Probably it is a classic issue with useState which is not updating.
So there is a tree with some checkboxes, some of them are already checked as they map some data from an endpoint.
The user has the possibility to check/uncheck them. There is a "cancel" button that should reset them to the original form.
Here is the code:
const [originalValues, setOriginalValues] = useState<string[]>([]);
...
const handleCancel = () => {
const originalValues = myData || []; //myData is the original data stored in a const
setOriginalValues(() => [...myData]);
};
...
useEffect(() => {
setOriginalValues(originalValues);
}, [originalValues]);
However, it is not working, the tree is not updating as it should. Is it something wrong here?
Just do the following, no need for ()=> the state will update inside the hook if called, plus change the constant it will cause confusion inside your code and protentional name clash later on, with the current state variable name, and also make sure your data are there and you are not injection empty array !!!! which could be the case as well !.
// Make sure data are available
console.log(myData)
// Then change the state
setOriginalValues([...myData]);
I have always felt like I have a pretty solid understanding of how JS closures work, but since using ReactJS for the last couple of months there is something I can't quite make sense of.
In the below example, will the query result object getData be stale if used in the success callback for the other query?
My closure instincts tell me yes, but my reactjs instincts tell me no for some reason and I can't make sense of why it works the way that it does.
All help is appreciated!
/*
* Custom query hooks
*/
function useGetData(someId, opts){
return useQuery(['getData', someId],
getDataQueryFn,
{
...opts,
},
);
}
function useOtherData(someId, opts){
return useQuery(['otherData', someId],
otherDataQueryFn,
{
...opts,
},
);
}
/*
* React component
*/
function MyComponent(){
const getData = useGetData("1");
const otherData = useOtherData("1", {
onSuccess: () => {
// QUESTION: if I use getData here, won't it be stale due to closure? why or why not?
}
});
// render and stuff...
}
It doesn't go stale. Here's a codesandbox that closures over a counter and the fetch needs 5 seconds to complete. relevant code from the sandbox:
const [count, inc] = React.useReducer((val) => val + 1, 0);
const { data } = useQuery(
"repoData",
async () => {
await waitFor(5000);
return Promise.resolve({ name: "foo " });
},
{
onSuccess: () => console.log(count)
}
);
if you click the counter a couple of times in the first 5 seconds, the latest count will still be logged. It doesn't really matter if you closure over local state or over some other query.
The reason is likely that react-query will only trigger a re-render after the Promise is successful (it's a simple forced re-render via useState under the hood), and at that time, react already "sees" the latest value, not the value from the time when the query started to fetch.
It depends on useQuery implementation. However I am pretty sure it wont use stale data. I expect following order:
Each time result of useGetData is updated - re-render happens, due to some internal state update
new onSuccess callback (capturing latest getData result in closure) is passed via props to useOtherData and picked up by library to handle next update
I am new to React and I have some doubt regarding useState hook.I was recently working on an API based recipe react app .The problem I am facing is when I submit something in search form a state change should happen but the state is not changing but if I resubmit the form the state changes.
import React,{useState,useEffect} from "react";
import Form from "./componnents/form";
import RecipeBlock from "./componnents/recipeblock"
import './App.css';
function App() {
const API_id=process.env.REACT_APP_MY_API_ID;
const API_key=process.env.REACT_APP_MY_API_KEY;
const [query,setQuery]=useState("chicken");
const path=`https://api.edamam.com/search?q=${query}&app_id=${API_id}&app_key=${API_key}`
const [recipe,setRecipe]=useState([]);
useEffect(() => {
console.log("use effect is running")
getRecipe(query);
}, []);
function search(queryString){
setQuery(queryString);
getRecipe();
}
async function getRecipe(){
const response=await fetch(path);
const data=await response.json();
setRecipe(data.hits);
console.log(data.hits);
}
queryString in search() function holds the value of form input,Every time I submit the form this value is coming correctly but setQuery(queryString) is not changing the query value or state and if I resubmit the form then it change the state.
The code you provided something doesn't make sense.
useEffect(() => {
console.log("use effect is running")
getRecipe(query);
}, []);
Your getRecipe doesn't take a variable. But from what I am understanding whenever you search you want to set the Query then get the recipe from that Query.
