Develop Reference

How to avoid unnecessary API calls with useEffect?

I'm still beginner to ReactJS and I'm having trouble rendering a list.
I don't know why, all the time calls are being made to my API. Since I don't put any dependency on useEffect, that is, I should only render my function once.
I don't understand why this is happening. Can you tell me what I'm doing wrong?
Here's my code I put into codesandbox.io
import React from "react";
import axios from "axios";
import "./styles.css";
const App = () => {
const BASE_URL = "https://pokeapi.co/api/v2";
const [pokemons, setPokemons] = React.useState([]);
const getAllPokemons = async () => {
const { data } = await axios.get(`${BASE_URL}/pokemon`);
data.results.map((pokemon) => getPokeType(pokemon));
};
const getPokeType = async (pokemon) => {
const { data } = await axios.get(pokemon.url);
setPokemons((prev) => [...prev, data]);
};
React.useEffect(() => {
getAllPokemons();
}, []);
console.log(pokemons);
return (
<div className="App">
<h1>Hello CodeSandbox</h1>
{pokemons.map((pokemon) => (
<p key={pokemon.id} style={{ color: "blue" }}>
{pokemon.name}
</p>
))}
</div>
);
};
export default App;
Thank you very much in advance.

Your issue is that you are calling setPokemons inside getPokeType (which is called for each data in part). Your useEffect is called just once (as expected) and the ${BASE_URL}/pokemon call is executed just once too. But getPokeType is called 20 times and the pokemons state is changed 20 times as well (once for each instance from data.results).
What I would recommend in your case (instead of what you have now) is:
Create a list of all the pokemons and
Set the state just once at the end.
So something like:
...
const getPokeType = async (pokemon) => {
const { data } = await axios.get(pokemon.url);
return data;
};
const getAllPokemons = async () => {
const { data } = await axios.get(`${BASE_URL}/pokemon`);
const pokemons = await Promise.all(
data.results.map((pokemon) => getPokeType(pokemon))
);
setPokemons(pokemons);
};
React.useEffect(() => {
getAllPokemons();
}, []);
...

I was just having the same issue in my project the way I solved is by moving the function definition inside the useEffect
React.useEffect(() => {
const getAllPokemons = async () => {
const { data } = await axios.get(`${BASE_URL}/pokemon`);
data.results.map((pokemon) => getPokeType(pokemon));
};
getAllPokemons();
}, []);
If this solves your problem please accept the answer.

React - Can console log object, but not specific property of said object

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));.

ReactJS how to update page after fetching data

I'm new to ReactJS and I'm now trying to do an interactive comments section (taken from frontendmentor.io), but the App component just doesn't show what it's supposed to show
This is my App component:
function App() {
const [data, setData] = useState([]);
useEffect(() => {
const getComm = async () => {
await fetchData();
};
getComm();
}, []);
console.log(data);
const fetchData = async () => {
const res = await fetch("db.json").then(async function (response) {
const comm = await response.json();
setData(comm);
return comm;
});
};
return (
<Fragment>
{data.length > 0 ? <Comments data={data} /> : "No Comments to Show"}
</Fragment>
);
}
export default App;
The console.log(data) logs two times:
the first time it's an empty Array;
the second time it's the Array with my datas inside.
As it follows:
If I force the App to print the Comments it just says that cannot map through an undefined variable
This is my Comments component:
function Comments({ data }) {
return (
<div>
{data.map((c) => (
<Comment key={c.id} />
))}
</div>
);
}
export default Comments;
I'm wondering why the page still displays No Comments to Show even if the log is correct

#Cristian-Irimiea Have right about response get from fetch. Response is an a object and can't be iterate. You need to store in state the comments from response
But you have multiple errors:
Take a look how use async function. Your function fetchData looks bad.
// Your function
const fetchData = async () => {
const res = await fetch("db.json").then(async function (response) {
const comm = await response.json();
setData(comm);
return comm;
});
};
// How can refactor
// fetchData function have responsibility to only fetch data and return a json
const fetchData = async () => {
const response = await fetch("./db.json");
const body = await response.json();
return body;
};
You are updating state inside fetch function but a good solution is update state then promise resolve:
useEffect(() => {
// here we use .then to get promise response and update state
fetchData().then((response) => setData(response.comments));
}, []);

The initial state of your data is an array.
After you fetch your data from the response you get an object. Changing state types is not a good practice. You should keep your data state as an array or as an object.
Considering you will keep it as an array, you need use an array inside of setData.
Ex.
comm && Array.isArray(comm.comments) && setData(comm.comments);
As for your Comments component you should consider expecting an array not an object.
Ex.
function Comments(data) {
return (
<div>
{data.map((c) => (
<Comment key={c.id} />
))}
</div>
);
}
export default Comments;

How can I return only the set-State from my fetched json 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>
);
};

Function to get data keep running in loop

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;