I have a simple react component that uses react hooks. I'm using useEffect and useState.
The problem is that i realized my API is getting huge amount of hits, after debugging i see that useEffect is running none stop!
This is my very simple code:
function DisplayUser({userId}) {
const [loggedUser, setLoggedUser] = React.useState(null);
React.useEffect(() => {
fetchData(userId).then(user => {
setLoggedUser(user);
})
});
return (
<div>
<div>{loggedUser}</div>
</div>
);
}
Whats happening here is that useEffect is running after each render
Does useEffect run after every render? Yes! By default, it runs both after the first render and after every update...
There is a way to do Optimizing Performance by Skipping Effects, you can pass an array as a second argument.
If the array is empty, that means there are no "dependencies" for this effect and it will run only once (quite simillar to componentDidMount).
If the array has values, then the effect will re-run only when those values have changeded (simillar to what we do with componentDidUpdate).
So in your case it is wise to pass an array with the userId value because you'll want to re-run the effect and fetch the user data only when the userId has changed.
function DisplayUser({userId}) {
const [loggedUser, setLoggedUser] = React.useState(null);
React.useEffect(() => {
fetchData(userId).then(user => {
setLoggedUser(user);
})
}, [userId]); // only run when userId changed
return (
<div>
<div>{loggedUser}</div>
</div>
);
}
Related
I have a counter and a console.log() in an useEffect to log every change in my state, but the useEffect is getting called two times on mount. I am using React 18. Here is a CodeSandbox of my project and the code below:
import { useState, useEffect } from "react";
const Counter = () => {
const [count, setCount] = useState(5);
useEffect(() => {
console.log("rendered", count);
}, [count]);
return (
<div>
<h1> Counter </h1>
<div> {count} </div>
<button onClick={() => setCount(count + 1)}> click to increase </button>
</div>
);
};
export default Counter;
useEffect being called twice on mount is normal since React 18 when you are in development with StrictMode. Here is an overview of what they say in the documentation:
In the future, we’d like to add a feature that allows React to add and remove sections of the UI while preserving state. For example, when a user tabs away from a screen and back, React should be able to immediately show the previous screen. To do this, React will support remounting trees using the same component state used before unmounting.
This feature will give React better performance out-of-the-box, but requires components to be resilient to effects being mounted and destroyed multiple times. Most effects will work without any changes, but some effects do not properly clean up subscriptions in the destroy callback, or implicitly assume they are only mounted or destroyed once.
To help surface these issues, React 18 introduces a new development-only check to Strict Mode. This new check will automatically unmount and remount every component, whenever a component mounts for the first time, restoring the previous state on the second mount.
This only applies to development mode, production behavior is unchanged.
It seems weird, but in the end, it's so we write better React code, bug-free, aligned with current guidelines, and compatible with future versions, by caching HTTP requests, and using the cleanup function whenever having two calls is an issue. Here is an example:
/* Having a setInterval inside an useEffect: */
import { useEffect, useState } from "react";
const Counter = () => {
const [count, setCount] = useState(0);
useEffect(() => {
const id = setInterval(() => setCount((count) => count + 1), 1000);
/*
Make sure I clear the interval when the component is unmounted,
otherwise, I get weird behavior with StrictMode,
helps prevent memory leak issues.
*/
return () => clearInterval(id);
}, []);
return <div>{count}</div>;
};
export default Counter;
In this very detailed article called Synchronizing with Effects, React team explains useEffect as never before and says about an example:
This illustrates that if remounting breaks the logic of your application, this usually uncovers existing bugs. From the user’s perspective, visiting a page shouldn’t be different from visiting it, clicking a link, and then pressing Back. React verifies that your components don’t break this principle by remounting them once in development.
For your specific use case, you can leave it as it's without any concern. And you shouldn't try to use those technics with useRef and if statements in useEffect to make it fire once, or remove StrictMode, because as you can read on the documentation:
React intentionally remounts your components in development to help you find bugs. The right question isn’t “how to run an Effect once”, but “how to fix my Effect so that it works after remounting”.
Usually, the answer is to implement the cleanup function. The cleanup function should stop or undo whatever the Effect was doing. The rule of thumb is that the user shouldn’t be able to distinguish between the Effect running once (as in production) and a setup → cleanup → setup sequence (as you’d see in development).
/* As a second example, an API call inside an useEffect with fetch: */
useEffect(() => {
const abortController = new AbortController();
const fetchUser = async () => {
try {
const res = await fetch("/api/user/", {
signal: abortController.signal,
});
const data = await res.json();
} catch (error) {
if (error.name !== "AbortError") {
/* Logic for non-aborted error handling goes here. */
}
}
};
fetchUser();
/*
Abort the request as it isn't needed anymore, the component being
unmounted. It helps avoid, among other things, the well-known "can't
perform a React state update on an unmounted component" warning.
*/
return () => abortController.abort();
}, []);
You can’t “undo” a network request that already happened, but your cleanup function should ensure that the fetch that’s not relevant anymore does not keep affecting your application.
