I was wondering Why this isn't an infinite loop?
I set the state inside of the useEffect hook and that should re-render the component and useEffect will be ran again and again and agian...
why this isn't what i expected?
import React, { useState, useEffect } from "react";
import axios from "axios";
const App = () => {
const [data, setData] = useState();
useEffect(() => {
axios.get("https://jsonplaceholder.typicode.com/comments").then((r) => {
console.log("Checking...");
setData(r.data[0]);
});
});
return (
<div>
<h1>{data}</h1>
</div>
);
};
export default App;
useEffect also accepts an array of dependencies i.e., useEffect will get executed when any values inside the array changes. So, if you want useEffect to be called whenever data changes, do this:
useEffect(() => {
axios.get("https://jsonplaceholder.typicode.com/comments").then((r) => {
console.log("Hello world");
setData(r.data[0]);
});
}, [data]);
The useEffect hook needs a second parameter, the dependency array.
When this dependency array is empty it will have the same behavior as the componentDidMount
If you add some dependencies to your array as data this useEffect will be executed every time that the data state changes.
So, if you want to fetch the data when the component loads you must do something like this.
useEffect(() => {
axios.get("https://jsonplaceholder.typicode.com/comments").then((r) => {
console.log("Hello world");
setData(r.data[0]);
});
}, []);
I have a navigation component where I'm passing a parameter to another page, the parameter is getting passed, however, the data in the dropdown is not updating for the passed ID:
nav:
<Link to='/service/ServiceAppointment/${car.Make}'> { serviceAppointment } </Link>
appointment page:
const ScheduleAppointment = () => {
const { id } = useParams();
console.log (id); //I can see the ID passed to the page in the console
useEffect(() => {
console.log(id); //the ID is not there
scheduleAppointment(id);
});
const Appointment= () => {
//call to API for open dates
//the ID never gets here
}
}
Router:
<Route exact path='/service/appointment/:id' component={ ScheduleAppointment } />
how can I get the appointment page to change when a new ID is passed to it?
Dependencies argument of useEffect is useEffect(callback, dependencies)
Let's explore side effects and runs:
Not provided: the side-effect runs after every rendering.
import { useEffect } from 'react';
function MyComponent() {
useEffect(() => {
// Runs after EVERY rendering
});
}
An empty array []: the side-effect runs once after the initial rendering.
import { useEffect } from 'react';
function MyComponent() {
useEffect(() => {
// Runs ONCE after initial rendering
}, []);
}
Has props or state values [prop1, prop2, ..., state1, state2]: the side-effect runs only when any dependency value changes.
import { useEffect, useState } from 'react';
function MyComponent({ prop }) {
const [state, setState] = useState('');
useEffect(() => {
// Runs ONCE after initial rendering
// and after every rendering ONLY IF `prop` or `state` changes
}, [prop, state]);
}
in your case try this way
useEffect(() => {
console.log(id); //the ID is not there
scheduleAppointment(id);
},[id]);
Please update the below link instead of your code
<Link to=`/service/ServiceAppointment/${car.Make}`> { serviceAppointment } </Link>
I hope it will work for you! Thanks.
I have some React Components that are going to be used a lot all over my App.
I'd like that only the first time they get printed in the DOM, they return as
<symbol id="SVGComponent"><SVGComponent></symbol><use href="#SVGComponent" />
while every other following printing should be only
<use href="#SVGComponent" /> since the symbol has already been declared in the page
const [firstTime, setFirstTime] = useState(true);
useEffect(() => { setFirstTime(false) }, []);
if (firstTime) {
return (<symbol id="Component"><Path /></symbol>);
}
else {
return(<use href="#Component"/>);
}
This is my code so far but the condition always return false.
How is that possibile in React with useEffect?
Thanks in advance for your help
It is not that your condition always returns false. The issue is that you change the state immediately after your first render(in the useEffect where you set the state variable value to false) which causes another immediate rerender so you skip the true condition of the variable(you technically don't skip it, you just render the true state and rerender with false straight away after).
