I'm very confused, I have parent and a child component. What It's doing is if condition is matched in a child component, then it add's child component to array. When there is transitionis in child component it calls a deleteMe function in parent component. Real problem what I deal with is a line deleteMe function, line where I have console.log. This line ALWAYS prints me that there is [{...}] one element in array. While useEffect hook shows me that updating an array happens just fine.
Why when I call same variable it gives me different results from deleteMe func and usEffect ?
import { FC, useRef, useState, ReactElement, useEffect} from "react"
import "./marquee.scss"
import MarqueeItem from "./Item/Item"
const Marquee: FC <{text: string}> = ({ text }) => {
let cnt = useRef<number>(0)
const [marquees, setMarquees] = useState< ReactElement[]>(
[<MarqueeItem
key={cnt.current}
launchNext={() => launchNext()}
deleteMe={() => deleteMe()}
text={text}/>]
)
const deleteMe = () => {
console.log(marquees, "inside deleteMe")
if(marquees.length > 5) { // Here I have an issue. Cause "marquees" is ALWAYS 1 element !
console.log("More then 5")
}
}
const launchNext = () => {
cnt.current = cnt.current + 1;
setMarquees(oldArr => [...oldArr, <MarqueeItem key={cnt.current} deleteMe={() => deleteMe()} launchNext={() => launchNext()} text={text}/>])
}
useEffect(() => {
console.log(marquees, "useEffect")
})
return <div className="marquee">
{ marquees }
</div>
}
export default Marquee
Child component :
const MarqueeItem: FC<IProps> = ({text, launchNext, deleteMe }) => {
let observed = useRef<HTMLParagraphElement>(null)
const isReady = <T extends number>(currLeft: T, header_width: T , p_width: T): boolean => {
return currLeft < 0 && (p_width + currLeft) < header_width ? true : false
}
useEffect(() => {
const marquee = observed.current!
const transitionendListener = () => deleteMe()
marquee.addEventListener("transitionend", transitionendListener) // calling deleteMe when transition is over
const parantNode = marquee.parentNode as Element
const header_width = parseInt(getComputedStyle(parantNode).width)
const p_width = parseInt(getComputedStyle(marquee).width)
marquee.style.left = -p_width + "px"
const intervalRead = setInterval(() => {
let currLeft = parseInt(getComputedStyle(marquee).left)
if(isReady(currLeft, header_width, p_width)) {
launchNext() // adding a new element to array in parent when it's good condition to add
clearInterval(intervalRead)
}
}, 400)
return () => {
marquee.removeEventListener("transitionend", transitionendListener)
}
}, [observed, launchNext, deleteMe])
return <p ref={observed} className="marquee__text">{text}</p>
}
export default MarqueeItem
According to the docs:
componentDidUpdate() is invoked immediately after updating occurs. This method is not called for the initial render.
We can use the new useEffect() hook to simulate componentDidUpdate(), but it seems like useEffect() is being ran after every render, even the first time. How do I get it to not run on initial render?
As you can see in the example below, componentDidUpdateFunction is printed during the initial render but componentDidUpdateClass was not printed during the initial render.
function ComponentDidUpdateFunction() {
const [count, setCount] = React.useState(0);
React.useEffect(() => {
console.log("componentDidUpdateFunction");
});
return (
<div>
<p>componentDidUpdateFunction: {count} times</p>
<button
onClick={() => {
setCount(count + 1);
}}
>
Click Me
</button>
</div>
);
}
class ComponentDidUpdateClass extends React.Component {
constructor(props) {
super(props);
this.state = {
count: 0,
};
}
componentDidUpdate() {
console.log("componentDidUpdateClass");
}
render() {
return (
<div>
<p>componentDidUpdateClass: {this.state.count} times</p>
<button
onClick={() => {
this.setState({ count: this.state.count + 1 });
}}
>
Click Me
</button>
</div>
);
}
}
ReactDOM.render(
<div>
<ComponentDidUpdateFunction />
<ComponentDidUpdateClass />
</div>,
document.querySelector("#app")
);
<script src="https://unpkg.com/react#16.7.0-alpha.0/umd/react.development.js"></script>
<script src="https://unpkg.com/react-dom#16.7.0-alpha.0/umd/react-dom.development.js"></script>
<div id="app"></div>
We can use the useRef hook to store any mutable value we like, so we could use that to keep track of if it's the first time the useEffect function is being run.
