How do I prevent my React component from fetching images every time the component is rendered and to fetch them from the store instead? I don't want to get rate limited by doing the API call over and over again, but the problem with useEffect is that it seems to be unaware of the variables being set "outside" of the effect. It seems to completely ignore !images.length and when I log the value for images.length it is always 0 :(
Images.tsx
const dispatch = useDispatch();
const images = useSelector(selectedImages);
useEffect(() => {
if (!images.length) {
dispatch(fetchImages());
}
}, []);
With React hook useMemo you will only call the API whenever you need it
https://www.robinwieruch.de/react-usememo-hook
https://reactjs.org/docs/hooks-reference.html#usememo
It is always logging 0 is because you didn't put anything inside dependency array of useEffect so that whenever React re-render your component, it looks inside the useEffect and see nothing inside the dep array, so it just skip running that useEffect.
When you use useEffect, you should declare dependencies in the array of useEffect that are used inside useEffect
const dispatch = useDispatch();
const images = useSelector(selectedImages);
useEffect(() => {
if (!images.length) {
dispatch(fetchImages());
}
}, [images]); // Our deps
So, let's say:
First, by default it will run the useEffect and do all the logics inside that effect. If the images array is empty, calling the API to get images.
Second, images array is not empty anymore, and the 2nd images is different from 1st images array which is empty, so it still run to the useEffect. But it won't fetch images anymore because you have if condition there.
Related
Am using useEffect in a react functional component to fetch data from an external API but it keeps calling the API endpoint on render on the page .
Am looking for a way to stop the useeffect from running on render on the component
Use the dependency array (second argument to the useEffect), so you can specify when you need to run the useEffect.
The problem here is that you have not used the dependency array, so that it executes every time. By adding a dependency array, you specify the changes where you want useEffect to run.
useEffect(()=>{
},[<dependency array: which contains the properties>]);
If you leave the dependency array empty, it will run only once. Therefore if you want the API call to run only once, add an empty array as the second argument to your useEffect. This is your solution.
Like this:
useEffect(()=>{
//Your API Call
},[]);
useEffect is always meant to run after all the changes or render effects are update in the DOM. It will not run while or before the DOM is updated. You may not have given the second argument to useEffect, which if u do not provide will cause the useEffect to execute on each and every change. Assuming you only want to call the API just once when on after the first render you should provide an empty array.
Runs on all updates, see no second argument to useEffect:
useEffect(() => { /* call API */ });
Runs when the prop or state changes, see the second argument:
useEffect(() => { /* call API */ }, [prop, state]);
Runs only once, see the empty second argument:
useEffect(() => { /* call API */ }, []);
I recommend you to read the full documentation about the React useEffect hook.
Here is a easy example of using useEffect
function functionalComponent() {
const [data, setData] = React.useState(null);
React.useEffect(() => {
const url = 'https://randomuser.me/api/?results=10';
fetch(url)
.then(data => {
setData(data);
})
.catch(error => console.error(error))
}, []); // it's necessary to use [] to avoid the re-rendering
return <React.Fragment>
{data !== null && (
<React.Fragment>
{data.results.map(data => (
<div>
{data.gender}
</div>
))}
</React.Fragment>
)}
</React.Fragment>;
}
Maybe in your useEffect implementation you are avoiding the [] dependencies, this is a bit hard to understand if you come from class states. This on hooks review when a state element inside the hook change, for example if you are using an element that always change like a prop that you pass throught another component you might be setting inside the dependencies or another state, if you do not need any dependency just use it empty like the example above. As you can see in the documentation sometimes the dependencies are not used, this might generate an infinite loop.
Inside a React Project. Let's say you start at the FavoriteList-page, from there you go to the ItemInfo-page. My question is, when you go back to the Favorites-page, is the useEffect(() => {}, []) hook (similar to the ComponentDidMount) load again?
Because it's not loading for me :'(
It only loads the very first time.
Use effect always runs on the first render of a component, then after that depends on what you pass as your second argument.
If you want useEffect to run ONLY on the first render of the component, use this:
useEffect(() => {}, [])
If you want useEffect to run on EVERY render, use this (no second argument):
useEffect(() => {})
If you want useEffect to run on the first render of the component, AND on state(s) changes, use this:
useEffect(() => {}, [state, anotherState])
If you are trying to avoid useEffect rendering on the first render, you need to use a UseRef hook. This post will tell you how to go about doing this.
I am now trying to call an API using data from the redux store.
Let say I got 2 API calls, Api A and Api B Inside the parent component I already called the API A and save the data inside the redux already.
Now I am in another component. I need to call Api B. But API B has a params, which I will get from API A. So Inside the Second component, I am using useEffect hook to call the data.
To get the params from the redux, I am using useSelector Hook.
