Imagine two components like this in React:
import MyComponent2 from "./components/MyComponent2";
import React from "react";
export default function App() {
const [myState, setMyState] = React.useState([]);
React.useEffect(() => {
console.log("useEffect triggered");
}, [myState]);
return <MyComponent2 myState={myState} setMyState={setMyState} />;
}
import React from "react";
export default function MyComponent2(props) {
const [inputValue, setInputValue] = React.useState("");
function handleChange(e) {
setInputValue(e.target.value);
let list = props.myState;
list.push(`${e.target.value}`);
props.setMyState(list);
console.log(props.myState);
}
return (
<div>
<input
type="text"
value={inputValue}
name="text"
onChange={handleChange}
/>
</div>
);
}
As you can see I am making changes with props.setMyState line in second component. State is changing but Somehow I could not trigger React.useEffect in first component even tough It is connected with [myState]. Why ?
In short form of my question : I can not get "useEffect triggered" on my console when i make changes in input
Instead of providing myState and setMyState to MyComponent2, you should only provide setMyState and use the functional update argument in order to access the current state.
In your handleChange function you are currently mutating the React state (modifying it directly):
let list = props.myState; // This is an array that is state managed by React
list.push(`${e.target.value}`); // Here, you mutate it by appending a new element
props.setMyState(list);
// ^ You update the state with the same array here,
// and since they have the same object identity (they are the same array),
// no update occurs in the parent component
Instead, you should set the state to a new array (whose object identity differs from the current array):
props.setMyState(list => {
const newList = [...list];
newList.push(e.target.value);
return newList;
});
// A concise way to write the above is like this:
// props.setMyState(list => [...list, e.target.value]);
I've been playing around with the new hook system in React 16.7-alpha and get stuck in an infinite loop in useEffect when the state I'm handling is an object or array.
First, I use useState and initiate it with an empty object like this:
const [obj, setObj] = useState({});
Then, in useEffect, I use setObj to set it to an empty object again. As a second argument I'm passing [obj], hoping that it wont update if the content of the object hasn't changed. But it keeps updating. I guess because no matter the content, these are always different objects making React thinking it keep changing?
useEffect(() => {
setIngredients({});
}, [ingredients]);
The same is true with arrays, but as a primitive it wont get stuck in a loop, as expected.
Using these new hooks, how should I handle objects and array when checking weather the content has changed or not?
Passing an empty array as the second argument to useEffect makes it only run on mount and unmount, thus stopping any infinite loops.
useEffect(() => {
setIngredients({});
}, []);
This was clarified to me in the blog post on React hooks at https://www.robinwieruch.de/react-hooks/
Had the same problem. I don't know why they not mention this in docs. Just want to add a little to Tobias Haugen answer.
To run in every component/parent rerender you need to use:
useEffect(() => {
// don't know where it can be used :/
})
To run anything only one time after component mount(will be rendered once) you need to use:
useEffect(() => {
// do anything only one time if you pass empty array []
// keep in mind, that component will be rendered one time (with default values) before we get here
}, [] )
To run anything one time on component mount and on data/data2 change:
const [data, setData] = useState(false)
const [data2, setData2] = useState('default value for first render')
useEffect(() => {
// if you pass some variable, than component will rerender after component mount one time and second time if this(in my case data or data2) is changed
// if your data is object and you want to trigger this when property of object changed, clone object like this let clone = JSON.parse(JSON.stringify(data)), change it clone.prop = 2 and setData(clone).
// if you do like this 'data.prop=2' without cloning useEffect will not be triggered, because link to data object in momory doesn't changed, even if object changed (as i understand this)
}, [data, data2] )
How i use it most of the time:
export default function Book({id}) {
const [book, bookSet] = useState(false)
const loadBookFromServer = useCallback(async () => {
let response = await fetch('api/book/' + id)
response = await response.json()
bookSet(response)
}, [id]) // every time id changed, new book will be loaded
useEffect(() => {
loadBookFromServer()
}, [loadBookFromServer]) // useEffect will run once and when id changes
if (!book) return false //first render, when useEffect did't triggered yet we will return false
return <div>{JSON.stringify(book)}</div>
}
I ran into the same problem too once and I fixed it by making sure I pass primitive values in the second argument [].
If you pass an object, React will store only the reference to the object and run the effect when the reference changes, which is usually every singe time (I don't now how though).