If we want the effect to run in the same phase that componentDidUpdate does, we can use useLayoutEffect instead.
Example
const { useState, useRef, useLayoutEffect } = React;
function ComponentDidUpdateFunction() {
const [count, setCount] = useState(0);
const firstUpdate = useRef(true);
useLayoutEffect(() => {
if (firstUpdate.current) {
firstUpdate.current = false;
return;
}
console.log("componentDidUpdateFunction");
});
return (
<div>
<p>componentDidUpdateFunction: {count} times</p>
<button
onClick={() => {
setCount(count + 1);
}}
>
Click Me
</button>
</div>
);
}
ReactDOM.render(
<ComponentDidUpdateFunction />,
document.getElementById("app")
);
<script src="https://unpkg.com/react#16.7.0-alpha.0/umd/react.development.js"></script>
<script src="https://unpkg.com/react-dom#16.7.0-alpha.0/umd/react-dom.development.js"></script>
<div id="app"></div>
You can turn it into custom hooks, like so:
import React, { useEffect, useRef } from 'react';
const useDidMountEffect = (func, deps) => {
const didMount = useRef(false);
useEffect(() => {
if (didMount.current) func();
else didMount.current = true;
}, deps);
}
export default useDidMountEffect;
Usage example:
import React, { useState, useEffect } from 'react';
import useDidMountEffect from '../path/to/useDidMountEffect';
const MyComponent = (props) => {
const [state, setState] = useState({
key: false
});
useEffect(() => {
// you know what is this, don't you?
}, []);
useDidMountEffect(() => {
// react please run me if 'key' changes, but not on initial render
}, [state.key]);
return (
<div>
...
</div>
);
}
// ...
I made a simple useFirstRender hook to handle cases like focussing a form input:
import { useRef, useEffect } from 'react';
export function useFirstRender() {
const firstRender = useRef(true);
useEffect(() => {
firstRender.current = false;
}, []);
return firstRender.current;
}
It starts out as true, then switches to false in the useEffect, which only runs once, and never again.
In your component, use it:
const firstRender = useFirstRender();
const phoneNumberRef = useRef(null);
useEffect(() => {
if (firstRender || errors.phoneNumber) {
phoneNumberRef.current.focus();
}
}, [firstRender, errors.phoneNumber]);
For your case, you would just use if (!firstRender) { ....
Same approach as Tholle's answer, but using useState instead of useRef.
const [skipCount, setSkipCount] = useState(true);
...
useEffect(() => {
if (skipCount) setSkipCount(false);
if (!skipCount) runYourFunction();
}, [dependencies])
EDIT
While this also works, it involves updating state which will cause your component to re-render. If all your component's useEffect calls (and also all of its children's) have a dependency array, this doesn't matter. But keep in mind that any useEffect without a dependency array (useEffect(() => {...}) will be run again.
Using and updating useRef will not cause any re-renders.
#ravi, yours doesn't call the passed-in unmount function. Here's a version that's a little more complete:
/**
* Identical to React.useEffect, except that it never runs on mount. This is
* the equivalent of the componentDidUpdate lifecycle function.
*
* #param {function:function} effect - A useEffect effect.
* #param {array} [dependencies] - useEffect dependency list.
*/
export const useEffectExceptOnMount = (effect, dependencies) => {
const mounted = React.useRef(false);
React.useEffect(() => {
if (mounted.current) {
const unmount = effect();
return () => unmount && unmount();
} else {
mounted.current = true;
}
}, dependencies);
// Reset on unmount for the next mount.
React.useEffect(() => {
return () => mounted.current = false;
}, []);
};
a simple way is to create a let, out of your component and set in to true.
then say if its true set it to false then return (stop) the useEffect function
like that:
import { useEffect} from 'react';
//your let must be out of component to avoid re-evaluation
let isFirst = true
function App() {
useEffect(() => {
if(isFirst){
isFirst = false
return
}
//your code that don't want to execute at first time
},[])
return (
<div>
<p>its simple huh...</p>
</div>
);
}
its Similar to #Carmine Tambasciabs solution but without using state :)
function useEffectAfterMount(effect, deps) {
const isMounted = useRef(false);
useEffect(() => {
if (isMounted.current) return effect();
else isMounted.current = true;
}, deps);
// reset on unmount; in React 18, components can mount again
useEffect(() => {
isMounted.current = false;
});
}
We need to return what comes back from effect(), because it might be a cleanup function. But we don't need to determine if it is or not. Just pass it on and let useEffect figure it out.