In development, you will see two fetches in the Network tab. There is nothing wrong with that. With the approach above, the first Effect will immediately get cleaned... So even though there is an extra request, it won’t affect the state thanks to the abort.
In production, there will only be one request. If the second request in development is bothering you, the best approach is to use a solution that deduplicates requests and caches their responses between components:
function TodoList() {
const todos = useSomeDataFetchingLibraryWithCache(`/api/user/${userId}/todos`);
// ...
Update: Looking back at this post, slightly wiser, please do not do this.
Use a ref or make a custom hook without one.
import type { DependencyList, EffectCallback } from 'react';
import { useEffect } from 'react';
const useClassicEffect = import.meta.env.PROD
? useEffect
: (effect: EffectCallback, deps?: DependencyList) => {
useEffect(() => {
let subscribed = true;
let unsub: void | (() => void);
queueMicrotask(() => {
if (subscribed) {
unsub = effect();
}
});
return () => {
subscribed = false;
unsub?.();
};
}, deps);
};
export default useClassicEffect;
I have a React application. I am using Jest and React Testing library for unit testing.
I have to test a component. In the useEffect of the component, there is an API call made and once the response is received, we update the component's local state.
const [data, setData] = useState({})
useEffect(()=>{
// Make API call to a custom fetch hook
},[])
useEffect(()=>{
setData(response.data) //response data is a JSON object
},[response])
The test files code snippet is as below -
const {getByTestId} = render(<MyComponent></MyComponent>)
I have not put any assertions yet because of the inifinite running test cases
What have I done? I have been able to mock the fetch call and execute setData.
The problem - The tests keep running forever. But if I change the response.data to some boolean or string or number, the tests do not run infinitly.
Also, if I put a dummy object in the initialization of the state, the tests run fine.
const [data, setData] = useState({
name: 'Test',
Age: '99'
})
Providing an object as dependency in useEffect is not a good idea, since even if the data in object remains same, on every render -- object reference changes - the effect will run again (even if the data within stays same).
A workaround for this would be stringifying the dependency with JSON.stringify. (although doing on data containing some objects like dates, symbols, null or undefined etc. isn't recommended)
useEffect(() => {
setData(response.data)
}, [JSON.stringify(response)]);
Doing above shouldn't affect your UI.
Other solution would be to store the previous value of response and compare before you do setData. You can use usePrevious hook:
function usePrevious(value) {
const ref = useRef();
useEffect(() => {
ref.current = value;
});
return ref.current;
}
I have UseEffect hook that fetches data from DB and I want it to run FIRST in my component, but it runs last.
How do I make it run before "console.log(titleee)"?
Code:
const [cPost, setCPost] = useState([]);
const postId = id.match.params.id;
useEffect(() => {
axios.get('http://localhost:5000/posts/'+postId)
.then(posts => {
setCPost(posts.data);
console.log("test");
})
}, []);
const titleee = cPost.title;
console.log(titleee);
I don't think that's the correct path that you want to take.
In order to show the cPost on your page after the request /posts/+postId finished you can opt-out for two following options.
You can show a "loader" to the user if the cPost data is crucial for your whole component.
const [fetchingCPost, setFetchingCPost] = useState(false)
const [cPost, setCPost] = useState({});
const postId = id.match.params.id;
useEffect(() => {
setFetchingCPost(true)
axios.get('http://localhost:5000/posts/'+postId)
.then(posts => {
setFetchingCPost(false)
setCPost(posts.data);
})
}, []);
return fetchingCPost && <div>Loading</div>
Or you can have some default values set from the start for cPost. Just to make sure that your code doesn't break. I think the first solution might be more UX acceptable.
const [cPost, setCPost] = useState({title: '', description: ''});
If you want to store title as a separate variable you can use useMemo for instance or do it via useState same as with cPost. But even then you can't "create" it after the request finishes, you can simply change its value.
In case you want to use useMemo you can make it dependent on your cPost.
const cPostTitle = useMemo(() => {
return !!cPost.title ? cPost.title : ''
}, [cPost])
You have to change you'r way of thinking when programming in react. It is important to know how react works. React does not support imperative programming, it rather support declarative and top down approach , in which case you have to declare your markup and feed it with you'r data then the only way markup changes is by means of changing you'r data. So in you'r case you are declaring a watched variable using useState hook const [cPost, setCPost] = useState([]); , this variable (cPost) has initial values of [] then react continues rendering you'r markup using initial value, to update the rendered title to something you get from a network request (eg: a rest API call) you use another hook which is called after you'r component is rendered (useEffect). Here you have chance to fetch data and update you'r state. To do so you did as following :
useEffect(() => {
axios.get('http://localhost:5000/posts/'+postId)
.then(posts => {
setCPost(posts.data);
})
}, []);
this code results in a second render because part of data is changes. Here react engine goes ahead and repaint you'r markup according data change.
If you check this sandbox you'll see two console logs , in first render title is undefined in second render it's something we got from network.
