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/${pokeName.pokemon_name}`
);
console.log(response.data);
setPokeList(response.data);
};
fetchData();
}, []);
console.log(pokeList);
Thank you
When the code runs first the pokeList is an empty array and it cannot find the property name. You should create a second state and do something like this
const pokeName = useParams();
const [pokeList, setPokeList] = useState([]);
const [specificPokemon, setSpecificPokemon] = useState({});
useEffect(() => {
const fetchData = async () => {
const response = await axios.get(
"https://pokeapi.co/api/v2/pokemon?limit=151"
);
setPokeList(response.data.results);
const selectedPokemon = response.data.results.find((pokemon) => {
return pokemon.name === pokeName.pokemon_name;
});
setSpecificPokemon(selectedPokemon)
};
fetchData();
}, [])
And don't forget to make the specificPokemon property optional like this specificPokemon?.name
When your component is mounted, pokeList is an empty array.
React will run the following block before the useEffect hook has finished running:
const specificPokemon = pokeList.find((pokemon) => {
return pokemon.name === pokeName.pokemon_name;
});
console.log(specificPokemon);
console.log(specificPokemon.name);
As long as your array is empty, specificPokemon will be undefined and calling specificPokemon.name will trigger your error.
Beware with console.log, its behavior is not always synchronous.
You might think specificPokemon is properly defined because console.log won't necessarily show undefined.
To verify this, use console.log(JSON.stringify(specificPokemon));.
Related
I am new with react hooks, i'm trying to get info from an API but when i do the request i get 2 responses first an empty array and then the data of the API, why am i getting that empty array! , this is my first question, i'm sorry.
Thanks for helping me !
import {useState, useEffect} from 'react';
const getSlides = (API) => {
const[data,setData] = useState([]);
const getData = () =>
fetch(`${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
I've been trying to use the data I get from an Async function inside of another function I use to display HTML on a react project. I have made several attempts but nothing seems to work for me. Hope any of you could help me. Please correct me if I did anything wrong.
I've tried it with a useEffect as well:
import React, { useState, useEffect } from 'react';
import { getGenres } from './api/functions';
const ParentThatFetches = () => {
const [data, updateData] = useState();
useEffect(() => {
const getData = async () => {
const genres = await getGenres('tv');
updateData(genres);
}
getData();
}, []);
return data && <Screen data={data} />
}
const Screen = ({data}) => {
console.log({data}); //logs 'data: undefined' to the console
return (
<div>
<h1 className="text-3xl font-bold underline">H1</h1>
</div>
);
}
export default Screen;
The Error I get from this is: {data: undefined}.
The getGenres function that makes the HTTP Request:
const apiKey = 'key';
const baseUrl = 'https://api.themoviedb.org/3';
export const getGenres = async (type) => {
const requestEndpoint = `/genre/${type}/list`;
const requestParams = `?api_key=${apiKey}`;
const urlToFetch = baseUrl + requestEndpoint + requestParams;
try {
const response = await fetch(urlToFetch);
if(response.ok) {
const jsonResponse = await response.json();
const genres = jsonResponse.genres;
return genres;
}
} catch(e) {
console.log(e);
}
}
I want to use the data inside my HTML, so the H1 for example.
Once again, haven't been doing this for a long time so correct me if I'm wrong.
There are a few conceptual misunderstandings that I want to tackle in your code.
In modern React, your components should typically render some type of jsx, which is how React renders html. In your first example, you are using App to return your genres to your Screen component, which you don't need to do.
If your goal is to fetch some genres and then ultimately print them out onto the screen, you only need one component. Inside that component, you will useEffect to call an asynchronous function that will then await the api data and set it to a react state. That state will then be what you can iterate through.
When genres is first rendered by react on line 6, it will be undefined. Then, once the api data is retrieved, React will update the value of genre to be your array of genres which will cause the component to be re-rendered.
{genres && genres.map((genre) ... on line 20 checks to see if genres is defined, and only if it is, will it map (like looping) through the genres. At first, since genres is undefined, nothing will print and no errors will be thrown. After the genres are set in our useEffect hook, genres will now be an array and we can therefore loop through them.
