patternreactCritical
useEffect() is called twice even if an empty array is used as an argument
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argumenttwicearrayusedevenemptyuseeffectcalled
Problem
I am writing code so that before the data is loaded from DB, it will show loading message, and then after it is loaded, render components with the loaded data. To do this, I am using both useState hook and useEffect hook. Here is the code:
The problem is, useEffect is triggered twice when I check with console.log. The code is thus querying the same data twice, which should be avoided.
Below is the code that I wrote:
Why it is called twice, and how to fix the code properly?
The problem is, useEffect is triggered twice when I check with console.log. The code is thus querying the same data twice, which should be avoided.
Below is the code that I wrote:
import React from 'react';
import './App.css';
import {useState,useEffect} from 'react';
import Postspreview from '../components/Postspreview'
const indexarray=[]; //The array to which the fetched data will be pushed
function Home() {
const [isLoading,setLoad]=useState(true);
useEffect(()=>{
/*
Query logic to query from DB and push to indexarray
*/
setLoad(false); // To indicate that the loading is complete
})
},[]);
if (isLoading===true){
console.log("Loading");
return This is loading...
}
else {
console.log("Loaded!"); //This is actually logged twice.
return (
{indexarray.map(indexarray=>
)}
);
}
}
export default Home;Why it is called twice, and how to fix the code properly?
Solution
Put the console.log inside the useEffect
Probably you have other side effects that cause the component to rerender but the useEffect itself will only be called once. You can see this for sure with the following code.
If the log "i fire once" is triggered more than once it means your issue is
one of 3 things.
This component appears more than once in your page
This one should be obvious, your component is in the page a couple of times and each one will mount and run the useEffect
Something higher up the tree is unmounting and remounting
The component is being forced to unmount and remount on its initial render. This could be something like a "key" change happening higher up the tree. you need to go up each level with this useEffect until it renders only once. then you should be able to find the cause or the remount.
React.Strict mode is on
StrictMode renders components twice (on dev but not production) in order to detect any problems with your code and warn you about them (which can be quite useful).
This answer was pointed out by @johnhendirx and written by @rangfu, see link and give him some love if this was your problem. If you're having issues because of this it usually means you're not using useEffect for its intended purpose. There's some great information about this in the docs you can read that here
Probably you have other side effects that cause the component to rerender but the useEffect itself will only be called once. You can see this for sure with the following code.
useEffect(()=>{
// Query logic
console.log('i fire once');
},[]);
If the log "i fire once" is triggered more than once it means your issue is
one of 3 things.
This component appears more than once in your page
This one should be obvious, your component is in the page a couple of times and each one will mount and run the useEffect
Something higher up the tree is unmounting and remounting
The component is being forced to unmount and remount on its initial render. This could be something like a "key" change happening higher up the tree. you need to go up each level with this useEffect until it renders only once. then you should be able to find the cause or the remount.
React.Strict mode is on
StrictMode renders components twice (on dev but not production) in order to detect any problems with your code and warn you about them (which can be quite useful).
This answer was pointed out by @johnhendirx and written by @rangfu, see link and give him some love if this was your problem. If you're having issues because of this it usually means you're not using useEffect for its intended purpose. There's some great information about this in the docs you can read that here
Context
Stack Overflow Q#60618844, score: 1265
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