I am new with react hooks, i'm trying to get info from an API but when i do the request i get 2 responses first an empty array and then the data of the API, why am i getting that empty array! , this is my first question, i'm sorry.
Thanks for helping me !
import {useState, useEffect} from 'react';
const getSlides = (API) => {
const[data,setData] = useState([]);
const getData = () =>
fetch(`${API}`)
.then((res) => res.json())
useEffect(() => {
getData().then((data) => setData(data))
},[])
return data
}
export default getSlides;
The useEffect() hook runs after the first render. Since you've initialized the data state with an empty array, the first render returns an empty array.
If you're component depends on data to render, you can always conditionally return null until your data is loaded.
Also, I recommend using an async function for api requests, it allows you to use the await keyword which makes your code easier to read. The only caveat, is that you cannot pass an async function to useEffect, instead define an async function inside your hook, and then call it.
import React, { useState, useEffect } from "react";
const API = "https://example.com/data";
const GetSlides = (props) => {
const [data, setData] = useState();
useEffect(() => {
async function getData() {
const request = fetch(API);
const response = await request;
const parsed = await response.json();
setData(parsed);
}
getData();
// eslint-disable-next-line react-hooks/exhaustive-deps
}, []);
if (data === undefined) {
return null;
}
return <>data</>;
};
export default GetSlides;
Of course, you can still use Promise chaining if you desire.
useEffect(() => {
async function getData() {
await fetch(API)
.then((res) => res.json())
.then((data) => setData(data));
}
getData();
// eslint-disable-next-line react-hooks/exhaustive-deps
}, []);
<GetSlides api="https://yay.com" />
react components need to be title case
import React, { useState, useEffect } from 'react'
const GetSlides = ({ api }) => {
const [data, setData] = useState(null)
const getData = async () =>
await fetch(`${api}`)
.then((res) => res.json())
.then((data) => setData(data))
useEffect(() => {
getData()
// eslint-disable-next-line react-hooks/exhaustive-deps
}, [])
console.log(data)
return <div>slides</div>
}
export default GetSlides
The effect callback function is called after the render of your component. (Just like componentDidMount) So during the first render phase, the data state has not been set yet.
You initialize your data with and empty array here:
const[data,setData] = useState([] <- empty array);
useEffect runs after your component is mounted, and then calls the API, that it might take a few seconds or minutes to retrieve the data, but you return the data right away before knowing if the API finished its call.
If you want to return the data after it has been retrieved from the API, you should declare and async method
const getSlides = async (API) => {
try {
const res = await fetch(API);
const data = await res.json();
return data;
} catch (e) {
throw new Error(e);
}
}
Note that it is not necessary hooks for this function
Related
Hello i'm newbie here...
I found my friend's code when he using useState instead of using useEffect to fetch the API.
I tried it and it worked, the code didn't cause an error and infinite loops.
here is for the code
import { useState } from "react";
import { IN_THEATER, POSTER } from "../../../constant/movies";
import { GlobalGet } from "../../../utilities/fetch";
const Service = () => {
const [movieData, setMovieData] = useState({ data: null, poster: null });
const fetchMovieData = async () => {
try {
let movieRes = await GlobalGet({ url: `${IN_THEATER}` });
return movieRes;
} catch (error) {
console.log(error);
}
};
const fetchPoster = async () => {
try {
let posterRes = await GlobalGet({ url: `${POSTER}` });
return posterRes;
} catch (error) {
console.log(error);
}
};
const fetchData = async () => {
setMovieData({
data: await fetchMovieData(),
poster: await fetchPoster(),
});
};
useState(() => { //<=here it is
fetchData();
}, []);
return {
movieData,
};
};
export default Service;
And my question is, why it could be happen ? why using useState there doesn't cause an infinite loops ?
The useState() function can accept an initializer function as its first argument:
const [state, setState] = useState(initializerFunction)
When a function is passed to useState(), that function is only called once before the component initially mounts. In your case below, the initializer function is an anonymous arrow function:
useState(() => { // <-- Initializer function invoked once
fetchData();
}, []);
Here, the initializer function is () => { fetchData(); }, which is invoked once before the initial mount, so the fetchData() method is only called once. The array that is passed as the second argument [] is useless and doesn't do anything in this case as it's ignored by useState(). The above useState would behave the same if you did useState(fetchData);. Because fetchData() is only called once on the initial mount, any state updates of your component don't cause the fetchData() function to execute again as it's within the initializer function.