The solution is to pass the values in the object. You can try,
const obj = { keyA: 'a', keyB: 'b' }
useEffect(() => {
// do something
}, [Object.values(obj)]);
or
const obj = { keyA: 'a', keyB: 'b' }
useEffect(() => {
// do something
}, [obj.keyA, obj.keyB]);
If you are building a custom hook, you can sometimes cause an infinite loop with default as follows
function useMyBadHook(values = {}) {
useEffect(()=> {
/* This runs every render, if values is undefined */
},
[values]
)
}
The fix is to use the same object instead of creating a new one on every function call:
const defaultValues = {};
function useMyBadHook(values = defaultValues) {
useEffect(()=> {
/* This runs on first call and when values change */
},
[values]
)
}
If you are encountering this in your component code the loop may get fixed if you use defaultProps instead of ES6 default values
function MyComponent({values}) {
useEffect(()=> {
/* do stuff*/
},[values]
)
return null; /* stuff */
}
MyComponent.defaultProps = {
values = {}
}
Your infinite loop is due to circularity
useEffect(() => {
setIngredients({});
}, [ingredients]);
setIngredients({}); will change the value of ingredients(will return a new reference each time), which will run setIngredients({}). To solve this you can use either approach:
Pass a different second argument to useEffect
const timeToChangeIngrediants = .....
useEffect(() => {
setIngredients({});
}, [timeToChangeIngrediants ]);
setIngrediants will run when timeToChangeIngrediants has changed.
I'm not sure what use case justifies change ingrediants once it has been changed. But if it is the case, you pass Object.values(ingrediants) as a second argument to useEffect.
useEffect(() => {
setIngredients({});
}, Object.values(ingrediants));
As said in the documentation (https://reactjs.org/docs/hooks-effect.html), the useEffect hook is meant to be used when you want some code to be executed after every render. From the docs:
Does useEffect run after every render? Yes!
If you want to customize this, you can follow the instructions that appear later in the same page (https://reactjs.org/docs/hooks-effect.html#tip-optimizing-performance-by-skipping-effects). Basically, the useEffect method accepts a second argument, that React will examine to determine if the effect has to be triggered again or not.
useEffect(() => {
document.title = `You clicked ${count} times`;
}, [count]); // Only re-run the effect if count changes
You can pass any object as the second argument. If this object remains unchanged, your effect will only be triggered after the first mount. If the object changes, the effect will be triggered again.
I'm not sure if this will work for you but you could try adding .length like this:
useEffect(() => {
// fetch from server and set as obj
}, [obj.length]);
In my case (I was fetching an array!) it fetched data on mount, then again only on change and it didn't go into a loop.
If you include empty array at the end of useEffect:
useEffect(()=>{
setText(text);
},[])
It would run once.
If you include also parameter on array:
useEffect(()=>{
setText(text);
},[text])
It would run whenever text parameter change.
I often run into an infinite re-render when having a complex object as state and updating it from useRef:
const [ingredients, setIngredients] = useState({});
useEffect(() => {
setIngredients({
...ingredients,
newIngedient: { ... }
});
}, [ingredients]);
In this case eslint(react-hooks/exhaustive-deps) forces me (correctly) to add ingredients to the dependency array. However, this results in an infinite re-render. Unlike what some say in this thread, this is correct, and you can't get away with putting ingredients.someKey or ingredients.length into the dependency array.
The solution is that setters provide the old value that you can refer to. You should use this, rather than referring to ingredients directly:
const [ingredients, setIngredients] = useState({});
useEffect(() => {
setIngredients(oldIngedients => {
return {
...oldIngedients,
newIngedient: { ... }
}
});
}, []);
If you use this optimization, make sure the array includes all values from the component scope (such as props and state) that change over time and that are used by the effect.
I believe they are trying to express the possibility that one could be using stale data, and to be aware of this. It doesn't matter the type of values we send in the array for the second argument as long as we know that if any of those values change it will execute the effect. If we are using ingredients as part of the computation within the effect, we should include it in the array.
const [ingredients, setIngredients] = useState({});
// This will be an infinite loop, because by shallow comparison ingredients !== {}
useEffect(() => {
setIngredients({});
}, [ingredients]);
// If we need to update ingredients then we need to manually confirm
// that it is actually different by deep comparison.
useEffect(() => {
if (is(<similar_object>, ingredients) {
return;
}
setIngredients(<similar_object>);
}, [ingredients]);
The main problem is that useEffect compares the incoming value with the current value shallowly. This means that these two values compared using '===' comparison which only checks for object references and although array and object values are the same it treats them to be two different objects. I recommend you to check out my article about useEffect as a lifecycle methods.