In an earlier version of this post I said resetting the ref (isMounted.current = false) wasn't necessary. But in React 18 it is, because components can remount with their previous state (thanks #Whatabrain).
I thought creating a custom hook would be overkill and I didn't want to muddle my component's readability by using the useLayoutEffect hook for something unrelated to layouts, so, in my case, I simply checked to see if the value of my stateful variable selectedItem that triggers the useEffect callback is its original value in order to determine if it's the initial render:
export default function MyComponent(props) {
const [selectedItem, setSelectedItem] = useState(null);
useEffect(() => {
if(!selectedItem) return; // If selected item is its initial value (null), don't continue
//... This will not happen on initial render
}, [selectedItem]);
// ...
}
This is the best implementation I've created so far using typescript. Basically, the idea is the same, using the Ref but I'm also considering the callback returned by useEffect to perform cleanup on component unmount.
import {
useRef,
EffectCallback,
DependencyList,
useEffect
} from 'react';
/**
* #param effect
* #param dependencies
*
*/
export default function useNoInitialEffect(
effect: EffectCallback,
dependencies?: DependencyList
) {
//Preserving the true by default as initial render cycle
const initialRender = useRef(true);
useEffect(() => {
let effectReturns: void | (() => void) = () => {};
// Updating the ref to false on the first render, causing
// subsequent render to execute the effect
if (initialRender.current) {
initialRender.current = false;
} else {
effectReturns = effect();
}
// Preserving and allowing the Destructor returned by the effect
// to execute on component unmount and perform cleanup if
// required.
if (effectReturns && typeof effectReturns === 'function') {
return effectReturns;
}
return undefined;
}, dependencies);
}
You can simply use it, as usual as you use the useEffect hook but this time, it won't run on the initial render. Here is how you can use this hook.
useNoInitialEffect(() => {
// perform something, returning callback is supported
}, [a, b]);
If you use ESLint and want to use the react-hooks/exhaustive-deps rule for this custom hook:
{
"rules": {
// ...
"react-hooks/exhaustive-deps": ["warn", {
"additionalHooks": "useNoInitialEffect"
}]
}
}
#MehdiDehghani, your solution work perfectly fine, one addition you have to do is on unmount, reset the didMount.current value to false. When to try to use this custom hook somewhere else, you don't get cache value.
import React, { useEffect, useRef } from 'react';
const useDidMountEffect = (func, deps) => {
const didMount = useRef(false);
useEffect(() => {
let unmount;
if (didMount.current) unmount = func();
else didMount.current = true;
return () => {
didMount.current = false;
unmount && unmount();
}
}, deps);
}
export default useDidMountEffect;
Simplified implementation
import { useRef, useEffect } from 'react';
function MyComp(props) {
const firstRender = useRef(true);
useEffect(() => {
if (firstRender.current) {
firstRender.current = false;
} else {
myProp = 'some val';
};
}, [props.myProp])
return (
<div>
...
</div>
)
}
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
};
For people who are having trouble with React 18 strict mode calling the useeffect on the initial render twice, try this:
// The init variable is necessary if your state is an object/array, because the == operator compares the references, not the actual values.
const init = [];
const [state, setState] = useState(init);
const dummyState = useRef(init);
useEffect(() => {
// Compare the old state with the new state
if (dummyState.current == state) {
// This means that the component is mounting
} else {
// This means that the component updated.
dummyState.current = state;
}
}, [state]);
Works in development mode...
function App() {
const init = [];
const [state, setState] = React.useState(init);
const dummyState = React.useRef(init);
React.useEffect(() => {
if (dummyState.current == state) {
console.log('mount');
} else {
console.log('update');
dummyState.current = state;
}
}, [state]);
return (
<button onClick={() => setState([...state, Math.random()])}>Update state </button>
);
}
ReactDOM.createRoot(document.getElementById("app")).render(
<React.StrictMode>
<App />
</React.StrictMode>
);
<script crossorigin src="https://unpkg.com/react#18/umd/react.development.js"></script>
<script crossorigin src="https://unpkg.com/react-dom#18/umd/react-dom.development.js"></script>
<div id="app"></div>
And in production.