If you don't change the state in the mounting useEffect you'll notice that the code works properly. In order to do this, though, you'll have to use useRef which basically enables you to create a variable that persists between rerenders but the mutation of its value doesn't cause a rerender(unlike state variables from useState).
I've added a button to simulate change of state or in other words - simulate a second render:
import React, { useState, useRef, useEffect } from 'react';
function App() {
const [buttonClicked, setButtonClicked] = useState(false);
const isFirstRender = useRef(true);
useEffect(() => {
isFirstRender.current = false;
}, []);
const renderRepresentation = () => {
if (isFirstRender.current) {
return (
<symbol id="Component">
<Path />
</symbol>
);
}
else {
return (
<use href="#Component" />
);
}
}
return (
<>
{renderRepresentation()}
<button onClick={() => setButtonClicked(true)}>SIMULATE STATE CHANGE</button>
</>
);
}
And here is a codesandbox demo which you can try.
The component itself won't be able to tell if others of itself exist. You will need to store this information at a higher level in the application, and then tell the component via props whether it is the first instance or not.
So I ended up replying to myself again for anyone coming. Fortunately what I want to achieve is possibile with just a few lines of code. Thanks this guy for the code: https://stackoverflow.com/a/57331421/3937587
let instancesCount = 0
function Component(props) {
const [firstRender, setFirstRender] = useState(false)
useEffect(() => {
setFirstRender(instancesCount === 0)
instancesCount++
return () => { instancesCount-- }
}, [])
if (firstRender)
//do something only the first time the component appears
}
Pretty much what it says on the title. When I console.log(repos) it returns an empty array. Why is the repos state not updating?
import React, { useEffect, useState } from "react";
import axios from "axios";
export default () => {
const [repos, setRepos] = useState([]);
useEffect(() => {
(async () => {
try {
let repo_lists = await axios.get(
"https://api.github.com/users/Coddielam/repos"
// { params: { sort: "created" } }
);
setRepos(repo_lists.data.slice(1).slice(-10));
console.log(repo_lists.data.slice(1).slice(-10));
console.log(repos);
} catch (err) {
setRepos([
"Something went wrong while fetching GitHub Api./nPlease use the link below to view my git repos instead ",
]);
}
})();
}, []);
return (
<div className="content">
<h2>View my recent git repos:</h2>
<ul>
...
</ul>
</div>
);
};
Answer is very simple. Your useState is updating .. believe me. The reason why you don't see it when you console.log() is because SetRespos is an asynchronous function.
Basically when you declare a function to update you useState value, react will use it as an async function
EXAMPLE
const [example, setExample] = useState('');
useEffect(() => {
setExample('Hello');
console.log('I'm coming first'); // This will be executed first
console.log(example); // This will come after this
}, [])
The output will be :
I'm coming first
// Blank Line
But still your useState will update after this. If you want to see that do this :
useEffect(() => {
console.log(respose); // This will give you the value
}, [respos])
I'm using a separate useEffect to console.log() the value. In the [] (dependency array) we pass respos which simply means that the useEffect will run every time the value of respos changes.
Read more about useStates and useEffects in react's documentation
State updates are async. You will only see them reflected on the next render. If you console log the state immediately after calling setState it will always log the current state, not the future state.
You can log the state in an effect every time it changes and you will see it changing:
useEffect(() => console.log(repos), [repos]);
This effect will be called after the state update has been applied.
With React's new Effect Hooks, I can tell React to skip applying an effect if certain values haven't changed between re-renders - Example from React's docs:
useEffect(() => {
document.title = `You clicked ${count} times`;
}, [count]); // Only re-run the effect if count changes
But the example above applies the effect upon initial render, and upon subsequent re-renders where count has changed. How can I tell React to skip the effect on the initial render?
As the guide states,
The Effect Hook, useEffect, adds the ability to perform side effects from a function component. It serves the same purpose as componentDidMount, componentDidUpdate, and componentWillUnmount in React classes, but unified into a single API.