Try adding async/await and see if it works. Here is the link btw for your reference https://medium.com/javascript-in-plain-english/how-to-use-async-function-in-react-hook-useeffect-typescript-js-6204a788a435
I've got the following component (simplified) which, given a note ID, would load and display it. It would load the note in useEffect and, when a different note is loaded or when the component gets unmounted, it saves the note.
const NoteViewer = (props) => {
const [note, setNote] = useState({ title: '', hasChanged: false });
useEffect(() => {
const note = loadNote(props.noteId);
setNote(note);
return () => {
if (note.hasChanged) saveNote(note); // bug!!
}
}, [props.noteId]);
const onNoteChange = (event) => {
setNote({ ...note, title: event.target.value, hasChanged: true });
}
return (
<input value={note.title} onChange={onNoteChange}/>
);
}
The issue is that within the useEffect I use note, which is not part of the dependencies so it means I always get stale data.
However, if I put the note in the dependencies then the loading and saving code will be executed whenever the note is modified, which is not what I need.
So I'm wondering how can I access the current note, without making it a dependency? I've tried to replace the note with a ref, but it means the component no longer updates when the note is changed, and I'd rather not use references.
Any idea what would be the best way to achieve this? Maybe some special React Hooks pattern?
You can't get the current state because this component does not render on the app render that removes it. Which means your effect never runs that last time.
Using an effect cleanup function is not a good place for this sort of thing. That should really be reserved for cleaning up that effect and nothing else.
Instead, whatever logic you have in the app that changes the state to close the NoteViewer should also save the note. So in some parent component (perhaps a NoteList or something) you'd save and close like:
function NoteList() {
const [viewingNoteId, setViewingNoteId] = useState(null)
// other stuff...
function closeNote() {
if (note.hasChanged) saveNote(note)
setViewingNoteId(null)
}
return <>{/* ... */}</>
}
I am trying to get realtime data from bitstamp API and store in my orders state. The page gets stuck in a loop and drains resources and react doesn't re-render the page when the orders state change. I can see the data if I log it to the console
This is what I have implemented so far.
const [loading, setLoading] = useState(true);
const [orders, setOrders] = useState([]);
const [subscription, setSubscription] = useState({
event: 'bts:subscribe',
data: {
channel: 'order_book_btcusd'
}
});
const ws = new WebSocket('wss://ws.bitstamp.net');
const initWebsocket = () => {
ws.onopen = () => {
ws.send(JSON.stringify(subscription));
};
ws.onmessage = (event) => {
const response = JSON.parse(event.data);
switch (response.event) {
case 'data':
setOrders(response.data);
setLoading(false);
break;
case 'bts:request_reconnect':
initWebsocket();
break;
default:
break;
}
};
ws.onclose = () => {
initWebsocket();
};
};
useEffect(() => {
initWebsocket();
}, [orders, subscription]);
console.log(orders);
const showResult = () => {
orders.bids.map((el, index) => (
<tr key={index}>
<td> {el[0]} </td>
<td> {el[1]} </td>
</tr>
));
};
This is happening because useEffect execute its callback after each render cycle i.e it runs both after the first render and after every update. So for every first message received it is opening a new WebSocket connection and storing the data in the state which is causing a loop.
You can read more about useEffect here
Edited:-
useEffect(() => {
initWebsocket();
}, [orders, subscription]);
The optional second argument to useEffect is used to detect if anything has changed or not (basically it compares prev state/props and given state/props) and it calls the effect whenever there is a change in value.
So on every orders state update, this effect will get called and which in turn causes a loop.
Solution:-
But in your case, you want to establish WebSocket connection only once after the component has mounted and keep listening to the incoming data irrespective of any state or prop change.
You can pass an empty [] such that it gets called only once on mount and unmount.
useEffect(() => {
initWebsocket();
// cleanup method which will be called before next execution. in your case unmount.
return () => {
ws.close
}
}, []);
From doc:-
This requirement is common enough that it is built into the useEffect Hook API. You can tell React to skip applying an effect if certain values haven’t changed between re-renders. To do so, pass an array as an optional second argument to useEffect.
If you want to run an effect and clean it up only once (on mount and unmount), you can pass an empty array ([]) as a second argument. This tells React that your effect doesn’t depend on any values from props or state, so it never needs to re-run. This isn’t handled as a special case — it follows directly from how the dependencies array always works.
If you pass an empty array ([]), the props and state inside the effect will always have their initial values. While passing [] as the second argument is closer to the familiar componentDidMount and componentWillUnmount mental model, there are usually better solutions to avoid re-running effects too often.
In useEffect, check if the WebSocket connection is closed before initializing it.
If you are confused with the working of react hooks, you can use class components and initialize your WebSocket connection in componentDidMount and componentDidUpdate(Check if the connection is closed and initialize it).
PS:
I have implemented a simple Chat Application using React and WebSockets.
https://github.com/Nikhil-Kumaran/ChatApp
Go through the repo to have a better idea.
Related component: https://github.com/Nikhil-Kumaran/ChatApp/blob/master/src/WebSockets.js