With the useEffect you can pass in a parameters to check if they changed before running a function. So update the setQuery then when the component reloads it will fire the useEffect if query has changed. Here is the code to explain:
useEffect(() => {
console.log("use effect is running")
getRecipe(query); <-- this doesn't make sense on your code
}, [query]);
function search(queryString){
setQuery(queryString);
}
By doing this when the state updates it causes the component to re-render and therefore if query has changed it will call your getRecipe function.
The main issue in your code is that you are running getRecipe() directly after setQuery(queryString). setQuery(queryString) is asynchronous and will queue a state change. When you then run getRecipe() directly after, the state will still hold the old value of query (and path) and therefore does not fetch the new data correctly.
One solution would be to call getRecipe() within a useEffect() dependent on path.
useEffect(() => {
getRecipe();
}, [path]);
function search(queryString){
setQuery(queryString);
// getRecipe() <- removed
}
With [path] given as dependencies for useEffect(), getRecipe() will be called automatically whenever path changes. So we don't have to call it manually from search() and therefore can remove getRecipe() from the function body. This also makes the current useEffect() (without [path] dependency) redundant, so it can be removed.
Another solution would be to provide the new query value through the getRecipe() parameters, removing the dependency upon the state.
function search(queryString){
setQuery(queryString);
getRecipe(queryString);
}
async function getRecipe(query) {
const path = `https://api.edamam.com/search?q=${query}&app_id=${API_id}&app_key=${API_key}`;
const response = await fetch(path); // <- is no longer dependent upon the state
const data = await response.json();
setRecipe(data.hits);
}
This does require moving the path definition inside getRecipe().
I have a function inside of my functional component that uses a value saved in state. However, when it is called, it has the original value in state, not the updated value. When I look at my component in Chrome React Dev Tools, I see that the updated value is stored in state. Aren't functions supposed to get the latest state value in React? I didn't think I'd have to wrap my functions in a useEffect every time some value in state they depend on changes. Why is this happening?
const Editor = (props) => {
const [template, setTemplate] = useState(null);
const [openDialog, setOpenDialog] = useState(false);
useEffect(() => {
if (props.templateId) {
getTemplate(props.templateId));
}
},[]);
const getTemplate = (templateId) => {
{...make API to get template...}
.then((response) => {
if (response.template) setTemplate(response.template);
});
}
/* THIS FUNCTION SAYS TEMPLATE IS ALWAYS NULL */
const sendClick = async () => {
if (template) {
await updateTemplate();
} else {
await initializeTemplate();
}
setOpenDialog(true);
};
}
UPDATE: I figured out the issue. The sendClick function is being used inside an object that I have in state. When that object is created, it creates a version of the sendClick function based on the state at that time. I realized I needed to refactor my code so that the function is not stored within my object in state so that the function will always have the latest state values.
Please correct the code there its setTemplate(template)); not getTemplate(template));
I'm guessing that you have that right in the source code... if Yes then,
You have got into a trap that all developers new to React fall into.
This code is betraying you ...
useEffect(() => {
if (props.template) {
setTemplate(template)); // Mentioned as getTemplate(template));
}
},[]); // Here is where you make the mistake
The second argument you pass to the useEffect is called as Dependencies. Meaning if your useEffect is dependent on any state or any variable or function, Ii should be pass as the second argument inside the []. By now you should have got the answer.
Clearly, your useEffect is dependent on template. You should pass that inside the [].
So the code will be : -
useEffect(() => {
if (props.template) {
setTemplate(template)); // Mentioned as getTemplate(template));
}
},[template]);
Now React will automatically run the function every time the value of template changes therefore, updates template.
For more information about useEffect ...
Refer React Documentation
Refer the useEffect API
The method componentWillUnmount() is invoked immediately before a component is unmounted and destroyed. If we use useEffect with an empty array ([]) as the second argument and put our function in return statement it will be executed after the component is unmounted and even after another component will be mounted. This is done for performance reasons as far as I understand. In order not to delay rendering.
So the question is - how can we call some function using hooks before a component gets unmounted?
What I am trying to do is an application which saves user's input as he types (without submitting form). I use setInterval to save updated text every N seconds. And I need to force save updates before the component will unmount. I don't want to use prompt by react router before navigating. This is an electron application. I appreciate any thoughts or advice on how to implement such functionality.
Update
Unfortunately, Effects with Cleanup run after letting the browser paint. More details can be found here: So What About Cleanup?. It basically means that cleanup is run after a component is unmounted and it is not the same as executing code in componentWillUnmount(). I can clearly see the sequence of calls if I put console.log statements in the cleanup code and in another component. The question is whether we can execute some code before a component is unmounted using hooks.
