Can't find documentation about this anywhere. Will this cause the useEffect to EVER run again? I don't want it to fetch twice, that would cause some issues in my code.
import React, { useEffect } from 'react'
import { useHistory } from 'react-router-dom'
const myComponent = () => {
const { push } = useHistory();
useEffect( () => {
console.log(" THIS SHOULD RUN ONLY ONCE ");
fetch(/*something*/)
.then( () => push('/login') );
}, [push]);
return <p> Hello, World! </p>
}
From testing, it doesn't ever run twice. Is there a case that it would?
For the sake of the question, assume that the component's parent is rerendering often, and so this component is as well. The push function doesn't seem to change between renders - will it ever?
Ciao, the way you write useEffect is absolutely right. And useEffect will be not triggered an infinite number of time. As you said, push function doesn't change between renders.
So you correctly added push on useEffect deps list in order to be called after fetch request. I can't see any error in your code.
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;
Just by instantiating the following code in another components
import React, { useState, useCallback, useEffect } from 'react';
import useWebSocket, { ReadyState } from 'react-use-websocket';
export const WebsocketHandler = () => {
const { lastJsonMessage, readyState } = useWebSocket('ws://127.0.0.1:8001/ws', {
shouldReconnect: (closeEvent) => {
return true;
},
reconnectAttempts: 99999,
reconnectInterval: 5000
});
const [isResponsive, setIsResponive] = useState(true);
useEffect(() => {
console.log("lastJsonMessage: " + JSON.stringify(lastJsonMessage));
}, [lastJsonMessage])
useEffect(() => {
console.log("readyState: " + readyState);
}, [readyState])
return isResponsive;
};
I got rerenders or to be more precise, i got some console output from my other components, so it looks like stuff is rerendered on every receive. I don't use the return value, so whats going on here? How can I avoid that?
Edit: The console log in this compoment is not the issue. The output will also come from other compoments in the application.
useEffect will run everytime lastJsonMessage or readyState value change. useEffect is a react hook and will run after side effects based parameters. For more info Using the Effect Hook
Data fetching, setting up a subscription, and manually changing the
DOM in React components are all examples of side effects. Whether or
not you’re used to calling these operations “side effects” (or just
“effects”), you’ve likely performed them in your components before.
If you’re familiar with React class lifecycle methods, you can think
of useEffect Hook as componentDidMount, componentDidUpdate, and
componentWillUnmount combined.
If you don't want to logging delete the both useEffects. if you want console log once
useEffect(() => {
console.log("lastJsonMessage: " + JSON.stringify(lastJsonMessage));
console.log("readyState: " + readyState);
}, [])
but this code may want to change based your want
I wrote a simple React to test how React render
import ReactDOM from 'react-dom';
import React, { useState, useEffect } from 'react';
const App = (props) => {
// State
const [trackIndex, setTrackIndex] = useState(1);
const [trackProgress, setTrackProgress] = useState(0);
const clickHandler = () =>{
setTrackIndex((trackIndex)=>trackIndex+1)
}
useEffect(() => {
console.log("effect; trackIndex=",trackIndex)
console.log("effect; trackProgress=",trackProgress)
if(trackIndex<5){
console.log("set trackProgress")
setTrackProgress(trackIndex)
}
});
console.log("render")
return (
<div>
<p>{trackIndex}</p>
<p>{trackProgress}</p>
<button onClick={clickHandler}>Click me</button>
</div>
);
};
ReactDOM.render(
<App />,
document.getElementById('root')
);
The following is the console output:
render
effect; trackIndex= 1
effect; trackProgress= 0
set trackProgress
render
effect; trackIndex= 1
effect; trackProgress= 1
set trackProgress
render
It seems that React renders three times before I click the button. The last rendering really confuses me. Could anyone explain to me why this render occurs and why no effect runs after this rendering? Thank you in advance
It seems that your useEffect is setting the state with the setTrackProgress on the initial render of the component. This is because trackIndex starts at 1 which is lower than 5. (seen by the if(trackIndex<5)) Notice that you haven't provided array dependency to useEffect as the second parameter. According to react-documentation this means the effect will only occur once after the first initial render of the component.