With that said, useState() shouldn't be used for fetching data on mount of your component, that's what useEffect() should be used for instead.
Generally it's possible to fetch data from outside of the useEffect hook.
Somewhere in the body of your component...
const [fetchedData, setFetchedData] = useState(false)
const someFetchFunc = asyunc (url) => {
setFetchedData(!fetchedData)
const res = await fetch(url)
const data = await res.json()
setMovieData(data)
}
!fetchedData && someFetchFunc()
But this is an antipattern. In this case developer lacks a whole toolset of dealing with possible issues. What if fetching fails?
So, it's generally a good idea to handle all the side effects like fetching in a place that was intended for that. It's useEffect hook)
I'm doing an exercise to learn React in which I have set up a page with a list of clickable pokemon names which are linking to the pokemons specific detail page. Below is the code of the details page
import { useState, useEffect } from "react";
import axios from "axios";
import { useParams } from "react-router-dom";
export default function DetailsPage() {
const pokeName = useParams();
console.log(pokeName);
const [pokeList, setPokeList] = useState([]);
useEffect(() => {
const fetchData = async () => {
const response = await axios.get(
"https://pokeapi.co/api/v2/pokemon?limit=151"
);
console.log(response.data);
setPokeList(response.data.results);
};
fetchData();
}, []);
const specificPokemon = pokeList.find((pokemon) => {
return pokemon.name === pokeName.pokemon_name;
});
console.log(specificPokemon);
console.log(specificPokemon.name);
return <div><p>{specificPokemon.name}</p></div>;
}
This code has an error I fail to understand
The console.log(specificPokemon) works fine, but the console.log(specificPokemon.name) gives me the following error
Uncaught TypeError: Cannot read properties of undefined (reading 'name')
The correct code is the following, but I wonder why my method doesn't work
const [pokeList2, setPokeList2] = useState([]);
useEffect(() => {
const fetchData = async () => {
const response = await axios.get(
`https://pokeapi.co/api/v2/pokemon/${pokeName.pokemon_name}`
);
console.log(response.data);
setPokeList(response.data);
};
fetchData();
}, []);
console.log(pokeList);
Thank you
When the code runs first the pokeList is an empty array and it cannot find the property name. You should create a second state and do something like this
const pokeName = useParams();
const [pokeList, setPokeList] = useState([]);
const [specificPokemon, setSpecificPokemon] = useState({});
useEffect(() => {
const fetchData = async () => {
const response = await axios.get(
"https://pokeapi.co/api/v2/pokemon?limit=151"
);
setPokeList(response.data.results);
const selectedPokemon = response.data.results.find((pokemon) => {
return pokemon.name === pokeName.pokemon_name;
});
setSpecificPokemon(selectedPokemon)
};
fetchData();
}, [])
And don't forget to make the specificPokemon property optional like this specificPokemon?.name
When your component is mounted, pokeList is an empty array.
React will run the following block before the useEffect hook has finished running:
const specificPokemon = pokeList.find((pokemon) => {
return pokemon.name === pokeName.pokemon_name;
});
console.log(specificPokemon);
console.log(specificPokemon.name);
As long as your array is empty, specificPokemon will be undefined and calling specificPokemon.name will trigger your error.
Beware with console.log, its behavior is not always synchronous.
You might think specificPokemon is properly defined because console.log won't necessarily show undefined.
To verify this, use console.log(JSON.stringify(specificPokemon));.
I am using React-native and in it, I have a custom Hook called useUser that gets the user's information from AWS Amplify using the Auth.getUserInfro method, and then gets part of the returned object and sets a state variable with it. I also have another Hook called useData hook that fetches some data based on the userId and sets it to a state variable.
useUser custom-Hook:
import React, { useState, useEffect } from "react";
import { Auth } from "aws-amplify";
const getUserInfo = async () => {
try {
const userInfo = await Auth.currentUserInfo();
const userId = userInfo?.attributes?.sub;
return userId;
} catch (e) {
console.log("Failed to get the AuthUserId", e);
}
};
const useUserId = () => {
const [id, setId] = useState("");
useEffect(() => {
getUserInfo().then((userId) => {
setId(userId);
});
}, []);
return id;
};
export default useUserId;
import useUserId from "./UseUserId";
// ...rest of the necessary imports
const fetchData = async (userId) = > { // code to fetch data from GraphQl}
const useData = () => {
const [data, setData] = useState();
useEffect(() => {
const userId = useUser();
fetchData(userId).then( // the rest of the code to set the state variable data.)
