I've been playing around with the new hook system in React 16.7-alpha and get stuck in an infinite loop in useEffect when the state I'm handling is an object or array.
First, I use useState and initiate it with an empty object like this:
const [obj, setObj] = useState({});
Then, in useEffect, I use setObj to set it to an empty object again. As a second argument I'm passing [obj], hoping that it wont update if the content of the object hasn't changed. But it keeps updating. I guess because no matter the content, these are always different objects making React thinking it keep changing?
useEffect(() => {
setIngredients({});
}, [ingredients]);
The same is true with arrays, but as a primitive it wont get stuck in a loop, as expected.
Using these new hooks, how should I handle objects and array when checking weather the content has changed or not?
Passing an empty array as the second argument to useEffect makes it only run on mount and unmount, thus stopping any infinite loops.
useEffect(() => {
setIngredients({});
}, []);
This was clarified to me in the blog post on React hooks at https://www.robinwieruch.de/react-hooks/
Had the same problem. I don't know why they not mention this in docs. Just want to add a little to Tobias Haugen answer.
To run in every component/parent rerender you need to use:
useEffect(() => {
// don't know where it can be used :/
})
To run anything only one time after component mount(will be rendered once) you need to use:
useEffect(() => {
// do anything only one time if you pass empty array []
// keep in mind, that component will be rendered one time (with default values) before we get here
}, [] )
To run anything one time on component mount and on data/data2 change:
const [data, setData] = useState(false)
const [data2, setData2] = useState('default value for first render')
useEffect(() => {
// if you pass some variable, than component will rerender after component mount one time and second time if this(in my case data or data2) is changed
// if your data is object and you want to trigger this when property of object changed, clone object like this let clone = JSON.parse(JSON.stringify(data)), change it clone.prop = 2 and setData(clone).
// if you do like this 'data.prop=2' without cloning useEffect will not be triggered, because link to data object in momory doesn't changed, even if object changed (as i understand this)
}, [data, data2] )
How i use it most of the time:
export default function Book({id}) {
const [book, bookSet] = useState(false)
const loadBookFromServer = useCallback(async () => {
let response = await fetch('api/book/' + id)
response = await response.json()
bookSet(response)
}, [id]) // every time id changed, new book will be loaded
useEffect(() => {
loadBookFromServer()
}, [loadBookFromServer]) // useEffect will run once and when id changes
if (!book) return false //first render, when useEffect did't triggered yet we will return false
return <div>{JSON.stringify(book)}</div>
}
I ran into the same problem too once and I fixed it by making sure I pass primitive values in the second argument [].
If you pass an object, React will store only the reference to the object and run the effect when the reference changes, which is usually every singe time (I don't now how though).
The solution is to pass the values in the object. You can try,
const obj = { keyA: 'a', keyB: 'b' }
useEffect(() => {
// do something
}, [Object.values(obj)]);
or
const obj = { keyA: 'a', keyB: 'b' }
useEffect(() => {
// do something
}, [obj.keyA, obj.keyB]);
If you are building a custom hook, you can sometimes cause an infinite loop with default as follows
function useMyBadHook(values = {}) {
useEffect(()=> {
/* This runs every render, if values is undefined */
},
[values]
)
}
The fix is to use the same object instead of creating a new one on every function call:
const defaultValues = {};
function useMyBadHook(values = defaultValues) {
useEffect(()=> {
/* This runs on first call and when values change */
},
[values]
)
}
If you are encountering this in your component code the loop may get fixed if you use defaultProps instead of ES6 default values
function MyComponent({values}) {
useEffect(()=> {
/* do stuff*/
},[values]
)
return null; /* stuff */
}
MyComponent.defaultProps = {
values = {}
}
Your infinite loop is due to circularity
useEffect(() => {
setIngredients({});
}, [ingredients]);
setIngredients({}); will change the value of ingredients(will return a new reference each time), which will run setIngredients({}). To solve this you can use either approach:
Pass a different second argument to useEffect
const timeToChangeIngrediants = .....
useEffect(() => {
setIngredients({});
}, [timeToChangeIngrediants ]);
setIngrediants will run when timeToChangeIngrediants has changed.
I'm not sure what use case justifies change ingrediants once it has been changed. But if it is the case, you pass Object.values(ingrediants) as a second argument to useEffect.
useEffect(() => {
setIngredients({});
}, Object.values(ingrediants));
As said in the documentation (https://reactjs.org/docs/hooks-effect.html), the useEffect hook is meant to be used when you want some code to be executed after every render. From the docs:
Does useEffect run after every render? Yes!