Also, I would suggest adding useCallback to your clickHandler in order to prevent re-defining the function for every render. (According to react-documentation)
You're practically making an infinite loop, but React saves you. The first render log is the initial render, then the useEffect is executed and your component re-renders, leading to the 2nd render log and the first effect; ... log. Now a 2nd useEffect is executed. However, the value of trackIndex hasn't changed, so your if statement will evaluate true and updates trackProgress with the same state/value(1). This leads to another re-render and the third render log. Now, you would think that a 3rd useEffect would be executed, but React is smart enough to know when state hasn't changed and thus doesn't execute the useEffect.
Add dependencies to your useEffect as stated above. That'll solve your problem.
useEffect(
...
, [trackIndex])
Why does my HomeTypes component unmount and remount every time I use setDrawerOpen?
Does a state change inside a component always cause that component to unmount and then re-mount?
import React, { useEffect, useState } from 'react';
import { FontAwesomeIcon } from '#fortawesome/react-fontawesome';
import Drawer from '../../components/DrawerComponent/Drawer';
function HomeTypes() {
const [drawerOpen, setDrawerOpen] = useState(null);
useEffect(() => {
console.log('Home Types Mounted');
return () => {
console.log('Home Types Unmounted');
};
});
return (
<div className={`dashboard-page`}>
<h1 className="dashboard-page-title">
Home Types
<button
className={`btn bright primary`}
onClick={() => {
setDrawerOpen(true);
}}>
<FontAwesomeIcon icon={['fas', 'plus']} />
<span>add home type</span>
</button>
</h1>
<Drawer
drawerOpen={drawerOpen}
closeDrawer={() => {
setDrawerOpen(false);
}}
title="Add Home Type"
drawerContent="Hello World"
/>
</div>
);
}
export default HomeTypes;
You're missing a dependencies array in your useEffect, that might be causing the behavior
useEffect(() => {
console.log('Home Types Mounted');
return () => {
console.log('Home Types Unmounted');
};
}. []); // this array right here
documentation see the Note in orange below: Using the Effect Hook
The hook useEffect will run every time the component renders.
to run the effect once on mount (end cleanup on dismount) run it with a second parameter [] in this form:
useEffect(() => {
// Side Effect
return () => {
// Cleanup
}
}, [])
Edit: Better to just quote the docs
Note
If you use this optimization, make sure the array includes all values from the component scope (such as props and state) that change over time and that are used by the effect. Otherwise, your code will reference stale values from previous renders. Learn more about how to deal with functions and what to do when the array changes too often.
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. Also, don’t forget that React defers running useEffect until after the browser has painted, so doing extra work is less of a problem.
We recommend using the exhaustive-deps rule as part of our eslint-plugin-react-hooks package. It warns when dependencies are specified incorrectly and suggests a fix.
I need to do a request when the page loaded (only once). I tried to do something like
if(!array.length){doRequest())
But with this method, react does more than 1 request, and I think that it will reduce my app speed. So I need the solution of this problem.
Thanks
When working with Class Components, the solution would be to make that request in the componentDidMount lifecycle, which is only called once.
class Example extends React.Component {
componentDidMount() {
doRequest();
}
render() {
return null;
}
}
If you're trying to use Hooks in a functional component instead of Class components then you should use:
function Example() {
useEffect(() => doRequest(), []);
return null;
}
Hoping of course, that doRequest is declared in an upper scope or imported somehow.
Hope this solves your question, if you need more insight into this I found a great article about replacing lifecycle with hooks.
https://dev.to/trentyang/replace-lifecycle-with-hooks-in-react-3d4n
just use useEffect Hook
import React, {useEffect} from 'react'
const func = () => {
useEffect(()=> {
doRequest()
},[]) // empty array means it only run once
}