 'Shashi' }
fn1() === fn2(); // true
fn1() === fn3(); // false, because the objects are different, with different references
In practice, redux changes the wrapping object if either a key (or key of a nested object) is changed, or you manually change the entire object using a dispatch action (This is related to the immer library integration). This is similar to how you would do in regular React.
/* See how most keys are spread in, and will hence maintain reference equality.
While certain keys like 'first', 'first.second', 'first.second[action.someId]'
are changed with new objects, and so will break reference equality */
function handwrittenReducer(state, action) {
return {
...state,
first: {
...state.first,
second: {
...state.first.second,
[action.someId]: {
...state.first.second[action.someId],
fourth: action.someValue,
},
},
},
}
}
Otherwise it maintains the same objects within its state, and returns the exact same objects with the same references, when you access them. To verify this, if you access your cart twice, its literally going to be the same object.
const cart1 = useSelector(state => state.cart)
const cart2 = useSelector(state => state.cart)
cart1 === cart2; // true
What does it do with this comparison?
If the comparison returns true, i.e. the new value is the same as the old value, Redux tells that instance of useSelector to chill tf out, and not do anything. If it returns false however, it tells that component to re-render. After all, the value you are accessing from state has "changed"(according to Redux laws), so you probably want to show the new value.
With this information, we can make change the kind of function we pass to the useSelector, in order to get certain optimization benefits.
Optimization Levels
Level 0: Accessing slice data inline
const cart = useSelector(state => state.cart)
// extract the information you need from within the cart
const itemInCart = cart.items[itemId];
const count = itemInCart?.count || 0;
This is not a good way to access the data. You actually need a subset of the data from the cart, but you are fetching the whole thing, and doing the calculation outside the selector.
Problems:
When you put stuff like this inline, what happens if you change the shape of your data in the future? You have to go to every place that uses useSelector and manually change it. Not so good.
More importantly, every time any part of the cart changes, the entire cart object actually changes. So Redux sees your component that asks for the cart, and thinks
The cart has changed. This component is asking for the cart. It should probably re-render.
BAM Every single instance of this component rerenders. And for what? The count of the item you're referencing probably didn't change. So ideally there shouldn't have been a re-render.
Level 1: Centralize the selector
An easy optimization is to put the selector function in a centralized location next to your slice. That way, if your data shape changes in the future, you can just change it in one place, and your whole app (wherever it uses that data) will work with the new shape.
// inside or next to the slice file
const selectCart = (state) => state.cart
//...
// somewhere inside a react component
const cart = useSelector(selectCart)
Level 2: Access the relevant data
Since redux is comparing the results of your selector function, if you want to avoid unnecessary rerenders, you want to make sure the results have reference equality (===). So target the exact value you wish to look at, in your selector.
// extract the information you need from within the cart, *within the selector*
const count = useSelector(state => state.cart.items[itemId]?.count || 0)
// You don't have to use a one-liner, a multi-line function is better for readability
When Redux executes these functions, it keeps a record of the value returned from these selector functions, for each individual useSelector. This time the values are going to be the same for every single counter, except the one that actually changed. All those other counters that didn't actually change in value don't have to unnecessarily re-render anymore!
And if any of you folks think this is premature optimization, the answer's no. This is more along the lines of putting a dependancy array on your useEffects to avoid infinite loops.
Not forgetting the Level 1 optimization, we can also extract this function centrally
const selectItemById = (state, itemId) => (state.cart.items[itemId]?.count || 0);
function CartItemCounter({ itemId }){
//...
// somewhere inside a react component
const count = useSelector((state) => selectCart(state, itemId))
//...
}
So that solves all of our problems right?
For now, yes. But what if this selector function has to run some expensive computation.
const selectSomething = (state) => reallyExpensiveFn(state.cart)
//...
// somewhere inside a react component
const cart = useSelector(selectSomething)
You don't want to keep running that do you?
Or what if you have no option but to return new objects from your select function. A common scenario for this case would be returning a subset of data from the state.
const selectFilteredItems = (state) => state.itemsArray.filter(checkCondition) // the filter method will always return a new array
//...
// somewhere inside a react component
const cart = useSelector(selectFilteredItems) // re-renders every time
To solve this you would have to memoize or cache the results from the function call. Essentially you would need to make sure that if the input arguments are the same, the result will maintain reference equality with the previous result. This introduces the need to maintain some kind of cache state.
Level 3: createSelector
Fortunately, Reselect library, which is reexported with Redux Tookit, does this work for you. You can take a look at the redux toolkit for the syntax.
const selectFilteredItems = createSelector(
(state) => state.itemsArray, // the first argument accesses relevant data from global state
(itemsArray) => itemsArray.filter(checkCondition) // the second parameter conducts the transformation
)
//...