The best way is to compare previous value with current value by using usePrevious() and _.isEqual() from Lodash.
Import isEqual and useRef. Compare your previous value with current value inside the useEffect(). If they are same do nothing else update. usePrevious(value) is a custom hook which create a ref with useRef().
Below is snippet of my code. I was facing problem of infinite loop with updating data using firebase hook
import React, { useState, useEffect, useRef } from 'react'
import 'firebase/database'
import { Redirect } from 'react-router-dom'
import { isEqual } from 'lodash'
import {
useUserStatistics
} from '../../hooks/firebase-hooks'
export function TMDPage({ match, history, location }) {
const usePrevious = value => {
const ref = useRef()
useEffect(() => {
ref.current = value
})
return ref.current
}
const userId = match.params ? match.params.id : ''
const teamId = location.state ? location.state.teamId : ''
const [userStatistics] = useUserStatistics(userId, teamId)
const previousUserStatistics = usePrevious(userStatistics)
useEffect(() => {
if (
!isEqual(userStatistics, previousUserStatistics)
) {
doSomething()
}
})
In case you DO need to compare the object and when it is updated here is a deepCompare hook for comparison. The accepted answer surely does not address that. Having an [] array is suitable if you need the effect to run only once when mounted.
Also, other voted answers only address a check for primitive types by doing obj.value or something similar to first get to the level where it is not nested. This may not be the best case for deeply nested objects.
So here is one that will work in all cases.
import { DependencyList } from "react";
const useDeepCompare = (
value: DependencyList | undefined
): DependencyList | undefined => {
const ref = useRef<DependencyList | undefined>();
if (!isEqual(ref.current, value)) {
ref.current = value;
}
return ref.current;
};
You can use the same in useEffect hook
React.useEffect(() => {
setState(state);
}, useDeepCompare([state]));
You could also destructure the object in the dependency array, meaning the state would only update when certain parts of the object updated.
For the sake of this example, let's say the ingredients contained carrots, we could pass that to the dependency, and only if carrots changed, would the state update.
You could then take this further and only update the number of carrots at certain points, thus controlling when the state would update and avoiding an infinite loop.
useEffect(() => {
setIngredients({});
}, [ingredients.carrots]);
An example of when something like this could be used is when a user logs into a website. When they log in, we could destructure the user object to extract their cookie and permission role, and update the state of the app accordingly.
my Case was special on encountering an infinite loop, the senario was like this:
I had an Object, lets say objX that comes from props and i was destructuring it in props like:
const { something: { somePropery } } = ObjX
and i used the somePropery as a dependency to my useEffect like:
useEffect(() => {
// ...
}, [somePropery])
and it caused me an infinite loop, i tried to handle this by passing the whole something as a dependency and it worked properly.
Another worked solution that I used for arrays state is:
useEffect(() => {
setIngredients(ingredients.length ? ingredients : null);
}, [ingredients]);
import React, {useState, useEffect} from 'react';
const Test = ( {numar}) => {
const [likeStatus, setLikeStatus] = useState(true);
const [likeNumber, setLikeNumber] = useState(100);
const onLikeHandler = () => {
setLikeStatus(prevState => !prevState);
if(likeStatus){
setLikeNumber(prevState=> prevState +1)
} else {
setLikeNumber(prevState=>prevState-1);
}
}
console.log(likeStatus);
console.log(likeNumber);
return <button className={`like ${likeStatus ? 'liked' : ""}`} onClick={onLikeHandler}>{`Like | ${ likeNumber}`}</button>
}
export default Test;
I am trying to make a like button that likes/unlikes based on the click.
How can I make the second change state function wait for my first state function to finish? I tried using an use effect hook, and I am using the likeStatus in the dependecy array, but for some reason "the unlike" function triggers twice upon refresh"
State update is async, and state is being accessed from closure which recreates only after rerender - meaning you can not set state and then in next line to expect that you will get that new state immediately, you will get new state but only in next function call(after element rerenders). Solution is to preserve new state in temp variable.