In this example from the guide it's expected that count is 0 only on initial render:
const [count, setCount] = useState(0);
So it will work as componentDidUpdate with additional check:
useEffect(() => {
if (count)
document.title = `You clicked ${count} times`;
}, [count]);
This is basically how custom hook that can be used instead of useEffect may work:
function useDidUpdateEffect(fn, inputs) {
const didMountRef = useRef(false);
useEffect(() => {
if (didMountRef.current) {
return fn();
}
didMountRef.current = true;
}, inputs);
}
Credits go to #Tholle for suggesting useRef instead of setState.
Here's a custom hook that just provides a boolean flag to indicate whether the current render is the first render (when the component was mounted). It's about the same as some of the other answers but you can use the flag in a useEffect or the render function or anywhere else in the component you want. Maybe someone can propose a better name.
import { useRef, useEffect } from 'react';
export const useIsMount = () => {
const isMountRef = useRef(true);
useEffect(() => {
isMountRef.current = false;
}, []);
return isMountRef.current;
};
You can use it like:
import React, { useEffect } from 'react';
import { useIsMount } from './useIsMount';
const MyComponent = () => {
const isMount = useIsMount();
useEffect(() => {
if (isMount) {
console.log('First Render');
} else {
console.log('Subsequent Render');
}
});
return isMount ? <p>First Render</p> : <p>Subsequent Render</p>;
};
And here's a test for it if you're interested:
import { renderHook } from '#testing-library/react-hooks';
import { useIsMount } from '../useIsMount';
describe('useIsMount', () => {
it('should be true on first render and false after', () => {
const { result, rerender } = renderHook(() => useIsMount());
expect(result.current).toEqual(true);
rerender();
expect(result.current).toEqual(false);
rerender();
expect(result.current).toEqual(false);
});
});
Our use case was to hide animated elements if the initial props indicate they should be hidden. On later renders if the props changed, we did want the elements to animate out.
I found a solution that is more simple and has no need to use another hook, but it has drawbacks.
useEffect(() => {
// skip initial render
return () => {
// do something with dependency
}
}, [dependency])
This is just an example that there are others ways of doing it if your case is very simple.
The drawback of doing this is that you can't have a cleanup effect and will only execute when the dependency array changes the second time.
This isn't recommended to use and you should use what the other answers are saying, but I only added this here so people know that there is more than one way of doing this.
Edit:
Just to make it more clear, you shouldn't use this approach to solving the problem in the question (skipping the initial render), this is only for teaching purpose that shows you can do the same thing in different ways.
If you need to skip the initial render, please use the approach on other answers.
I use a regular state variable instead of a ref.
// Initializing didMount as false
const [didMount, setDidMount] = useState(false)
// Setting didMount to true upon mounting
useEffect(() => { setDidMount(true) }, [])
// Now that we have a variable that tells us wether or not the component has
// mounted we can change the behavior of the other effect based on that
const [count, setCount] = useState(0)
useEffect(() => {
if (didMount) document.title = `You clicked ${count} times`
}, [count])
We can refactor the didMount logic as a custom hook like this.
function useDidMount() {
const [didMount, setDidMount] = useState(false)
useEffect(() => { setDidMount(true) }, [])
return didMount
}
Finally, we can use it in our component like this.
const didMount = useDidMount()
const [count, setCount] = useState(0)
useEffect(() => {
if (didMount) document.title = `You clicked ${count} times`
}, [count])
UPDATE Using useRef hook to avoid the extra rerender (Thanks to #TomEsterez for the suggestion)
This time our custom hook returns a function returning our ref's current value. U can use the ref directly too, but I like this better.
function useDidMount() {
const mountRef = useRef(false);
useEffect(() => { mountRef.current = true }, []);
return () => mountRef.current;
}
Usage
const MyComponent = () => {
const didMount = useDidMount();
useEffect(() => {
if (didMount()) // do something
else // do something else
})
return (
<div>something</div>
);
}
On a side note, I've never had to use this hook and there are probably better ways to handle this which would be more aligned with the React programming model.
