I'm trying to make an infinite scroll function with IntersectionObserver. When the callback is called I make a test to see if there is some loading happens and if so I want to cancel that new call. The function is that:
const loadMore = useCallback(() => {
if (loading) {
return
}
setLoading(true);
console.log('Loading');
setTimeout(() => {
console.log('Finished');
setLoading(false);
}, 5000);
}, []);
The problem is that when the watcher calls loadMore again, and there is a load going on at that moment, the loading state sometimes has the old value (false), and setTimeOut is called again even if there is another one running. If I put the loading in the dependency array, the function is called many times (in loop) because I change it's value inside it. I tried to use a useRef to control loading, it works in the function, but I also want to use the loading state in JSX to show or not a text, but with the useRef variable it doesn't work in JSX and I find it a little ugly having to use two variables to control almost the same thing. Is there a way to do this just with the state?
The observer code:
useEffect(() => {
const options = {
root: null,
rootMargin: '10px',
threshold: 1.0
}
const observer = new IntersectionObserver(loadMore, options);
if (loaderRef && loaderRef.current) {
observer.observe(loaderRef.current);
}
}, [loadMore]);
For some odd reason the value of props in my "unmount" useEffect hook is always at the original state (true), I can console and see in the devtools that it has changed to false but when the useEffect is called on unmount it is always true.
I have tried adding the props to the dependancies but then it is no longer called only on unmount and does not serve it's purpose.
Edit: I am aware the dependancy array is empty, I cannot have it triggered on each change, it needs to be triggered ONLY on unmount with the update values from the props. Is this possible?
React.useEffect(() => {
return () => {
if (report.data.draft) { // this is ALWAYS true
report.snapshot.ref.delete();
}
};
}, []);
How can I conditionally run my code on unmount with the condition being dependant on the updated props state?
If you want code to run on unmount only, you need to use the empty dependency array. If you also require data from the closure that may change in between when the component first rendered and when it last rendered, you'll need to use a ref to make that data available when the unmount happens. For example:
const onUnmount = React.useRef();
onUnmount.current = () => {
if (report.data.draft) {
report.snapshot.ref.delete();
}
}
React.useEffect(() => {
return () => onUnmount.current();
}, []);
If you do this often, you may want to extract it into a custom hook:
export const useUnmount = (fn): => {
const fnRef = useRef(fn);
fnRef.current = fn;
useEffect(() => () => fnRef.current(), []);
};
// used like:
useUnmount(() => {
if (report.data.draft) {
report.snapshot.ref.delete();
}
});
The dependency list of your effect is empty which means that react will only create the closure over your outer variables once on mount and the function will only see the values as they have been on mount. To re-create the closure when report.data.draft changes you have to add it to the dependency list:
React.useEffect(() => {
return () => {
if (report.data.draft) { // this is ALWAYS true
report.snapshot.ref.delete();
}
};
}, [report.data.draft]);
There also is an eslint plugin that warns you about missing dependencies: https://www.npmjs.com/package/eslint-plugin-react-hooks
Using custom js events you can emulate unmounting a componentWillUnmount even when having dependency. Here is how I did it.
Problem:
useEffect(() => {
//Dependent Code
return () => {
// Desired to perform action on unmount only 'componentWillUnmount'
// But it does not
if(somethingChanged){
// Perform an Action only if something changed
}
}
},[somethingChanged]);
Solution:
// Rewrite this code to arrange emulate this behaviour
// Decoupling using events
useEffect( () => {
return () => {
// Executed only when component unmounts,
let e = new Event("componentUnmount");
document.dispatchEvent(e);
}
}, []);
useEffect( () => {
function doOnUnmount(){
if(somethingChanged){
// Perform an Action only if something changed
}
}
document.addEventListener("componentUnmount",doOnUnmount);
return () => {
// This is done whenever value of somethingChanged changes
document.removeEventListener("componentUnmount",doOnUnmount);
}
}, [somethingChanged])
Caveats: useEffects have to be in order, useEffect with no dependency have to be written before, this is to avoid the event being called after its removed.
I have a problem when using the useEffect hook, it is generating an infinite loop.
I have a list that is loaded as soon as the page is assembled and should also be updated when a new record is found in "developers" state.