Update2
As I can see I should better describe my use case. Let's imagine a theoretical app which holds its data in a Redux store. And we have two components with some forms. For simplicity, we don't have any backend or any async logic. We use only Redux store as data storage.
We don't want to update Redux store on every keystroke. So we keep actual values in the local component's state which we initialize with values from the store when a component mounts. We also create an effect which sets up a setInterval for 1s.
We have the following process. A User types something. Updates are stored in the local component state until our setInterval callback is called. The callback just puts data in the store (dispatches action). We put our callback in the useEffect return statement to force save to store when the component gets unmounted because we want to save data to store in this case as soon as possible.
The problem comes when a user types something in the first component and immediately goes to the second component (faster than 1s). Since the cleanup in our first component will be called after re-rendering, our store won't be updated before the second component gets mounted. And because of that, the second component will get outdated values to its local state.
If we put our callback in componentWillUnmount() it will be called before unmounting and the store will be updated before the next component mounts. So can we implement this using hooks?
componentWillUnmount can be simulated by returning a function inside the useEffect hook. The returned function will be called just before every rerendering of the component. Strictly speaking, this is the same thing but you should be able to simulate any behaviour you want using this.
useEffect(() => {
const unsubscribe = api.createSubscription()
return () => unsubscribe()
})
Update
The above will run every time there is a rerender. However, to simulate the behaviour only on mounting and unmounting (i.e. componentDidMount and componentWillUnmount). useEffect takes a second argument which needs to be an empty array.
useEffect(() => {
const unsubscribe = api.createSubscription()
return () => unsubscribe()
}, [])
See a more detailed explanation of the same question here.
Since the introduction of the useLayoutEffect hook, you can now do
useLayoutEffect(() => () => {
// Your code here.
}, [])
to simulate componentWillUnmount. This runs during unmount, but before the element has actually left the page.
The question here is how do you run code with hooks BEFORE unmount? The return function with hooks runs AFTER unmount and whilst that doesn’t make a difference for most use cases, their are some where it is a critical difference.
Having done a bit of investigation on this, I have come to the conclusion that currently hooks simply does not provide a direct alternative to componentWillUnmount. So if you have a use case that needs it, which is mainly for me at least, the integration of non-React libs, you just have to do it the old way and use a component.
Update: see the answer below about UseLayoutEffect() which looks like it may solve this issue.
I agree with Frank, but the code needs to look like this otherwise it will run only on the first render:
useLayoutEffect(() => {
return () => {
// Your code here.
}
}, [])
This is equivalent to ComponentWillUnmount
Similar to #pritam's answer, but with an abstracted code example. The whole idea of useRef is to allow you to keep track of the changes to the callback and not have a stale closure at the time of execution. Hence, the useEffect at the bottom can have an empty dependency array to ensure it only runs when the component unmounts. See the code demo.
Reusable hook:
type Noop = () => void;
const useComponentWillUnmount = (callback: Noop) => {
const mem = useRef<Noop>();
useEffect(() => {
mem.current = callback;
}, [callback]);
useEffect(() => {
return () => {
const func = mem.current as Noop;
func();
};
}, []);
};
After a bit of research, found that - you could still accomplish this. Bit tricky but should work.
You can make use of useRef and store the props to be used within a closure such as render useEffect return callback method
function Home(props) {
const val = React.useRef();
React.useEffect(
() => {
val.current = props;
},
[props]
);
React.useEffect(() => {
return () => {
console.log(props, val.current);
};
}, []);
return <div>Home</div>;
}
DEMO
However a better way is to pass on the second argument to useEffect so that the cleanup and initialisation happens on any change of desired props
React.useEffect(() => {
return () => {
console.log(props.current);
};
}, [props.current]);
I got in a unique situation where the useEffect(() => () => { ... }, []); answers did not work for me. This is because my component never got rendered — I was throwing an exception before I could register the useEffect hook.
function Component() {
useEffect(() => () => { console.log("Cleanup!"); }, []);
if (promise) throw promise;
if (error) throw error;
return <h1>Got value: {value}</h1>;
}
In the above example, by throwing a Promise<T> that tells react to suspend until the promise is resolved. However, once the promise is resolved, an error is thrown. Since the component never gets rendered and goes straight to an ErrorBoundary, the useEffect() hook is never registered!
