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])
I am looking to stop an interval function when a user on my website changes the page. I searched online for "how to tell if a react element is visible on the screen," but I can only find results for intersection observers. Is there a way to stop an interval function when a react component dismounts? (ps. I believe dismounting is when the component is no longer rendered?)
Heres my component interval:
useEffect(() => {
const updatePostInfo =
inView &&
setInterval(() => {
dispatch(getPostInfo(data._id));
}, 3500);
!inView && clearInterval(updatePostInfo);
}, [inView]);
Currently, I have it set up with an intersection observer so that it doesn't dispatch the function when it isn't visible. But the interval still runs when the page changes?
You need to return a cleanup function from your effect:
useEffect(() => {
const updatePostInfo = setInterval(() => {
dispatch(getPostInfo(data._id));
}, 3500);
return () => clearInterval(updatePostInfo);
}, []);
I'm currently running into some issues whilst developing a Typescript React App.
Underneath is my current code..
But it's not behaving like I would want it to behave. :)
So what I would like to achieve is that the data with getData(depth) runs whenever the component is being loaded and afterwards every 5 seconds.
But when the Depth changes with the Dropdown.item buttons, it should re-render and the getData() should be ran with the new depth value that we just set in the state.. and keep on rendering afterwards with the new value...
I've been struggling with this, so any help is very much appreciated!!
Thank you!
import React, { useState, useEffect } from "react";
const chart = () => {
const [depth, setDepth] = useState(20);
const [chartData, setChartData] = useState({})
//Getting the data when the app initially renders and should keep rendering every 5 seconds after that.
//When the value of the depth changes, we should stop getting the data with the old depth //value and should start a new interval of 5 seconds and just keep running with the new //depth value
//When first entering the app, this should run immediately with the initial depth state //(20)
useEffect(() => {
const interval = setInterval(() => {
//this code is not the actual code, just an example of what is running
const data = getData(depth)
//just fetched the new data, now setting it..
setChartData(data)
}, 5000);
return () => clearInterval(interval);
}, []);
return (
<div>
<div>
<DropdownButton id="dropdown-basic-button" title="Depth Percentage">
<Dropdown.Item onClick={() => setDepth(5)}>5%</Dropdown.Item>
<Dropdown.Item onClick={() => setDepth(20)}>20%</Dropdown.Item>
</DropdownButton>
</div>
<div>
//Rendering the Chart here....
</div>
</div>
);
};
export default chart;
That's because useEffect hook take a second params called dependency array, where this dependency array is what matter for the inner callback(inisde useEffect) to access the latest values you want.
So your are not being totally truthful here, if the inner callback depends on depth to be in its latest update then you should include it in the dependency array
useEffect(() => { ... }, [ depth ]);
that's for the depth but writing this code will immediately cause problems because for each new depth value the inner callback will be called and the setInterval will re-run again (causing many many...many of intervals).
To solve this you should avoid using setInterval alll together in hooks based code.
If having interval is really important I have a suggestion for you
const [intervalCount, setIntervalCount] = useState(0);
const [depth, setDepth] = useState(20);
const [chartData, setChartData] = useState({})
useEffect(() => {
// when depth change re-fetch data and set it
const data: any = getData(depth);
setChartData(data);
}, [depth])
// simulate set interval behavior
// each 5 s this function will be re-invoked
useEffect(() => {
// re-fetch data and set it
const data: any = getData(depth);
setChartData(data);
// wait 5 s before cause a re-render
setTimeout(() => {
setIntervalCount(count => count + 1);
}, 5000);
}, [intervalCount]);
Updated: After rading from Dan Abramov blog
you can find a better elegant solution that use setInterval and hooks
Making setInterval Declarative with React Hooks
He made a custom hook called useInterval
function useInterval(callback, delay) {
const savedCallback = useRef();
// Remember the latest callback.
useEffect(() => {
savedCallback.current = callback;
}, [callback]);
// Set up the interval.
useEffect(() => {
function tick() {
savedCallback.current();
}
if (delay !== null) {
let id = setInterval(tick, delay);
return () => clearInterval(id);
}
}, [delay]);
}
Usage be like
useInterval(() => {
// Your custom logic here
setCount(count + 1);
}, 1000);
I am trying to apply a parallax effect to an .svg image by using useRef() to grab bubblesRef and translateY() onScroll.
The parallax works but when I navigate to the next page I receive error "TypeError: Cannot read property 'style' of null". I think it is because the addEventListener is still listening and trying to useRef() on bubblesRef while navigating to the next page. So I added the cleanup function in useEffect() but that doesn't seem to fix it.
Any help is appreciated. Thanks!
p.s. If anyone can share their approach to a simple parallax effect like this that would be great too. This is the only approach I've figured that won't rerender everything else on the page onScroll.
const HomePage = () => {
const [loadedPosts, setLoadedPosts] = useState([]);
const { sendRequest } = useHttpClient();
console.log("loadedPosts homePage", loadedPosts);
const bubblesRef = useRef();
useEffect(() => {
if (loadedPosts.length === 0) {
//api call
}
}, [sendRequest, loadedPosts]);
useEffect(() => {
const parallax = () => {
let scrolledValue = window.scrollY / 3.5;
bubblesRef.current.style.transform = `translateY(
-${scrolledValue + "px"}
)`;
console.log("scrolling...", scrolledValue);
};
window.addEventListener("scroll", parallax);
return () => window.removeEventListener("scroll", parallax);
}, []);
return (
<HomePageContainer>
<Header />
<SectionOne posts={loadedPosts} />
<SectionTwo />
<BubbleBlobs className="bubbleBlobs" ref={bubblesRef} />
<BlobTop className="backBlobBottom" preserveAspectRatio="none" />
</HomePageContainer>
);
};
export default HomePage;
You definitely need the cleanup function any time you add a listener to the window, or the handler (and thus the component instance itself) will live on forever. However, since React runs those cleanup hooks asynchronously, it might not happen until after other window events. The value of the ref is set to null when the component unmounts, so you need to check that it is still defined before using the value.
useEffect(() => {
const handler = () => {
if (ref.current) {
// perform update
}
}
window.addEventListener('scroll', handler)
return () => window.removeEventListener('scroll', handler)
}, [])
When you call useEffect, your reference has not been instantiated, so the error message appears, in your useEffect dependency array, insert your ref and before running the code in useEffect, make sure your current reference is defined.
I am writing a react application in which I need to set the zoom level of a particular page to 90%. I know that I can do it using document.body.style.zoom = '90%' as given below:
useEffect(() => {
document.body.style.zoom = "90%";
}, []);
It's a basic componentDidMount function in react using useEffect. But the problem is how to set the zoom level back to the default as it was before loading the page using componentDidUnmount?
Add cleanup effect as callback returned from useEffect.
The clean-up function runs before the component is removed from the UI to prevent memory leaks.
const Zoom = () => {
useEffect(() => {
const initialValue = document.body.style.zoom;
// Change zoom level on mount
document.body.style.zoom = "150%";
return () => {
// Restore default value
document.body.style.zoom = initialValue;
};
}, []);
return <></>;
};
Such callback acts like componentWillUnmount.