If you want to customize this, you can follow the instructions that appear later in the same page (https://reactjs.org/docs/hooks-effect.html#tip-optimizing-performance-by-skipping-effects). Basically, the useEffect method accepts a second argument, that React will examine to determine if the effect has to be triggered again or not.
useEffect(() => {
document.title = `You clicked ${count} times`;
}, [count]); // Only re-run the effect if count changes
You can pass any object as the second argument. If this object remains unchanged, your effect will only be triggered after the first mount. If the object changes, the effect will be triggered again.
I'm not sure if this will work for you but you could try adding .length like this:
useEffect(() => {
// fetch from server and set as obj
}, [obj.length]);
In my case (I was fetching an array!) it fetched data on mount, then again only on change and it didn't go into a loop.
If you include empty array at the end of useEffect:
useEffect(()=>{
setText(text);
},[])
It would run once.
If you include also parameter on array:
useEffect(()=>{
setText(text);
},[text])
It would run whenever text parameter change.
I often run into an infinite re-render when having a complex object as state and updating it from useRef:
const [ingredients, setIngredients] = useState({});
useEffect(() => {
setIngredients({
...ingredients,
newIngedient: { ... }
});
}, [ingredients]);
In this case eslint(react-hooks/exhaustive-deps) forces me (correctly) to add ingredients to the dependency array. However, this results in an infinite re-render. Unlike what some say in this thread, this is correct, and you can't get away with putting ingredients.someKey or ingredients.length into the dependency array.
The solution is that setters provide the old value that you can refer to. You should use this, rather than referring to ingredients directly:
const [ingredients, setIngredients] = useState({});
useEffect(() => {
setIngredients(oldIngedients => {
return {
...oldIngedients,
newIngedient: { ... }
}
});
}, []);
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.
I believe they are trying to express the possibility that one could be using stale data, and to be aware of this. It doesn't matter the type of values we send in the array for the second argument as long as we know that if any of those values change it will execute the effect. If we are using ingredients as part of the computation within the effect, we should include it in the array.
const [ingredients, setIngredients] = useState({});
// This will be an infinite loop, because by shallow comparison ingredients !== {}
useEffect(() => {
setIngredients({});
}, [ingredients]);
// If we need to update ingredients then we need to manually confirm
// that it is actually different by deep comparison.
useEffect(() => {
if (is(<similar_object>, ingredients) {
return;
}
setIngredients(<similar_object>);
}, [ingredients]);
The main problem is that useEffect compares the incoming value with the current value shallowly. This means that these two values compared using '===' comparison which only checks for object references and although array and object values are the same it treats them to be two different objects. I recommend you to check out my article about useEffect as a lifecycle methods.
The best way is to compare previous value with current value by using usePrevious() and _.isEqual() from Lodash.
Import isEqual and useRef. Compare your previous value with current value inside the useEffect(). If they are same do nothing else update. usePrevious(value) is a custom hook which create a ref with useRef().
Below is snippet of my code. I was facing problem of infinite loop with updating data using firebase hook
import React, { useState, useEffect, useRef } from 'react'
import 'firebase/database'
import { Redirect } from 'react-router-dom'
import { isEqual } from 'lodash'
import {
useUserStatistics
} from '../../hooks/firebase-hooks'
export function TMDPage({ match, history, location }) {
const usePrevious = value => {
const ref = useRef()
useEffect(() => {
ref.current = value
})
return ref.current
}
const userId = match.params ? match.params.id : ''
const teamId = location.state ? location.state.teamId : ''
const [userStatistics] = useUserStatistics(userId, teamId)
const previousUserStatistics = usePrevious(userStatistics)
useEffect(() => {
if (
!isEqual(userStatistics, previousUserStatistics)
) {
doSomething()
}
})
In case you DO need to compare the object and when it is updated here is a deepCompare hook for comparison. The accepted answer surely does not address that. Having an [] array is suitable if you need the effect to run only once when mounted.
Also, other voted answers only address a check for primitive types by doing obj.value or something similar to first get to the level where it is not nested. This may not be the best case for deeply nested objects.