If you're in a similar situation as myself, this little code may help:
To solve this, I modified my ErrorBoundary code to run a list of teardowns once it was recovered
export default class ErrorBoundary extends Component {
// ...
recover() {
runTeardowns();
// ...
}
// ...
}
Then, I created a useTeardown hook which would add teardowns that needed to be ran, or make use of useEffect if possible. You'll most likely need to modify it if you have nesting of error boundaries, but for my simple usecase, it worked wonderfully.
import React, { useEffect, useMemo } from "react";
const isDebugMode = import.meta.env.NODE_ENV === "development";
const teardowns: (() => void)[] = [];
export function runTeardowns() {
const wiped = teardowns.splice(0, teardowns.length);
for (const teardown of wiped) {
teardown();
}
}
type Teardown = { registered?: boolean; called?: boolean; pushed?: boolean } & (() => unknown);
/**
* Guarantees a function to run on teardown, even when errors occur.
*
* This is necessary because `useEffect` only runs when the component doesn't throw an error.
* If the component throws an error before anything renders, then `useEffect` won't register a
* cleanup handler to run. This hook **guarantees** that a function is called when the component ends.
*
* This works by telling `ErrorBoundary` that we have a function we would like to call on teardown.
* However, if we register a `useEffect` hook, then we don't tell `ErrorBoundary` that.
*/
export default function useTeardown(onTeardown: () => Teardown, deps: React.DependencyList) {
// We have state we need to maintain about our teardown that we need to persist
// to other layers of the application. To do that, we store state on the callback
// itself - but to do that, we need to guarantee that the callback is stable. We
// achieve this by memoizing the teardown function.
const teardown = useMemo(onTeardown, deps);
// Here, we register a `useEffect` hook to run. This will be the "happy path" for
// our teardown function, as if the component renders, we can let React guarantee
// us for the cleanup function to be ran.
useEffect(() => {
// If the effect gets called, that means we can rely on React to run our cleanup
// handler.
teardown.registered = true;
return () => {
if (isDebugMode) {
// We want to ensure that this impossible state is never reached. When the
// `runTeardowns` function is called, it should only be ran for teardowns
// that have not been able to be hook into `useEffect`.
if (teardown.called) throw new Error("teardown already called, but unregistering in useEffect");
}
teardown();
if (isDebugMode) {
// Because `teardown.registered` will already cover the case where the effect
// handler is in charge of running the teardown, this isn't necessary. However,
// this helps us prevent impossible states.
teardown.called = true;
}
};
}, deps);
// Here, we register the "sad path". If there is an exception immediately thrown,
// then the `useEffect` cleanup handler will never be ran.
//
// We rely on the behavior that our custom `ErrorBoundary` component will always
// be rendered in the event of errors. Thus, we expect that component to call
// `runTeardowns` whenever it deems it appropriate to run our teardowns.
// Because `useTeardown` will get called multiple times, we want to ensure we only
// register the teardown once.
if (!teardown.pushed) {
teardown.pushed = true;
teardowns.push(() => {
const useEffectWillCleanUpTeardown = teardown.registered;
if (!useEffectWillCleanUpTeardown) {
if (isDebugMode) {
// If the useEffect handler was already called, there should be no way to
// re-run this teardown. The only way this impossible state can be reached
// is if a teardown is called multiple times, which should not happen during
// normal execution.
const teardownAlreadyCalled = teardown.called;
if (teardownAlreadyCalled) throw new Error("teardown already called yet running it in runTeardowns");
}
teardown();
if (isDebugMode) {
// Notify that this teardown has been called - useful for ensuring that we
// cannot reach any impossible states.
teardown.called = true;
}
}
});
}
}
It does not matter wether the returned function from useEffect gets called before or after the component unmounted: You still have access to the states valuey through the closure:
const [input, setInput] = useState(() => Store.retrieveInput());
useEffect(() => {
return () => Store.storeInput(input); // < you can access "input" here, even if the component unmounted already
}, []);
If you don't manage the input in the components state, your whole structure is broken and should be changed to manage state at the right place. In your case, you should lift the shared input state of the components to the parent.
ReactJS docs on hooks specify this:
Effects may also optionally specify how to “clean up” after them by
returning a function.
So any function you return in your useEffect hook, will be executed when the component unmounts, as well as before re-running the effect due to a subsequent render.