Let me introduce to you react-use.
npm install react-use
Wanna run:
only after first render? -------> useUpdateEffect
only once? -------> useEffectOnce
check is it first mount? -------> useFirstMountState
Want to run effect with deep compare, shallow compare or throttle? and much more here.
Don't want to install a library? Check the code & copy. (maybe a star for the good folks there too)
Best thing is one less thing for you to maintain.
A TypeScript and CRA friendly hook, replace it with useEffect, this hook works like useEffect but won't be triggered while the first render happens.
import * as React from 'react'
export const useLazyEffect:typeof React.useEffect = (cb, dep) => {
const initializeRef = React.useRef<boolean>(false)
React.useEffect((...args) => {
if (initializeRef.current) {
cb(...args)
} else {
initializeRef.current = true
}
// eslint-disable-next-line react-hooks/exhaustive-deps
}, dep)
}
Here is my implementation based on Estus Flask's answer written in Typescript. It also supports cleanup callback.
import { DependencyList, EffectCallback, useEffect, useRef } from 'react';
export function useDidUpdateEffect(
effect: EffectCallback,
deps?: DependencyList
) {
// a flag to check if the component did mount (first render's passed)
// it's unrelated to the rendering process so we don't useState here
const didMountRef = useRef(false);
// effect callback runs when the dependency array changes, it also runs
// after the component mounted for the first time.
useEffect(() => {
// if so, mark the component as mounted and skip the first effect call
if (!didMountRef.current) {
didMountRef.current = true;
} else {
// subsequent useEffect callback invocations will execute the effect as normal
return effect();
}
}, deps);
}
Live Demo
The live demo below demonstrates the different between useEffect and useDidUpdateEffect hooks
I was going to comment on the currently accepted answer, but ran out of space!
Firstly, it's important to move away from thinking in terms of lifecycle events when using functional components. Think in terms of prop/state changes. I had a similar situation where I only wanted a particular useEffect function to fire when a particular prop (parentValue in my case) changes from its initial state. So, I created a ref that was based on its initial value:
const parentValueRef = useRef(parentValue);
and then included the following at the start of the useEffect fn:
if (parentValue === parentValueRef.current) return;
parentValueRef.current = parentValue;
(Basically, don't run the effect if parentValue hasn't changed. Update the ref if it has changed, ready for the next check, and continue to run the effect)
So, although other solutions suggested will solve the particular use-case you've provided, it will help in the long run to change how you think in relation to functional components.
Think of them as primarily rendering a component based on some props.
If you genuinely need some local state, then useState will provide that, but don't assume your problem will be solved by storing local state.
If you have some code that will alter your props during a render, this 'side-effect' needs to be wrapped in a useEffect, but the purpose of this is to have a clean render that isn't affected by something changing as it's rendering. The useEffect hook will be run after the render has completed and, as you've pointed out, it's run with every render - unless the second parameter is used to supply a list of props/states to identify what changed items will cause it to be run subsequent times.
Good luck on your journey to Functional Components / Hooks! Sometimes it's necessary to unlearn something to get to grips with a new way of doing things :)
This is an excellent primer: https://overreacted.io/a-complete-guide-to-useeffect/
Below solution is similar to above, just a little cleaner way i prefer.
const [isMount, setIsMount] = useState(true);
useEffect(()=>{
if(isMount){
setIsMount(false);
return;
}
//Do anything here for 2nd render onwards
}, [args])
You can use custom hook to run use effect after mount.
const useEffectAfterMount = (cb, dependencies) => {
const mounted = useRef(true);
useEffect(() => {
if (!mounted.current) {
return cb();
}
mounted.current = false;
}, dependencies); // eslint-disable-line react-hooks/exhaustive-deps
};
Here is the typescript version:
const useEffectAfterMount = (cb: EffectCallback, dependencies: DependencyList | undefined) => {
const mounted = useRef(true);
useEffect(() => {
if (!mounted.current) {
return cb();
}
mounted.current = false;
}, dependencies); // eslint-disable-line react-hooks/exhaustive-deps
};
Example:
useEffectAfterMount(() => {
document.title = `You clicked ${count} times`;
}, [count])