},[])
return data
}
When I try to do this I get an error telling me
*Error: Invalid hook call. Hooks can only be called inside of the body of a function component. This could happen for one of the following reasons:
You might have mismatching versions of React and the renderer (such as React DOM)
You might be breaking the Rules of Hooks
You might have more than one copy of React in the same app
See https://reactjs.org/link/invalid-hook-call for tips about how to debug and fix this problem.*
I think the problem is that I am calling the Hook useUser inside of the use effect, but using it inside the function will cause the problem described here, and I can't use it outside the body of the fetchData since the useData itself is a hook, and it can be only used inside a functional component's or Hook's body. So I don't know how to find a way around this problem.
Correct, React hooks can only be called from React function components and other React hooks. The useEffect hook's callback isn't a React hook, it's a callback. According to the Rules of Hooks, don't call hooks inside loops, conditions, or nested functions.
I suggest refactoring the useData hook to consume the userId as an argument, to be used in the dependency array of the useEffect.
const fetchData = async (userId) => {
// code to fetch data from GraphQl
};
const useData = (userId) => {
const [data, setData] = useState();
useEffect(() => {
fetchData(userId)
.then((....) => {
// the rest of the code to set the state variable data.
});
}, [userId]);
return data;
};
Usage in Function component:
const userId = useUser();
const data = useData(userId);
If this is something that is commonly paired, abstract into a single hook:
const useGetUserData = () => {
const userId = useUser();
const data = useData(userId);
return data;
};
...
const data = useGetUserData();
Though you should probably just implement as a single hook as follows:
const useGetUserData = () => {
const [data, setData] = useState();
useEffect(() => {
getUserInfo()
.then(fetchData) // shortened (userId) => fetchData(userId)
.then((....) => {
// the rest of the code to set the state variable data.
setData(....);
});
}, []);
return data;
};
You can't call hook inside useEffect, Hook should be always inside componet body not inside inner function/hook body.
import useUserId from "./UseUserId";
// ...rest of the necessary imports
const fetchData = async (userId) => {
// code to fetch data from GraphQl}
};
const useData = () => {
const [data, setData] = useState();
const userId = useUser();
useEffect(() => {
if (userId) {
fetchData(userId).then(setData);
}
}, [userId]);
return data;
};
im using this Api to get json data.
const FetchEarthquakeData = url => {
const [data, setData] = useState(null);
useEffect(() => {
fetch(url)
.then(res => res.json())
.then(jsonData => setData(jsonData.features))
}, [url]);
return data;
};
The problem is when I use this function like this:
const jsonData = FetchEarthquakeData(url)
console.log(jsonData);
I get following console.logs:
null
Array(17)
So my function FetchEarthquakeData returns the null variable and! the desired api. However if I want to map() over the jsonData, the null value gets mapped. How can I refactor my code so I get only the Array?
I'm not quite sure what useState() and setData() do. But in order to fetch the json data from the API, you can make the function as followed, then you can perform operations on the fetched data.
const url = "https://earthquake.usgs.gov/earthquakes/feed/v1.0/summary/4.5_day.geojson"
const FetchEarthquakeData = url => {
return new Promise(resolve => {
fetch(url)
.then(res => res.json())
.then(jsonData => resolve(jsonData.features))
})
}
FetchEarthquakeData(url).then(features => {
console.log(features)
// Do your map() here
})
as per the documentation of react hooks ,Only Call Hooks from React Functions Don’t call Hooks from regular JavaScript functions.
