// Fetch Products
const fetchProducts = async () => {
const { data } = await axios.get('/api/products')
setProducts(data)
}
// Retrieve all products at reload
useEffect(()=>{
fetchProducts()
},[])`
This is a simple function for retrieving products whenever the home page component loads. So every time it loads it should run fetchProducts(). I understood that an empty array means nothing/no dependency so when the component mounts it will run this function.
The error I receive is "Line 19:7: React Hook useEffect has a missing dependency: 'fetchProduct'. Either include it or remove the dependency array"
I'm new to React and am not really understanding the statement.
Why would removing the array or adding the function to the dependency change anything?
Isn't the way it's written doing exactly what I would need it to do?
That error is telling you that yow fetchProducts function is been used inside yow hook, if you want to fix that you have two choices .
Add it to the brackets of the hook
useEffect(()=>{},[fetchProducts]);
Or created it inside the hook.
useEffect(()=>{
const fetchProducts () =>{...};
fetchProducts()
},[]);
The way you have it is considered as a side effect
This Warning is basically an eslinting problem as in the dependency array takes primitive type but eslint does not correctly detect dependencies therefore you can fix this by a disable eslint above the lines where there is warning. If you async logic in use Effect put your fetchProducts inside React.useCallback hook and add fetchProducts to use Effect dependency array
Related
I have the below code and as far as I'm concerned it only relies upon [page] yet, I am getting the error
React Hook useEffect has a missing dependency
I've seen similar questions about commenting out a line in your eslint file but
I don't have an eslint file
I would rather understand and resolve the issue
const fetchStarWarsInfo = async () => {
const response = await getData(
`https://swapi.dev/api/people/?page=${dontReturnZero(page)}`
);
dispatch(setCurrentCharacters(response.results));
};
useEffect(() => {
fetchStarWarsInfo();
}, [page]);
Actually it might also ask you to put dispatch in dependencies and other stuff, so it's not so critical not to list all of the dependencies that he ask for.
Try to remove dispatch from your fetchStarWarsInfo and check. If error doesn't disappear, so try to remove also getData function.
You might also want to configure eslint for React, so those errors would have more information about error
If fetchStarWarsInfo is declared within the functional component, everytime state changes, the function is re-constructed and re-declared, therefore, it changes. Therefore the error message is notifying you that although the function is changing, it's not declared as a dependency. The problem though, is if you declare a function as a dependency, you can potentially create an infinite loop, since if you change state within the fetchStarWarsInfo, the useEffect hook will have to continuously keep running, as a new instance of fetchStarWarsInfo will be created.
The solutions to this are pretty straightforward. Declare the fetchStarWarsInfo within the useEffect hook or split up the logic within the function, so you can declare it outside the functional component completely. Then handle state changes within the useEffect hook. That way the function is declared only once, and state changes are handled within the hook, thereby causing no unwanted side effects
After doing this, you need to verify that all the functions used within fetchStarWarsInfo are not continuously changing either. (i.e dontReturnZero,setCurrCharacters, etc). You can check this by commenting them out and seeing if the error disappears. If it does disappear, you'll need to include them in your dependency array for useEffect.
Example 1:
useEffect(() => {
const fetchStarWarsInfo = async () => {
const response = await getData(
`https://swapi.dev/api/people/?page=${dontReturnZero(page)}`
);
dispatch(setCurrentCharacters(response.results));
};
fetchStarWarsInfo();
}, [page, dispatch]);
Example 2:
const fetchStarWarsInfo = async (page) => {
const response = await getData(
`https://swapi.dev/api/people/?page=${dontReturnZero(page)}`
);
return response
};
const FunctionalComponent=() => {
const dispatch = useDispatch()
useEffect(() => {
fetchStarWarsInfo(page).then((data)=> dispatch(setCurrentCharacters(data.results));
}, [page, dispatch]);
}
I'm making a checkbox component for my project, and I have it set so that whenever is isChecked state changes, useEffect will run a returnFunction (defined in the props), like so:
export function JCheckbox({ returnFunction }: JCheckboxProps) {
const [isChecked, setIsChecked] = React.useState(defaultState)
React.useEffect(() => {
returnFunction(isChecked)
}, [isChecked])
When I do this, my compiler wants me to include returnFunction in the useEffect dependency array. I'm confused why I would need to track that, it's not used at all anywhere else in the function, and it doesn't change either. The code works perfectly fine without it.
You should add your function (returnFunction) to useEffect dependency because your function reference (returnFunction) may be changed, So react ask you to add your function (returnFunction) in useEffect dependency to recognize that and update them by new function reference.
returnFunction is certainly used as it is called inside the effect, and it could certainly change as the parent component could pass in different props, in which case your component would continue to call the previously passed in function. So the linter is correct, returnFunction is a dependency. However adding it makes it slightly more difficult to detect changes of isChecked, as changing returnFunction would also trigger the effect. Thus it seems way more elegant to call returnFunction from within setIsChecked:
const [isChecked, _setIsChecked] = useState(false);
function setIsChecked(value: boolean) {
returnFunction(value);
_setIsChecked(value);
}
However in the case given it seems that the state should rather be maintained by the parent component, consider lifting it up.
returnFunction is read from props so you probably must include it in the array of dependencies.
You should almost always include every variable and function you declare inside React and use in useEffect callback in the array of dependencies.
Eslint does not expect you to include setState functions and refs that you create in the same component that you execute useEffect.
Edit:
If you have a handleChange event handler and isChecked is toggled whenever the onChange method is invoked, you probably do not need useEffect. Call returnFunction function with the new isChecked value.
