How dose the useEffect and useState hook works? - reactjs

Hi I am learning React Hooks I know useEffect when used without any dependency array should run after every render but what about render itself does it happen every time a state changes because I am trying following code where I am rending a static h1 to the DOM and in my useEffect I am using a fetch to call a json file on the success of the fetch I am setting a state loaded to true from false I have written this example for the learning purpose only I was expecting that this should have caused a infinite loop because useEffect without dependency array should run after every render and I am calling set state which according to me should force the render to happen but It only run the useEffect twice but
when I am using the other example where I am using useState create a name state and in the useeffect without dependency arry I am changing the name state with Math.random it is behaving as expected causing infinite loop what I understand is useState does not cause render state to happen untill the value of the state changed as in the first example the state value only changed once from false to true and in name example it is random so changes every time
Just need a to know is my understanding correct and also if you can share where I can learn these Kind of stuff as I did not find this on the react doc.
example 1 with fetch
`
const App = () => {
// const [name, setName] = useState('sumit');
const [loaded, setLoaded] =useState(false);
useEffect(() => {
console.log('sumit');
fetch('./option.json').then((res) => {
console.log(res);
return res.json();
}).then((res) => {
console.log(res);
setLoaded(true);
})
// setName(Math.random());
})
return (
<h1>sumit</h1>
);
}
example 2 with name
const App = () => {
const [name, setName] = useState('sumit');
// const [loaded, setLoaded] =useState(false);
useEffect(() => {
console.log('sumit');
/* fetch('./option.json').then((res) => {
console.log(res);
return res.json();
}).then((res) => {
console.log(res);
setLoaded(true);
}) */
// setcount(count+1);
setName(Math.random());
})
return (
<h1>sumit</h1>
);
}
`

Related

usestate can change the state value after axios in useEffect

I expected to get the url with category=business,but the web automatically reset my state to the url that dosent have the category.I dont know the reason behind
let {id}=useParams()
const [newsurl,setNewsurl]=useState(()=>{
const initialstate="https://newsapi.org/v2/top-headlines?country=us&apiKey=c75d8c8ba2f1470bb24817af1ed669ee"
return initialstate;})
//console.log(id);
const [articles, setActicles] = useState([]);
useEffect( ()=>{
if(id === 2)
console.log("condition")
setNewsurl("https://newsapi.org/v2/top-headlines?country=de&category=business&apiKey=c75d8c8ba2f1470bb24817af1ed669ee")},[])
useEffect(() => {
const getArticles = async () => {
const res = await Axios.get(newsurl);
setActicles(res.data.articles);
console.log(res);
};
getArticles();
}, []);
useEffect(() => {
console.log(newsurl)
// Whatever else we want to do after the state ha
s been updated.
}, [newsurl])
//return "https://newsapi.org/v2/top-headlines?country=us&apiKey=c75d8c8ba2f1470bb24817af1ed669ee";}
return (<><Newsnavbar />{articles?.map(({title,description,url,urlToImage,publishedAt,source})=>(
<NewsItem
title={title}
desciption={description}
url={url}
urlToImage={urlToImage}
publishedAt={publishedAt}
source={source.name} />
)) } </>
)
one more things is that when i save the code the page will change to have category but when i refresh it ,it change back to the inital state.Same case when typing the url with no id.May i know how to fix this and the reason behind?
Setting the state in React acts like an async function.
Meaning that the when you set the state and put a console.log right after it, it will likely run before the state has actually finished updating.
You can instead, for example, use a useEffect hook that is dependant on the relevant state in-order to see that the state value actually gets updates as anticipated.
Example:
useEffect(() => {
console.log(newsurl)
// Whatever else we want to do after the state has been updated.
}, [newsurl])
This console.log will run only after the state has finished changing and a render has occurred.
Note: "newsurl" in the example is interchangeable with whatever other state piece you're dealing with.
Check the documentation for more info about this.
setState is an async operation so in the first render both your useEffetcs run when your url is equal to the default value you pass to the useState hook. in the next render your url is changed but the second useEffect is not running anymore because you passed an empty array as it's dependency so it runs just once.
you can rewrite your code like the snippet below to solve the problem.
const [articles, setActicles] = useState([]);
const Id = props.id;
useEffect(() => {
const getArticles = async () => {
const newsurl =
Id === 2
? "https://newsapi.org/v2/top-headlines?country=de&category=business&apiKey=c75d8c8ba2f1470bb24817af1ed669ee"
: "https://newsapi.org/v2/top-headlines?country=us&apiKey=c75d8c8ba2f1470bb24817af1ed669ee";
const res = await Axios.get(newsurl);
setActicles(res.data.articles);
console.log(res);
};
getArticles();
}, []);

React useEffect is late runnig

I have a state map. map contains coordinates and goes to react-leaflet as props. I am updating the map using useEffect, but it works late, the inital value is gone as props.
const [map,setMap] = useState([0,0]);
useEffect(() => {
const data = getIp();
data.then(result => {
setMap([result.lat,result.lon]);
})
},[])
console.log(map);
console.log view:
Array [ 0, 0 ]
Array [ 36.8943, 30.7209 ]
useEffect hook is running after the first render so if you want to avoid the first render you could use a loader in your state and use it to conditionally render the content:
const [loading, setLoading] = useState(true);
const [map,setMap] = useState([0,0]);
useEffect(() => {
const data = getIp();
data.then(result => {
setMap([result.lat,result.lon]);
setLoading(false);
})
},[])
if (loading) return <p>Loading</p>;
return (
...your code
);
According to React documentation : https://reactjs.org/docs/hooks-reference.html
By default, effects run after every completed render, but you can
choose to fire them only when certain values have changed.
So your useEffect will work after the first render, that's normal behaivor.