See the code:
const [developers, setDevelopers] = useState<DevelopersData[]>([]);
const getDevelopers = async () => {
await api.get('/developers').then(response => {
setDevelopers(response.data);
});
};
// This way, the loop does not happen
useEffect(() => {
getDevelopers();
}, []);
// This way, infinte loop
useEffect(() => {
getDevelopers();
}, [developers]);
console.log(developers)
If I remove the developer dependency on the second parameter of useEffect, the loop does not happen, however, the list is not updated when a new record is found. If I insert "developers" in the second parameter of useEffect, the list is updated automatically, however, it goes into an infinite loop.
What am I doing wrong?
complete code (with component): https://gist.github.com/fredarend/c571d2b2fd88c734997a757bac6ab766
Print:
The dependencies for useEffect use reference equality, not deep equality. (If you need deep equality comparison for some reason, take a look at use-deep-compare-effect.)
The API call always returns a new array object, so its reference/identity is not the same as it was earlier, triggering useEffect to fire the effect again, etc.
Given that nothing else ever calls setDevelopers, i.e. there's no way for developers to change unless it was from the API call triggered by the effect, there's really no actual need to have developers as a dependency to useEffect; you can just have an empty array as deps: useEffect(() => ..., []). The effect will only be called exactly once.
EDIT: Following the comment clarification,
I register a developer in the form on the left [...] I would like the list to be updated as soon as a new dev is registered.
This is one way to do things:
The idea here is that developers is only ever automatically loaded on component mount. When the user adds a new developer via the AddDeveloperForm, we opportunistically update the local developers state while we're posting the new developer to the backend. Whether or not posting fails, we reload the list from the backend to ensure we have the freshest real state.
const DevList: React.FC = () => {
const [developers, setDevelopers] = useState<DevelopersData[]>([]);
const getDevelopers = useCallback(async () => {
await api.get("/developers").then((response) => {
setDevelopers(response.data);
});
}, [setDevelopers]);
useEffect(() => {
getDevelopers();
}, [getDevelopers]);
const onAddDeveloper = useCallback(
async (newDeveloper) => {
const newDevelopers = developers.concat([newDeveloper]);
setDevelopers(newDevelopers);
try {
await postNewDeveloperToAPI(newDeveloper); // TODO: Implement me
} catch (e) {
alert("Oops, failed posting developer information...");
}
getDevelopers();
},
[developers],
);
return (
<>
<AddDeveloperForm onAddDeveloper={onAddDeveloper} />
<DeveloperList developers={developers} />
</>
);
};
The problem is that your getDevelopers function, calls your setDevelopers function, which updates your developers variable. When your developers variable is updated, it triggers the useEffect function
useEffect(() => {
getDevelopers();
}, [developers]);
because developers is one of the dependencies passed to it and the process starts over.
Every time a variable within the array, which is passed as the second argument to useEffect, gets updated, the useEffect function gets triggered
Use an empty array [] in the second parameter of the useEffect.
This causes the code inside to run only on mount of the parent component.
useEffect(() => {
getDevelopers();
}, []);
The goal here is to change table data when data is modified in Firebase
The issue is that for some reason, within the scope of firebase listener, setState(...) does not trigger the useEffect(()=> {}, [...]) with the appropriate dependencies.
Any idea why, and is there a way to force the change?
const [actions, setActions] = useState([]);
....
firebase.
.firestore()
.collection("collection_name")
.onSnapshot(s => {
if (!s.empty) {
s.docChanges().forEach(change => {
if (change.type === "modified") {
const newActions = [...actions, change.doc.data()]
setActions(newActions) //The change
console.log("arrived here")
}...
...
useEffect(() => {
console.log("change in actions"); // Does not triggered on when modified
}, [actions]);
This does work without the dependency array:
useEffect(() => {
console.log("This does work")
});
I think you should look other your references.
The thing with react hook is that if the reference of the object before and after the setter is the same, the useEffect listening to this object wont trigger.
Meaning that if in newActions = ... you use the reference leading to actions to update the value of action and then setActions() the reference of the object stay the same, instead you should try to make a copy of actions and then modify this independent copy. You can then setActions(modifiedCopy) and this will normally trigger the useEffect.
note: that's only a guess cause i can't know what you putted behind newActions = ...
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]);