// somewhere inside a react component
const cart = useSelector(selectFilteredItems) // re-renders only when needed
Here the second function is called the transformation function, and is where we put the expensive computation, or the function that returns inconsistent references as a result (filter,map etc).
The createSelector caches
a) the arguments to the transformation function
b) the result of the transformation function
of the previous call to the selectFilteredItems function. If the arguments are the same, it skips executing the transformation function, and gives you the result you got the last time it was executed.
So when useSelector looks at the result, it gets reference equality. Hence the re-render is skipped!
One little caveat here is that createSelector only caches the very previous result. This makes sense if you think about a single component. In a single component you are only concerned about differences in values and results compared to the previous render. But in practice, you are likely to share selectors across multiple components. If this happens, you have a single cache location, and multiple components using this cache. i.e. Stuff breaks.
Level 4: createSelector factory function
Since the logic for your selector is the same, what you need to do is run createSelector for each component that uses it. This creates a cache for each component, giving us the desired behaviour. In order to do this, we use a factory function.
const makeSelectFilteredItems = () => createSelector(
(state) => state.itemsArray, // the first argument accesses relevant data from global state
(itemsArray) => itemsArray.filter(checkCondition) // the second parameter conducts the transformation
)
//...
// somewhere inside a react component
const selectFilteredItems = useMemo(makeSelectFilteredItems,[]); // make a new selector for each component, when it mounts
const cart = useSelector(selectFilteredItems) // re-renders only when needed
You intend to make a new selector (and by extension, cache) for each new component that mounts. So you put it inside the actual component function and not on the module scope. But this will re-run makeFilteredSelector for each render, and hence create a new selector for each render, and hence eliminate the cache. This is why you need to wrap the function in a useMemo with an empty dependency array. It runs on every mount.
And voila!
You now know where, why and how to use selectors in Redux. I personally feel that the createSelector syntaxes are slightly contrived. There is some discussion on changing cache sizes going on. But for now I feel that sticking to the docs should get you through most situations.
But I'm seeing all this information saying you should define your selectors beside your slice, use createSelector and all.
That's the way to go if you're deriving something from the state, which ends up being an expensive computation or something that's reused often throughout your app. Imagine, for example, your state.cart can contain 50.000 items and you need to sort them from most expensive item to least expensive. You don't want to re-calculate this all the time because it slows your app down. So you cache/memoize the result.
What is the right way, and why is it better?
The right way is to use memoization helpers like createSelector when/if you want to avoid expensive computation. Most people optimize prematurely, so I'd just stick to useSelector and keep it simple if in doubt.
I want to do some side effects like setState and update context after the data is fetched. However, the onSuccess will not be executed when the data is in cache. Also useEffect doesn't work because if the data is cached, it doesn't change from undefined to the real data. Therefore it doesn't get trigger either. What's the best way of doing this? Thanks
My usecase is to extract some values from the data returned from useQuery and set a new state on those.
usually, they’re shouldn’t be a need to be a need to copy state from react-query into local state. This will just lead to duplication of the source of truth. It is best to keep them separated, so that you can also profit from background updates.
If you want to transform the data or subscribe to parts of the data, use the select option of useQuery:
const { data } = useQuery(key, fn, { select: data => data.map(...) })
Alternatively, you can compute some new data depending on the returned data with useMemo, e.g.:
const { data } = useQuery(...)
const articles = useMemo(() => data?.map(...), [data])
// work with articles from here on
You can also put that nicely in a custom hook.
I know Redux solves this but I came up with an idea.
Imagine I have an app that gets some JSON on start. Based on this JSON I'm setting up the environment, so let's assume the app starts and it downloads an array of list items.
Of course as I'm not using Redux (the app itself is quite simple and Redux feels like a huge overkill here) if I want to use these list items outside of my component I have to pass them down as props and then pass them as props again as deep as I want to use them.
Why can't I do something like this:
fetch(listItems)
.then(response => response.json())
.then(json => {
window.consts = json.list;
This way I can access my list anywhere in my app and even outside of React. Is it considered an anti-pattern? Of course the list items WON'T be changed EVER, so there is no interaction or change of state.
What I usually do when I have some static (but requested via API) data is a little service that acts kind like a global but is under a regular import:
// get-timezones.js
import { get } from '../services/request'
let fetching = false
let timez = null
export default () => {
// if we already got timezones, return it
if (timez) {
return new Promise((resolve) => resolve(timez))
}
// if we already fired a request, return its promise
if (fetching) {
return fetching
}
// first run, return request promise
// and populate timezones for caching
fetching = get('timezones').then((data) => {
timez = data
return timez
})
return fetching
}
And then in the view react component:
// some-view.js
getTimezones().then((timezones) => {
this.setState({ timezones })
})
This works in a way it will always return a promise but the first time it is called it will do the request to the API and get the data. Subsequent requests will use a cached variable (kinda like a global).