Inside the second component, UseEffect Hook is something like this:
useEffect(() => {
let splitText = cartList?.OrderDTO?.DeliveryCountry;
let deliveryAddressId = splitText?.split(',');
if (cartList.OrderDTO?.DeliveryCountry !== '') {
dispatch(getShippingMethodById(token.access_token, deliveryAddressId));
} else {
dispatch(
getShippingMethodById(
token.access_token,
cartList.OrderDTO?.CustomerAddressId,
),
}
}, []);
So in the useEffect hook, I got the deliveryAddressId from redux. To draw in data from the redux into component, I am using useSelector hook
let cartList = useSelector((state) => state.productReducer.cartList);
The problem is that I always get undefined for cartlist when ever I tried to access it inside the useEffect hook
So the dispatch called are always getting undefined. So What can I do to make this hooks works?
You should add cartList to your dependency array, so the useEffect hook watches for updates to that piece of state. As it is written now, the useEffect only runs on the first render, where cartList is probably undefined.
React - useEffect Docs
useEffect(() => {
let splitText = cartList?.OrderDTO?.DeliveryCountry;
let deliveryAddressId = splitText?.split(',');
if (cartList.OrderDTO?.DeliveryCountry !== '') {
dispatch(getShippingMethodById(token.access_token, deliveryAddressId));
} else {
dispatch(
getShippingMethodById(
token.access_token,
cartList.OrderDTO?.CustomerAddressId,
),
}
}, [cartList]); // Add 'cartList' to your dependency array here
Solution is that you add cartList inside the dependency array.
useEffect(() => {
// All your logic inside
}, [cartList]);
I don't have info about the complete parent-child component structure, but from what I understood with the error I can explain the issue.
You are using [], for useEffect dependency, which means the callback inside useEffect will be triggered only once when the component mounts.
It is possible that when your component mounted, the API call in the parent was not complete, and you have still have undefined in store for cartList.
To check this hypothesis you can add console.log in API response and
inside the useEffect.
What else you can do?
You can not render the child component until you have data from the API call.
Why adding cartList in the dependency array fixed the issue?
By dependency array inside useEffect, your useEffect call back will be
called whenever the values in the dependency array change + on the
mount.
So, at the time of mount of the child component, useEffect's callback will trigger (cartList as undefined), then when the API call is successful and after that data is pushed in state, your child component will rerender and will retrigger the callback inside useEffect with the actual(which you got from API and pushed in store) cartList data.
I am trying to setup a functional component that allows the user to copy/paste their clipboard image into the app. In useEffect, I setup document.onpaste . This was working and I was able to get the image and upload it. But I need to have it update the state after paste, so that I can allow them to paste multiple images and keep them in array in state.
I've simplified the problem below. Basically when I paste I want to have it update the counter value, but it doesn't. Counter always = 0. If I use normal react button onclick event, it works fine. I'm guessing the onpaste event is outside of React, but then,
How can I process the paste event and also update the functional state?
import React, { useEffect, useState } from 'react';
const AddAttachmentsContainer = ({ zoneid, tableid, field }) => {
const [counter,setCounter]=useState(0);
useEffect(() => {
document.onpaste=(e)=>{GetImage(e);e.preventDefault()}
},[])
const GetImage = (e) => {
setCounter(counter+1)
console.log(counter)
}
}
If I am not mistaken, I'll bet that the state is updated. useState will update the state and trigger a re-render. Since it is asynchronous, console.log will be called with the old state value.
useEffect(() => console.log(counter), [counter]);
... don't forget to add counter in the second argument of useEffect. see here...
Also see here
Alternatively, of course, you can also use refs, but be aware of the use cases for them. It is easy to allow refs to unnecessarily spread through your code.
To be honest, it might be wise to just setCounter in the useEffect you already have and call your GetImage function afterwards... from there, you can retrieve counter. There are multiple approaches to this.
I have a redux action that fetches all data and stores it into a global Redux store.
I want to store that state in a local state using Hooks so that the actual state doesn't get changed when I change it locally.
What I am doing right now is,
const [filteredTable, setFilteredTable] = useState([])
useEffect(() => {
props.fetchDatabase();
props.fetchOptions();
setFilteredTable(props.filtered_table_data);
}, [])
In useEffect, the props.fetchDatabase() gets the props.filtered_table_data and I can see that when I console.log it out.
However, when I use Hooks to store it into a local state and check if it's in there,
console.log(filteredTable, 'filteredTable')
just gives me [].
What am I doing wrong?
I believe the props.fetchDatabase() call is asynchronous, so by the time you are attempting to setFilteredTable the props.filtered_table_data has not updated yet.
You can try something like this:
useEffect(() => {
props.fetchDatabase();
props.fetchOptions();
}, [])
useEffect(() => {
setFilteredTable(props.filtered_table_data);
}, [props.filtered_table_data]);
Note that this effect will run every time filtered_table_data changes, so you may need to wrap around the code in the callback with some sort of condition if you want to restrict setting the local state.
useEffect's callback with [] as hook's second argument is only being called once when component just mounted. Inside it fetchDatabase, and fetchOptions callbacks are called, and right after that (when data isn't yet fetched) you call setFilteredTable, that's why there are empty array occurs in filteredTable.
Not sure if this answers your question, but React-Redux provides some hooks for accessing the redux store.
The one that might be of interest to you is the useSelector() hook.
Here's an example usage:
import { useSelector } from 'react-redux';
const App = () => {
const tableData = useSelector(state => state.tableData);
...
}