import { useContext, useEffect, useState } from 'react';
const Log = () => {
useEffect(() => {
console.log('Running ...')
},[])
return(<p>here</p>)
}
export default Log;
Whenever this code runs, I get
Running... messages twice in the browser console.
I think it should run once, as I have an empty second parameter in useEffect.
Can anybody explain why it is getting run twice?
This is due to <StrictMode> most likely in your root tree.
What is strict mode?
StrictMode is a tool for highlighting potential problems in an application.
How does it make useEffect() run twice?
It activates additional checks and warnings for its descendants, or in other words... renders twice.
Note: Strict mode checks are run in development mode only; they do not impact the production build.
If useEffect doesn't have any dependencies, the component itself is called only once.
This answer will help you.
React Hooks: useEffect() is called twice even if an empty array is used as an argument
As the dependency list of useEffect() sets empty, "console.log" will automatically run whenever the Log component re-renders.
I think there might be some changes of context or parent component from component tree.
Please check your project structure.
<ContextProvider.Provider value={setGlobalState}>
<ParentComponent>
<Log/>
</Parent Component>
</ContextProvider.Provider>
You can prevent running useEffect() twice by using the following way,
import { useEffect, useRef } from 'react';
const Log = () => {
// initiate dataFetch
const dataFetch = useRef(false)
useEffect(() => {
console.log('Running ...')
// start:: prevent executing useEffect twice
if (dataFetch.current)
return
dataFetch.current = true
// end:: prevent executing useEffect twice
}, [])
return (<p>here</p>)
}
export default Log;
Related
I have a requirement not to execute the code inside useEffect() when the component renders for the first time. For that, I defined a variable outside of the component function and set it to true. And then I checked if the variable is true inside useEffect() hook and if it was true, I set the variable to false and set it to return as shown below. But the code is not working as I expected. The code inside useEffect() executes regardless.
import { useEffect, useState } from 'react';
let isInitial = true;
function App() {
const [message, setMessage] = useState('First');
useEffect(() => {
if (isInitial) {
isInitial = false;
return;
}
setMessage('Executed');
}, []);
return <p>{message}</p>;
}
export default App;
I wanted to print 'First' inside the <p>. But the result was 'Executed' inside <p> which is not what I expected.
Strict Mode would be the problem. It renders your component twice in development mode. So the result would be not what you need in your code.
In addition, I suggest you to change the let statement to useState. Changing mutable let in the useEffect would cause unexpectable side effects. Using useState would be more predictable, React way.
import { useEffect, useRef, useState } from "react";
function App() {
const [message, setMessage] = useState("First");
const [isInitial, setIsInitial] = useState(true);
useEffect(() => {
if (isInitial) {
setIsInitial(false);
} else {
// Do what you want excepts first render
}
}, [isInitial]);
return <p>{message}</p>;
}
export default App;
The code you wrote should result <p>First</p>, unless <React.StrictMode> has wrapped around your main component (StrictMode is a tool for highlighting potential problems in an application and the checks are run in development mode only).
It causes App component to render twice and useEffect callback function will be called twice too(although the useEffect has [] dependency).
You should remove that wrapper.
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;
An API call is being made from useEffect() and when the application is refreshed, useEffect() is called twice and API is call is made twice.
// App.js
import { useEffect, useCallback } from "react";
function App() {
const fetchUsers = useCallback(async () => {
try {
const response = await fetch(
"https://jsonplaceholder.typicode.com/users"
);
const data = await response.json();
console.log("data: ", data); // displayed two times in console.
} catch (error) {
console.log("Error in catch!");
}
}, []);
useEffect(() => {
fetchUsers();
}, [fetchUsers]);
}
export default App;
how to fix this? useCallback() also has no effect in not invoking fetchUsers() function twice.
Here is the full code. CodeSandBox
using react v18
Edit:
Removing <React.StrictMode> has solved the issue, but I don't want to remove it, as it is useful in warning potential issues in the application.
Is there any other solution?
New Strict Mode Behaviors
In short, there is no "fix" as this is intentional. Do not remove Strict mode as some users are suggesting. You will see it only in development mode.
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 would unmount and remount trees using the same component state as before.
This feature will give React apps 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 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.
I'm new to react.js hook. Not sure about why the following code doesn't work.
import React,{useEffect,useState} from "react"
function App() {
const [fruit,setFruit] = useState("orange");
setFruit("apple")
return (
<div>
<h1>{fruit}</h1>
</div>
);
}
export default App;
The error says that
Too many re-renders. React limits the number of renders to prevent an infinite loop.
You're setting the state inside the functional component body (The setFruit("apple") line) which will cause the component to re-render and when this happens all the component body will rerun again which will cause another set state call and so on which ultimately causes an infinite loop.
If you want to set the state when the component is mounting, You can use the useEffect hook to achieve that like so:
useEffect(() => {
setFruit("apple")
}, []);
Read more about the hook here.
you need to use useEffect
useEffect(() => {
setFruit("apple")
}, [])
The actual pattren of react is that when you set a state it cause rerender that means component re run again and recompile every line of code if it found a set state fucntion again it rerender and that's what happening in youe code. To prevent this you can use useEffect hook which accepts a callback function and a dependency array
For Example
useEffect(() => {}, []);
The code in every useEffect will run with first time component mount and if you want to run the code again you need to pass dependency in 2nd argument of useEffect which decide that if the dependency change it's value the code in useEffect will run again.
In your case you can set state like this
useEffect(() => {
setFruit("apple")
}, []);
In the MobX with React docs, in the Side effects and observables section there is a receipe to respond to changes inside a useEffect hook.
import React from 'react'
import { autorun } from 'mobx'
function useDocumentTitle(store: TStore) {
React.useEffect(
() =>
autorun(() => {
document.title = `${store.title} - ${store.sectionName}`
}),
[], // note empty dependencies
)
}
The example combines React.useEffect with mobx.autorun (but it could be mobx.reaction), but I can not see the benefit of autorun in the code. Once we are inside useEffect we can track our dependencies in the dependency array. The code is more clear, there is no need to dispose() and useEffect has a clear dependency array with what is needed.
import React from 'react'
import { autorun } from 'mobx'
function useDocumentTitle(store: TStore) {
React.useEffect(() => document.title = `${store.title} - ${store.sectionName}`
,[store.title, store.sectionName])
}
Is there any reason to follow the example as given?
Here is a Code Sandbox
autorun creates an observer, which means it will watch for any changes in store.title and store.sectionName, and automatically run whenever they change.
Setting it up in useEffect ensures that the autorun observer is only created once, and removed when the component is unmounted. Note that useEffect doesn't actually run when the store value changes; its effect is that autorun is (un)registered when the component (un)mounts.
Your second example without autorun would still run the effect and thus update the title if this component is re-rendered by other means, either because a parent component triggers a re-render, or if this component is wrapped in an observer, as in the Sandbox example:
function Test(props) {
// 2. rerendered by observer when the value changes
useEffect(() => {
// <-- 3. execute effect because (2) rendered with a different dependency
}, [props.store.size]); // <-- 1. observer notes this value is accessed
return ( ... );
}
// <-- 1. observe any store value that is accessed in the Test function
export default observer(Test);
(edited for clarification)