React Native + Firestore infinite loop, using hooks

just starting to learn hooks here.
I am getting data from firestore and trying to set it to state using hooks. when I uncomment the line doing so, I get stuck in an infinite loop. no error, but the console goes crazy with logging the state thousands of times.
Let me know if you need more info!
function Lists(props) {
const [lists, setLists] = useState([])
const [loading, setLoading] = useState(true)
useEffect(() => {
const subscriber =
firestore().collection('users').doc(props.user).collection('lists')
.onSnapshot(QuerySnapshot => {
const items = []
QuerySnapshot.forEach(documentSnapshot => {
items.push({
...documentSnapshot.data(),
key: documentSnapshot.id,
});
//setLists(items)
setLoading(false)
console.log(lists)
})
})
// unsubscribe from firestore
return () => subscriber();
})
//rest of func..
this issue happens becauase useEffect gets called over and over again. useEffect is like componentDidMount and componentDidUpdate if you are familiar with React class components.
so whenever you set the state inside the useEffect, you trigger an update, and then, useEffect gets called again, and thus the infinite loop.
to fix this, useEffect accepts a extra argument, which is an array of dependancies, which indicates that this useEffect call should only re-executed whenever a change happens to one of its dependancies. in your case you can provide an empty array, telling react that this useEffect should only be called one time.
useEffect(() => {
const subscriber =
firestore().collection('users').doc(props.user).collection('lists')
.onSnapshot(QuerySnapshot => {
const items = []
QuerySnapshot.forEach(documentSnapshot => {
items.push({
...documentSnapshot.data(),
key: documentSnapshot.id,
});
//setLists(items)
setLoading(false)
console.log(lists)
})
})
// unsubscribe from firestore
return () => subscriber();
}, []) // <------------ the second argument we talked about

Updating Hooks state resulting in infinite loop

I tried to clone the state, update the clone then set the state to cloned value, but getting an error "Too many re-renders. React limits the number of renders to prevent an infinite loop."
Here's my code
const [poketypes,setPoketypes] = useState([])
useEffect(()=>{
axios.get("https://pokeapi.co/api/v2/type")
.then(response => {
setPoketypes(response.data.results);
})
},[])
const all = {name:"all",url:""};
const updatedPoketypes = [...poketypes];
updatedPoketypes.push(all);
setPoketypes(updatedPoketypes);
const App = () => {
const [poketypes,setPoketypes] = useState([])
useEffect(()=>{
axios.get("https://pokeapi.co/api/v2/type")
.then(response => {
setPoketypes(response.data.results);
})
},[])
const all = {name:"all",url:""};
const updatedPoketypes = [...poketypes];
updatedPoketypes.push(all);
setPoketypes(updatedPoketypes);
return somediv;
}
If you can see the body of this component, setPoketypes will be invoked and entire body of App will be re-render, and when it's re-render, setPoketypes will be invoked again, and re-render again, and hence you're getting the infinite loop.
Depending on what you need, you should really scope your setPoketypes in some certain event or function call.
UPDATES
I'm guessing you want to have a all being added to poketypes at the end of the array, if so do take a look at below code.
const App = () => {
const [poketypes,setPoketypes] = useState([])
useEffect(()=>{
axios.get("https://pokeapi.co/api/v2/type")
.then(response => {
const all = {name:"all",url:""};
setPoketypes([...response.data.results, all]);
})
},[])
return somediv;
}
Either you want to assign all as one of the default value of poketypes, or you can place it in the useEffect as shown above

I cannot collect data from API using Axios + React

I'm beginner with React. I have 2 different cases where I'm using React Hooks which I cannot receive the data from my local API properly.
Case 1:
export const RegisterT = () => {
const [test, setTest] = useState()
const addrState = {}
axios.get('http://127.0.0.1:3333/states', { addrState })
.then(res => {
setTest(res.data)
console.log(test)
})
...
}
It works with the state test displaying correctly the content from the API but I don't know why/how the Axios continues calling the API infinity - endless. (Ps: the very first call it returns undefined, then the next ones it works) What am I doing wrong?
To fix this I've tried to use useEffect like this (Case 2):
export const RegisterT = () => {
const [test, setTest] = useState()
const addrState = {}
useEffect(() => {
axios.get('http://127.0.0.1:3333/states', { addrState })
.then(res => {
setTest(res.data)
console.log(test);
})
}, [])
...
}
Now the Axios works only once but no data is coming from the API. Maybe I should use async/wait for this case but I cannot make it work. Does anyone know how to fix that (Case 1 or/and Case 2)?
Thanks.
Updating the state is an asynchronous operation. So the state is not really updated until the next time the component gets rendered. If you want to capture the correct state, you can either console.log(res.data) or wrap that inside the useEffect hook with test as dependency.
export const RegisterT = () => {
const [test, setTest] = useState()
const addrState = {}
// effect only runs when component is mounted
useEffect(() => {
axios.get('http://127.0.0.1:3333/states', { addrState })
.then(res => {
setTest(res.data);
});
}, []);
// effect runs whenever value of test changes
useEffect(() => {
console.log(test);
}, [test]);
}
That way it is guaranteed that the console.log runs when the value of test is updated.
Also the reason the API request is invoked once is you have not mentioned anything in the dependency array. [] empty dependency array runs the effect when the component is mounted for the first time.
async/await is just a wrapper around Promise object. So they would behave similarly.
The solution with useEffect is good. If you don't use it each render will call the request. This is the same if you put there console.log with any information. The reason why you don't see the data in the useEffect is that the value of the state is not updated in current render but in the next which is called by setter of the state. Move the console.log(test); after useEffect to see the data. On init it will be undefined but in the next render, it should contain the data from the request.

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