Your approach may have a few issues:
If react renders before this window.consts is populated you won't
be able to access it, react won't know it should re-render.
You seem to be doing this request even when the data won't be used.
The only downside of my approach is setting state asynchronously, it may lead to errors if the component is not mounted anymore.
From the React point of view:
You can pass the list from top level via Context and you can see docs here.
Sample of using it is simple and exists in many libraries, such as Material UI components using it to inject theme across all components.
From engineering concept of everything is a trade of:
If you feel that it's gonna take so much time, and you are not going to change it ever, so keep it simple, set it to window and document it. (For your self to not forget it and letting other people know why you did this.)
If you're absolutely certain they won't ever change, I think it's quite ok to store them in a global, especially if you need to access the data outside of React. You may want to use a different name, maybe something like "appNameConfig"..
Otherwise, React has a feature called Context, which can also be used for "deep provision" - Reference
I'm new to React and Redux and I'm trying to write a simple application where a person can submit a URL for an image and it will show up on the page. Note that there is no backend to the application as of yet.
export const addImage = (url) => {
return {
type: ADD_IMAGE,
key: Guid.create().toString(),
payload: url
}
}
Adding an image creates an action of type ADD_IMAGE and my reducer updates the state consequently. However I also check if the URL is already in the list.
switch (action.type) {
case ADD_IMAGE:
if (state.find(image => image.url === action.payload)) {
return state;
} else {
return(
[
...state,
{key: action.key, url: action.payload}
]
);
}
break;
default:
}
The problem is that when I deny a post because the URL is already in the state I also want to convey that message to the user by showing it in a div next to the form. From what I've read I think I'm not supposed to try to access React state from reducers (if that is even possible) and... well.. I'm just stuck. I've been trying to find a simple guide on how to do this but I find nothing I can quite understand. After adding a database I guess I will have to do this as part of the async process but as I have it now I guess there should be some kind of simple solution.
You are starting to introduce logic into your reducer and this will inevitably lead to situation where you need to process some state outside of the reducer's scope.
The solution is to transfer your reducer logic into a thunk using a middleware package such redux-thunk (or similar package). This allows you to treat special kinds of actions as functions which means you can extend a plain action with specific action-related logic. The example you give of needing to dispatch an error action under certain conditions is an excellent use-case for redux-thunk.
Below is a example of how you might pull the logic out of your reducer into a thunk. You should note that, unlike reducers, thunks explicitly support fetching state and dispatching subsequent actions via the getState and dispatch functions.
Thunk example
export const addImage = (url) => {
return (dispatch, getState) => {
const key = Guid.create().toString()
dispatch({
type: ADD_IMAGE,
key,
payload: url
})
const state = getState()
// you would want to use a `selector` here to locate the existing image
// within the state tree
const exists = selectors.images.exists(state, url)
if (exists) {
dispatch(actions.ERROR_IMAGE_EXISTS({key, url}))
}
}
}
A note on selectors
You will see that I am using a selector to determine if the image exists. In the same way that thunks are the place to put your dispatch logic, a selector is the place to put your state-traversal logic. They are used to return portions of the state-tree or provide simple state-utilities such as the exists function shown above. Packages are available to help, for example reselect.
Follow on questions from comments
Are selectors not a built-in thing in Redux?
No they are not. Selectors are an idea that builds on top of redux and the concept exists as a place to put your state searching, caching, reading logic. This extracts the sometimes complex state traversal logic out of your thunks and components and into a nice tidy, structured collection of selectors.
Why use a selector instead of state.images.find(i => i.url === url)?
If you use a selector package then you get far more benefit than just a good separation of concerns, you get a big performance improvement (see usage example below).
Here are the headlines from the popular reselect package:
Selectors can compute derived data, allowing Redux to store the minimal possible state.
Selectors are efficient. A selector is not recomputed unless one of its arguments change.
Selectors are composable. They can be used as input to other selectors.
Why doesn't actions.ERROR_IMAGE_EXISTS(url) work for me
Because I just made that up for the example. The point is that you can dispatch actions from within the thunk, how you declare or get access to the action is up to you. I tend to centralise all my shared actions into an actions object that I import.
Selector usage example
Here is an example from my real-life code that shows how I use selectors to passing portions of the state as props to a react component:
const mapStateToProps = (state) => ({
model: services.model.selector.getSelected(state),
build: services.build.selector.getLastBuild(state),
recommendations: services.recommend.selector.getRecommendations(state)
})
Each of these selectors is finding the correct portion of the state tree and delivering it back ready for use. Nice and tidy, and if you use reselector, very efficient.