So here is one that will work in all cases.
import { DependencyList } from "react";
const useDeepCompare = (
value: DependencyList | undefined
): DependencyList | undefined => {
const ref = useRef<DependencyList | undefined>();
if (!isEqual(ref.current, value)) {
ref.current = value;
}
return ref.current;
};
You can use the same in useEffect hook
React.useEffect(() => {
setState(state);
}, useDeepCompare([state]));
You could also destructure the object in the dependency array, meaning the state would only update when certain parts of the object updated.
For the sake of this example, let's say the ingredients contained carrots, we could pass that to the dependency, and only if carrots changed, would the state update.
You could then take this further and only update the number of carrots at certain points, thus controlling when the state would update and avoiding an infinite loop.
useEffect(() => {
setIngredients({});
}, [ingredients.carrots]);
An example of when something like this could be used is when a user logs into a website. When they log in, we could destructure the user object to extract their cookie and permission role, and update the state of the app accordingly.
my Case was special on encountering an infinite loop, the senario was like this:
I had an Object, lets say objX that comes from props and i was destructuring it in props like:
const { something: { somePropery } } = ObjX
and i used the somePropery as a dependency to my useEffect like:
useEffect(() => {
// ...
}, [somePropery])
and it caused me an infinite loop, i tried to handle this by passing the whole something as a dependency and it worked properly.
Another worked solution that I used for arrays state is:
useEffect(() => {
setIngredients(ingredients.length ? ingredients : null);
}, [ingredients]);
I'm having an issue when trying to save to State an axios API call. I've tried
useState set method not reflecting change immediately 's answer and many other and I can't get the state saved. This is not a duplicate, because I've tried what the accepted answer is and the one below and it still doesn't work.
Here's the (rather simple) component. Any help will be appreciated
export const Home = () => {
const [widgets, setWidgets] = useState([]);
useEffect(() => {
axios
.get('/call-to-api')
.then((response) => {
const data = response.data;
console.log(data); // returns correctly filled array
setWidgets(widgets, data);
console.log(widgets); // returns '[]'
});
}, []); // If I set 'widgets' here, my endpoint gets spammed
return (
<Fragment>
{/* {widgets.map((widget) => { // commented because it fails
<div>{widget.name}</div>;
})} */}
</Fragment>
);
};
Welcome to stackoverflow, first thing first the setting call is incorrect you must use spread operator to combine to array into one so change it to setWidgets([...widgets, ...data]); would be correct (I assume both widgets and data are Array)
second, react state won't change synchronously
.then((response) => {
const data = response.data;
console.log(data); // returns correctly filled array
setWidgets(widgets, data);
console.log(widgets); // <--- this will output the old state since the setWidgets above won't do it's work till the next re-render
so in order to listen to the state change you must use useEffect hook
useEffect(() => {
console.log("Changed Widgets: ", widgets)
}, [widgets])
this will console log anytime widget changes
the complete code will look like this
export const Home = () => {
const [widgets, setWidgets] = useState([]);
useEffect(() => {
axios
.get('/call-to-api')
.then((response) => {
const data = response.data;
setWidgets([...widgets, ...data])
});
}, []);
useEffect(() => {
console.log("Changed Widgets: ", widgets)
}, [widgets])
return (
<Fragment>
{/* {widgets.map((widget) => { // commented because it fails
<div>{widget.name}</div>;
})} */}
</Fragment>
);
};
Try:
setWidgets(data);
istead of
setWidgets(widgets, data);
Your widgets.map() probably fails because there isn't much to map over when the component is being rendered.
You should update it with a conditional like so, just for clarity:
widgets.length>0 ? widgets.map(...) : <div>No results</div>
And your call to setWidgets() should only take one argument, the data:
setWidgets(data)
or if you want to merge the arrays use a spread operator (but then you need to add widgets as the dependency to the useEffect dependency array.
setWidgets(...widgets, ...data)
You might also have to supply the setWidgets hook function to the useEffect dependency array.
Let me know if this helps..