Here is a working example of your code.
import React, { useState, useEffect } from "react";
import { getGenres } from "./api/functions";
function App() {
const [genres, setGenres] = useState();
useEffect(() => {
async function apiCall() {
const apiResponse = await getGenres("tv");
console.log(apiResponse);
setGenres(apiResponse);
}
apiCall();
}, []);
return (
<div>
<h1 className="text-3xl font-bold underline">H1</h1>
{genres && genres.map((genre) => <div key={genre}>{genre}</div>)}
</div>
);
}
export default App;
You should use a combination or useEffect and useState
You should use useEffect to launch the async get, without launching it each rerendering. If a async function is used to get some data from outside the component, this is called 'side effect' so use useEffect.
You should use useState to react to changes on theses side effects, re-rendering the component to get the data in the dom.
In the next example, Im using a dummy async function getGenres which returns an array of genres.
Here is an example and a WORKING EXAMPLE :
const {useState, useEffect} = React;
async function getGenres() {
var promise = new Promise(function(resolve, reject) {
window.setTimeout(function() {
resolve( ['genre1', 'genre2']);
});
});
return promise;
}
const Screen = () => {
const [genres, setGenres] = useState([])
useEffect(
() => {
getGenres().then(
res => setGenres(res)
)
}, [getGenres]
)
return (
<ul>
{
genres.map(
i => <li>{i}</li>
)
}
</ul>
);
}
const root = ReactDOM.createRoot(document.getElementById('root'))
root.render(<Screen/>)
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;
};
I am fetching an object from api using axios.get("url"). The object fetched successfully (in Animal state) but there is a component level state (imageState) which requires updation using setState with fetched data. Code:Component:
import React,{useEffect, useState} from 'react'
import axios from 'axios'
const AnimalDetail = ({match}) => {
const [Animal ,setAnimal ] = useState({})
const Id = parseInt(match.params.id)
const [imageState, setImageState] = useState ("");
useEffect(()=>{
const fetchAnimal = async () => {
const {data} = await axios.get(`/api/animals/${Id}`)
setAnimal(data)
}
fetchAnimal()
// setImageState(Animal.image[0]) // need to access first index of image object
},[])
useEffect(()=>{
setImageState(Object.values(Animal.image)[0]) // error cant convert undefined to object
}
return (
<>
<h2>imageState </h2> //undefined
<h2>{typeof(Animal.image)}</h2> //gives object
</>
)
}
export default AnimalDetail
Backend Api :
{"id":2,
"image":["/image.jpg","/image2.jpg"],
"price":60000,
"breed":"",
"isAvailable":true,
"weight":110,
}
How can i fetch the data and update the component level state periodically(after fetching)?
You can try following, maybe this can help you. Removed the second useEffect and updated the image state in the first useEffect.
And also I can see, you have declared const [imageState, setImageState] = useState (""); twice. You can remove the second one.
Also, make sure you handle the API error in useEffect otherwise this may break the application on API failure.
import React, { useEffect, useState } from 'react';
import axios from 'axios';
const AnimalDetail = ({ match }) => {
const [Animal, setAnimal] = useState({});
const Id = parseInt(match.params.id);
const [imageState, setImageState] = useState('');
useEffect(() => {
const fetchAnimal = async () => {
const { data } = await axios.get(`/api/animals/${Id}`);
setAnimal(data);
setImageState(data.image[0]);
};
if (Id) {
fetchAnimal();
}
}, [Id]);
return (
<>
<h2>imageState </h2> //undefined
<h2>{typeof Animal.image}</h2> //gives object
</>
);
};
export default AnimalDetail;
your code has some error in the second useEffect.
you can use this one :
useEffect(() => {
if (Animal) setImageState(Object.values(Animal.image)[0]); // error cant convert undefined to object
}, [Animal]);
this is because the Animal should have value first.
and you are defining imageState two times in your code! the first one is enough.
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;