This question already has answers here:
Understanding the React Hooks 'exhaustive-deps' lint rule
(2 answers)
Closed 11 months ago.
Code highlighted in GitHub
useEffect(() => {
async function getTok() {
await Gettestimon();
alldoc.map(forget => console.log(forget.name));
setcondi(true);
// eslint-disable-next-line react-hooks/exhaustive-deps
}
getTok();
}, []);
Whenever I compile the highlighted code, it says my function gettestimon which stands for getting a testimonial, not something else, is not imported in effect's, but then it ends up working anyway. I would like to know how to fix this, but I read somewhere and added:
// eslint-disable-next-line react-hooks/exhaustive-deps
How can I fix this?
The exact error is:
React Hook useEffect has a missing dependency: 'Gettestimon'. Either include it or remove the dependency array.eslintreact-hooks/exhaustive-deps
This is the website where it is used:
https://www.aayushgarg.net/testimonials
If you are using external functions/variables inside React.useEffect, you need to add them to its dependency array as well. This way, React will know to run the React.useEffect only when the values specified in the dependency array changes. Plus, you won't need to use the eslint comment.
While it still works without adding it to the dependencies array, it will decrease the performance of your component with too many unnecessary re-renders.
You could improve it to something like this
useEffect(() => {
async function getTok() {
await Gettestimon();
alldoc.map(forget => console.log(forget.name));
setcondi(true);
}
getTok();
}, [Gettestimon]);
To learn more about the React.useEffect, I'd suggest reading their official documentation here.
This is due to a missing dependency.
The useEffect method is called for each render with the second argument being empty. But if you need your useEffect function to execute when only something changes, we need to add that variable or object to the dependency array.
In using Effect, you used external objects out of function scope like alldoc and Gettestimon.
If you want to execute useEffect only with these object changes, alldoc and Gettestimo, you need to add it in the dependency array. Otherwise, you will send up in executing useEffect for each render cycle.
in what case I shouldn't put the empty array [] as dependcies in react useEffect? like
React.useEffect(() => {
if (true) {
doStuff()
}
return () => null;
});
Is above code even valid?
just remember 3 rules:
useEffect(()=>{},[]) //this is like componentDidMount
useEffect(()=>{}) //this will run every render
useEffect(()=>{},[dep1, dep2]) //this will run if one on de dependency change
Your code is valid.
undefined is passed as dependencies and it means useEffect runs on every render. So if you want to do some work that shouldn't be run on every render (like adding event listener), you should give a dependency array.
I am not sure if this is a valid warning by using useEffect around the dependency array, it seems like whenever variable, method or dispatch inside the useEffect are giving warning that
React Hook useEffect has missing dependencies: 'active', 'retrieveUser', and 'dispatch'. Either include them or remove the dependency array of following example, if I just leave it as blank array to perform the componentDidMount functionality
useEffect(() => {
setActive(active);
await retrieveUser(param1, param2);
dispatch(someAction);
}, []). // warning: React Hook useEffect has missing dependencies: 'active', 'retrieveUser', 'param1', 'param2', and 'dispatch'. Either include them or remove the dependency array, but here I just want to perform componentDidMount concept so my dependency list has to be empty
or
useEffect(() => {
await retrieveUser(param1, param2);
dispatch(someAction);
}, [userId]). // warning: React Hook useEffect has missing dependencies: 'retrieveUser', 'param1', 'param2', and 'dispatch'. Either include them or remove the dependency array
Are those valid warning? Especially that I just want to monitor on specific data field, what's the point of adding all inside dispatch or method into the dependency array if I can't add anything into the dependency list(for componentDidMOunt) or I just want to monitor userId, not param1, param2, etc
React is giving this warning because you are using part of your component's state or props in useEffect but have told it not to run the useEffect callback when that state/props changes. React assumes you will want to run that useEffect() whenever the referenced states change so it is giving you a warning. If you were only referencing variables outside of state or props, you would not get this warning. See below for info about conditionally running useEffect.
Since your goal is to only run useEffect only sometimes, you're likely going to need to restructure your component so that different data is in or out of state. It's difficult to recommend more specific solutions without knowing more about the component; but, a good goal is to have the state you utilize in useEffect match up with the changes that you want to trigger a rerender.
There might be other issues with state in your component. For example, I notice that you call setActive(active);. If active is part of the component's state, that means you are setting active to itself. Which isn't necessary. (this is assuming you are following typical naming patterns and likely have a line at the top of your component like:
const [active, setActive] = useState(initialValue);
Conditionally running useEffect
When you provide a second argument to useEffect(), it limits the situations when useEffect runs the callback function on rerenders. This is for performance reasons as explained in the React docs.
In the first useEffect where you provide an empty array, you are telling React to only run those actions (setActive(active)...etc.) when the component mounts and unmounts.
In the second useEffect where you provide an array with userId, you are telling React to only run the callback on mount, unmount, and whenever userId changes.
React is giving you that warning because it knows it won't run useEffect if active...etc or other values change. This can cause different parts of your component to be referencing different states. For example, one part of your component might be using userId = 1 and another part is using userId = 2. This is not how React is designed to work.
Create a hook that executes only once:
const useEffectOnce = effect => {
useEffect(effect, []);
};
use this hook inside your component
useEffectOnce(() => {
const request = async () => {
const result = await retrieveSum(param1, param2);
setResult(result);
};
request();
});
Now, even if your props (param1, param2) change, the effect is called only once. Be very careful when doing this, the component is probably buggy unless your initial props are truly special. I don't see how this is different from disabling the rule with
// eslint-disable-next-line react-hooks/exhaustive-deps
Demo: https://codesandbox.io/s/trusting-satoshi-fngrw?file=/src/App.js