I'm trying to pass an array of users as a props to a component, when I change something and click save, the array is showed in the component but when I hit refresh, the array is disappeared, here is my code:
First in my App.js I'm reading an array of users from the database (works perfectly shows the list of users) :
const [users,setUsers] = React.useState([]);
React.useEffect( () => {
async function fetchData() {
await axios.get('/api/users/')
.then(response => {
setUsers(response.data);
});
}
fetchData();
}
, []);
Then, also in App.js, I'm rendering a ListComponent that takes the users array and shows the users:
return (
<ListComponent users={users} />
);
}
In my ListComponent after a page refresh the console.log shows an empty array []
const ListComponent = (props) => {
React.useEffect(()=>{
console.log(props.users); // []
},[])
When you refresh the page, the ListComponent will be remounted, and what you are logging, is the state of the component just after it is mounted, so the user array is not already fetched. If you want log it when it is fetched, you should add the user array in the dependency array of the useEffect function:
const ListComponent = (props) => {
React.useEffect(()=>{
console.log(props.users); // Should be an empty array first, then updated if your fetch function is working properly
},[props.users]);
// ...
};
If you still cannot see the user array, it means that something is not happening as expected in your fetchData function I guess.
I verushc an array from one component to another component.
The initial array is filled by a DB and is not empty.
If I try to map over the array in my second component, it is empty (length = 0);
However, after I wrote a value in a search box to filter the array, all articles appear as intended.
What is that about?
export default function Einkäufe({ alleEinkäufe, ladeAlleEinkäufe, url }) {
const [searchTerm, setSearchTerm] = React.useState("");
const [searchResults, setSearchResults] = React.useState(alleEinkäufe);
const listeFiltern = (event) => {
setSearchTerm(event.target.value);
};
React.useEffect(() => {
setSearchResults(alleEinkäufe);
}, []);
React.useEffect(() => {
const results = alleEinkäufe.filter((eink) =>
eink.artikel.toLowerCase().includes(searchTerm.toLowerCase())
);
setSearchResults(results);
}, [searchTerm]);
[...]
{searchResults.map((artikel, index) => {
return ( ... );
})}
}
The problem is with your useEffect hook that sets the list of searchResults, it's not rerun when alleEinkäufe property is updated. You need to add alleEinkäufe as it's dependency.
React.useEffect(() => {
setSearchResults(alleEinkäufe);
}, [alleEinkäufe]);
My bet is that the parent component that renders Einkäufe is initially passing an empty array which is used as searchResults state and then never updated since useEffect with empty dependencies array is only run once on the component's mount.
I would also advise you to use English variable and function names, especially when you ask for assistance because it helps others to help you.
Your search term intially is "". All effects run when your components mount, including the effect which runs a filter. Initially, it's going to try to match any article to "".
You should include a condition to run your filter.
React.useEffect(() => {
if (searchTerm) {
const results = alleEinkäufe.filter((eink) =>
eink.artikel.toLowerCase().includes(searchTerm.toLowerCase())
);
setSearchResults(results);
}
}, [searchTerm]);
BTW, "" is falsy.
I have a component that makes a request and displays a list of jobs.
import React, { useState, useEffect, Fragment } from 'react';
import { Jobs } from '../components/Jobs.component';
export const Fixed = () => {
const [jobs, setJobs] = useState([]);
useEffect(() => {
getItems();
}, []);
async function getItems() {
const url = 'http://localhost:8081/api/fixed/list';
const res = await fetch(url, {
method: 'POST',
headers: {
'content-type': 'application/json',
},
});
const data = await res.json();
console.log(data);
setJobs(data.jobsList);
console.log(jobs);
}
return (
<Fragment>
{jobs.map(job => (
<div>
<Jobs job={job} />
</div>
))}
</Fragment>
);
};
My problem is that the first console outputs an array of jobs, but the second console displays an empty array. And an empty value is passed to the job component, which causes an error.
He does not have time to write the work in a state? Where am I wrong?
Method setJobs needs some time to change state so console.log runs faster than value changes.
You should render list if the array length is bigger than 0.
{jobs.length && jobs.map(job => <Jobs job={job} />)}
State updates are run asynchroniously
The reason your console.log shows an empty array is because setJobs runs asynchroniously and will update jobs value on next render. Looking at react setState documentation (same as useState react hooks) :
setState() enqueues changes to the component state and tells React that this component and its children need to be re-rendered with the updated state.
And so
setState() does not always immediately update the component. It may batch or defer the update until later. This makes reading this.state right after calling setState() a potential pitfall.
const ... jobs ... is a constant - it will be a different constant in 2 different renders, but it will not change value during a single render
The jobs inside getItems is a closure and will reference to the value from the first render, while setJobs will only change the value in second render.
It's similar to the following:
const rememberJobs = (jobs) => () => console.log(jobs)
const first = rememberJobs([])
const second = rememberJobs([1, 2, 3])
first()
second()