function App() {
const init = [];
const [state, setState] = React.useState(init);
const dummyState = React.useRef(init);
React.useEffect(() => {
if (dummyState.current == state) {
console.log('mount');
} else {
console.log('update');
dummyState.current = state;
}
}, [state]);
return (
<button onClick={() => setState([...state, Math.random()])}>Update state </button>
);
}
ReactDOM.createRoot(document.getElementById("app")).render(
<React.StrictMode>
<App />
</React.StrictMode>
);
<script crossorigin src="https://unpkg.com/react#18/umd/react.production.min.js"></script>
<script crossorigin src="https://unpkg.com/react-dom#18/umd/react-dom.production.min.js"></script>
<div id="app"></div>
If you want to skip the first render, you can create a state "firstRenderDone" and set it to true in the useEffect with empty dependecy list (that works like a didMount). Then, in your other useEffect, you can check if the first render was already done before doing something.
const [firstRenderDone, setFirstRenderDone] = useState(false);
//useEffect with empty dependecy list (that works like a componentDidMount)
useEffect(() => {
setFirstRenderDone(true);
}, []);
// your other useEffect (that works as componetDidUpdate)
useEffect(() => {
if(firstRenderDone){
console.log("componentDidUpdateFunction");
}
}, [firstRenderDone]);
All previous are good, but this can be achieved in a simplier way considering that the action in useEffect can be "skipped" placing an if condition(or any other ) that is basically not run first time, and still with the dependency.
For example I had the case of :
Load data from an API but my title has to be "Loading" till the date were not there, so I have an array, tours that is empty at beginning and show the text "Showing"
Have a component rendered with different information from those API.
The user can delete one by one those info, even all making the tour array empty again as the beginning but this time the API fetch is been already done
Once the tour list is empty by deleting then show another title.
so my "solution" was to create another useState to create a boolean value that change only after the data fetch making another condition in useEffect true in order to run another function that also depend on the tour length.
useEffect(() => {
if (isTitle) {
changeTitle(newTitle)
}else{
isSetTitle(true)
}
}, [tours])
here my App.js
import React, { useState, useEffect } from 'react'
import Loading from './Loading'
import Tours from './Tours'
const url = 'API url'
let newTours
function App() {
const [loading, setLoading ] = useState(true)
const [tours, setTours] = useState([])
const [isTitle, isSetTitle] = useState(false)
const [title, setTitle] = useState("Our Tours")
const newTitle = "Tours are empty"
const removeTours = (id) => {
newTours = tours.filter(tour => ( tour.id !== id))
return setTours(newTours)
}
const changeTitle = (title) =>{
if(tours.length === 0 && loading === false){
setTitle(title)
}
}
const fetchTours = async () => {
setLoading(true)
try {
const response = await fetch(url)
const tours = await response.json()
setLoading(false)
setTours(tours)
}catch(error) {
setLoading(false)
console.log(error)
}
}
useEffect(()=>{
fetchTours()
},[])
useEffect(() => {
if (isTitle) {
changeTitle(newTitle)
}else{
isSetTitle(true)
}
}, [tours])
if(loading){
return (
<main>
<Loading />
</main>
)
}else{
return (
<main>
<Tours tours={tours} title={title} changeTitle={changeTitle}
removeTours={removeTours} />
</main>
)
}
}
export default App
const [dojob, setDojob] = useState(false);
yourfunction(){
setDojob(true);
}
useEffect(()=>{
if(dojob){
yourfunction();
setDojob(false);
}
},[dojob]);
I'm trying to create a functional component that fetches data from an API and renders it to a list. After the data is fetched and rendered I want to check if the URL id and list item is equal, if they are then the list item should be scrolled into view.
Below is my code:
import React, { Fragment, useState, useEffect, useRef } from "react";
export default function ListComponent(props) {
const scrollTarget = useRef();
const [items, setItems] = useState([]);
const [scrollTargetItemId, setScrollTargetItemId] = useState("");
useEffect(() => {
const fetchData = async () => {
let response = await fetch("someurl").then((res) => res.json());
setItems(response);
};
fetchData();
if (props.targetId) {
setScrollTargetItemId(props.targetId)
}
if (scrollTarget.current) {
window.scrollTo(0, scrollTarget.current.offsetTop)
}
}, [props]);
let itemsToRender = [];
itemsToRender = reports.map((report) => {
return (
<li
key={report._id}
ref={item._id === scrollTargetItemId ? scrollTarget : null}
>
{item.payload}
</li>
);
});
return (
<Fragment>
<ul>{itemsToRender}</ul>
</Fragment>
);
}
My problem here is that scrollTarget.current is always undefined. Please advice what I'm doing wrong. Thanks in advance.