React Function Components -- which are basically just JavaScript Functions being React Components which return JSX (React's Syntax.)
for your requirement you can do as follows in your react component.
idea is to have state hook initialized with empty array then update it once json data available. the fetch logic can be moved inside useeffect .
const SampleComponent=()=>{
const [data,setData] = useState([])
useeffect (){
fetch(url).then((responsedata)=>setData(responseData) ,err=>console.log (err)
}
return (
<>
{
data.map((value,index)=>
<div key=index>{value}<div>
}
</>
)
}
if you find above fetching pattern repetitive in your app thou can use a custom hook for the same.
const useFetch = (url, options) => {
const [response, setResponse] = React.useState(null);
const [error, setError] = React.useState(null);
React.useEffect(() => {
const fetchData = async () => {
try {
const res = await fetch(url, options);
const json = await res.json();
setResponse(json);
} catch (error) {
setError(error);
}
};
fetchData();
}, []);
return { response, error };
};
in your React component you can use like
const {data,error} = useFetch(url , options)
You have to do it in an async fashion in order to achieve what you need.
I recommend you doing it separately. In case you need the data to load when the component mounts.
Another thing is that your function is a bit confusing.
Let's assume some things:
const Example = () => {
const [data, setData] = useState(null);
useEffect(() => {
const fetchData = async () => {
const result = await fetch(url);
setData(result.features);
};
fetchData();
}, []);
return (
<div>
{console.log(data)}
{data && <p>{JSON.stringify(data, null, 2)}</p>}
</div>
);
};
I am sure that the way you are doing it is storing the data properly on data, but you can not see it on your console.log. It is a bit tricky.
You can read a bit more here => https://medium.com/#wereHamster/beware-react-setstate-is-asynchronous-ce87ef1a9cf3
Adding a bit more of complexity in case you want to handle different states like loading and error.
const Example = () => {
const [data, setData] = useState(null);
const [isLoading, setIsLoading] = useState(false);
const [isError, setIsError] = useState(false);
useEffect(() => {
const fetchData = async () => {
setIsError(false);
setIsLoading(true);
try {
const result = await fetch(url);
setData(result.features);
} catch (error) {
setIsError(true);
}
setIsLoading(false);
};
fetchData();
}, []);
return (
<div>
{console.log(data)}
{data && <p>{JSON.stringify(data, null, 2)}</p>}
</div>
);
};
I have a function that gets some data from my backend and then I want simply to assign it to the state and display it in my browser. Everything works correctly, but I don't know why when I run a request the function keeps calling the API without stopping. What is the reason for this?
It seems that the function is stuck in some kind of while-true loop.
function App() {
const [orders, setOrders] = useState();
const getOrders = async () => {
const response = await axios.get("/api/orders/");
setOrders(response);
console.log(response);
};
getOrders();
return <div className="App">{JSON.stringify(orders)}</div>;
}
export default App;
What is the reason for this?
This happens because you are calling a function every render inside the functional component.
const getOrders = async () => {
const response = await axios.get("/api/orders/");
setOrders(response); // this will re render the component
console.log(response);
};
getOrders(); // this will be called every render and cause the infinity loop
When you render the component, you call getOrders and this functions calls setOrders wich will rerender the component, causing a infinity loop.
First render => call getOrders => call setOrders => Rerender =>
Second render => call getOrders => call setOrders => Rerender =>
...
You need to use useEffect hook or call the function on some event (maybe button click)
e.g. using useEffect
function App() {
const [orders, setOrders] = useState(null);
useEffect(() => {
const getOrders = async () => {
const response = await axios.get("/api/orders/");
setOrders(response);
console.log(response);
};
getOrders();
}, []);
return <div className="App">{JSON.stringify(orders)}</div>;
}
You want to use the Effect hook React offers:
https://reactjs.org/docs/hooks-effect.html
function App() {
const [orders, setOrders] = useState();
const getOrders = async () => {
const response = await axios.get("/api/orders/");
setOrders(response);
console.log(response);
};
useEffect(() => {
getOrders();
}, []); //Empty array = no dependencies = acts like componentDidMount / will only run once
return <div className="App">{JSON.stringify(orders)}</div>;
}
export default App;
I think you are using react-hooks here, so your getOrders function is getting render all the time.
Use useEffect from react to avoid this.
function App() {
const [orders, setOrders] = useState();
useEffect(() => {
const getOrders = async () => {
const response = await axios.get("/api/orders/");
setOrders(response);
console.log(response);
};
getOrders();
}, [])
return <div className="App">{JSON.stringify(orders)}</div>;
}
export default App;