Using useCallback, as #yagiro suggested, did the trick!
My code ended up like this:
const scroll = useCallback(node => {
if (node !== null) {
window.scrollTo({
top: node.getBoundingClientRect().top,
behavior: "smooth"
})
}
}, []);
And then I just conditionally set the ref={scroll} on the node that you want to scroll to.
That is because when a reference is changed, it does not cause a re-render.
From React's docs: https://reactjs.org/docs/hooks-reference.html#useref
Keep in mind that useRef doesn’t notify you when its content changes. Mutating the .current property doesn’t cause a re-render. If you want to run some code when React attaches or detaches a ref to a DOM node, you may want to use a callback ref instead.
constructor(props) {
thi.modal = React.createRef();
}
handleSwitch() {
// debugger
this.setState({ errors: [] }, function () {
this.modal.current.openModal('signup') // it will call function of child component of Modal
});
// debugger
}
return(
<>
<button className="login-button" onClick={this.handleSwitch}>Log in with email</button>
<Modal ref={this.modal} />
</>
)
React Hooks will delay the scrolling until the page is ready:
useEffect(() => {
const element = document.getElementById('id')
if (element)
element.scrollIntoView({ behavior: 'smooth' })
}, [])
If the element is dynamic and based on a variable, add them to the Effect hook:
const [variable, setVariable] = useState()
const id = 'id'
useEffect(() => {
const element = document.getElementById(id)
if (element)
element.scrollIntoView({ behavior: 'smooth' })
}, [variable])
I am trying to build Hanging man game and want to get value from useState inside the checkMatchLetter function, but not sure if that is possible and what I did wrong....
import React, { useState, useEffect } from 'react';
import { fetchButton } from '../actions';
import axios from 'axios';
import 'babel-polyfill';
const App = () => {
const [word, setWord] = useState([]);
const [underscore, setUnderscore] = useState([]);
const [data, setData] = useState([]);
useEffect(() => {
const runEffect = async () => {
const result = await axios('src/api/api.js');
setData(result.data)
}
runEffect();
}, []);
const randomWord = () => {
const chosenWord = data[Math.floor(Math.random() * data.length)];
replaceLetter(chosenWord.word);
}
const replaceLetter = (string) => {
let getString = string; // here it shows a valid string.
setWord(getString);
let stringToUnderScore = getString.replace(/[a-z]/gi, '_');
setUnderscore(stringToUnderScore);
}
useEffect(() => {
const checkLetter = (event) => {
if(event.keyCode >= 65 && event.keyCode <= 90) {
checkMatchLetter(word, String.fromCharCode(event.keyCode).toLowerCase());
}
};
document.addEventListener('keydown', checkLetter);
return () => {
document.removeEventListener('keydown', checkLetter);
}
}, []);
const checkMatchLetter = (keyButton) => {
console.log(keyButton);
let wordLength = word.length;
console.log(wordLength); // here it outputs '0'
/// here I want word of useState here....
}
return (
<div>
<p>{word}</p>
<p>{underscore}</p>
<button onClick={randomWord}></button>
</div>
)
}
export default App;
The reason why I want to obtain that value inside this function is so I can compare the clicked keybutton (a-z) to the current chosenword. And if there is something wrong with other functions, please feel free to share your feedback here below as well.
You're using a variable defined inside the component render function in a useEffect effect and that variable is missing in the hook's deps. Always include the deps you need (I highly recommend the lint rule react-hooks/exhaustive-deps). When you add checkMatchLetter to deps you'll always have the newest instance of the function inside your effect instead of always using the old version from the first render like you do now.
useEffect(() => {
const checkLetter = (event) => {
if(event.keyCode >= 65 && event.keyCode <= 90) {
checkMatchLetter(word, String.fromCharCode(event.keyCode).toLowerCase());
}
};
document.addEventListener('keydown', checkLetter);
return () => {
document.removeEventListener('keydown', checkLetter);
}
}, [checkMatchLetter, word]);
This change will make the effect run on every render. To rectify that, you can memoise your callbacks. However, that's a new can of worms.