Rewrite to this:
const onLikeHandler = () => {
const newLikeStatus = !likeStatus; // Here you are preserving that new state in temp variable
setLikeStatus(newLikeStatus);
if(newLikeStatus){
setLikeNumber(prevState=> prevState +1)
} else {
setLikeNumber(prevState=>prevState-1);
}
}
I know how to use prevState in new State, I want to know how does it actually works ?? How does this functionality works ?? why on putting in 'prevValue' in 'setFullName' function changes its value ?
import React, { useState } from "react";
function App() {
const [fullName, setFullName] = useState({
firstName: "",
lastName: ""
});
function handleChange(event) {
let value = event.target.value;
let name = event.target.name;
setFullName((prevValue) => {
if (name === "fName") {
return {
firstName: value,
lastName: prevValue.lastName
};
} else if (name === "lName") {
return {
firstName: prevValue.firstName,
lastName: value
};
}
});
}
return (
<div className="container">
<h1>
{fullName.firstName} {fullName.lastName}
</h1>
<form>
<input onChange={handleChange} name="fName" placeholder="First Name" />
<input onChange={handleChange} name="lName" placeholder="Last Name" />
<button>Submit</button>
</form>
</div>
);
}
export default App;
When a component is created (with React.createElement, or with JSX like <App />), React will see how many times useState is called in the component body, and will create an internal mapping of state indicies to state values. For example, with the following component:
const Comp = () => {
const [v1, setV1] = useState('a');
const [v2, setV12] = useState('b');
return <div>Text</div>;
};
When called, React will, internally, now have something like the following:
[ // array of all stateful values for this rendered component
'a', // corresponds to the stateful value of the first useState
'b', // corresponds to the stateful value of the second useState
]
These internal state values are changed when the state setter is called (eg setV1), and then when the component gets re-rendered, the new stateful values (coming from React internals) are then returned by useState.
When the state setter is passed a callback, eg, with your
setFullName((prevValue) => {
All that's really needed for this to work is for React to pass the current corresponding state value in React's internals as prevValue.
When state is set, the corresponding value in React's internals gets updated, but there may be other synchronous code that runs before the component gets re-rendered, and the new values (coming from React internals) get returned by the next invocations of useState. That's why using the callback form of the state setter sometimes gives different results - the callback argument always comes from React's internals, but the state value returned by useState only refers to the state at the time the component was rendered.
For example
const [count, setCount] = useState(0);
const clickHandler = () => {
setCount(count + 1); // sets the state in React's internals to 1
setCount(count + 1); // sets the state in React's internals to 1, not 2;
// because `count` still refers to the initial value of 0
};
const [count, setCount] = useState(0);
const clickHandler = () => {
setCount(count + 1); // sets the state in React's internals to 1
setCount(prevCount => prevCount + 1); // sets the state in React's internals to 2
// because `prevCount` comes directly from React's internals
};
But, for your code here, because multiple state changes are not occurring in the same render, there's no need to use the callback form - feel free to use the outside fullName identifier instead of using the prevValue and a callback.
I needed a hook to get the previous distinct value of a specific state. It looks like this and it seems to work:
function usePreviousDistinct(state) {
const prevRef = useRef();
useEffect(() => {
prevRef.current = state;
}, [state]);
return prevRef.current;
}
I've also seen there is a usePreviousDistinct hook in the react-use package but the approach is different than mine.
import { useRef } from 'react';
import { useFirstMountState } from './useFirstMountState';
export type Predicate<T> = (prev: T | undefined, next: T) => boolean;
const strictEquals = <T>(prev: T | undefined, next: T) => prev === next;
export default function usePreviousDistinct<T>(value: T, compare: Predicate<T> = strictEquals): T | undefined {
const prevRef = useRef<T>();
const curRef = useRef<T>(value);
const isFirstMount = useFirstMountState();
if (!isFirstMount && !compare(curRef.current, value)) {
prevRef.current = curRef.current;
curRef.current = value;
}
return prevRef.current;
}
I wonder if I have not understood something or am missing something. Is my version also correct?
In my test I could not find a difference:
https://codesandbox.io/s/distracted-mayer-zpym8?file=/src/App.js
useEffect() together with useRef() (your version) does not show the latest value.
useRef() without useEffect() gives you the correct value, because it runs synchronously, but doesn't have the advantages that come with asynchronicity.
useEffect() together with useState() gives you the correct value, but might trigger unnecessary renders (adds potentially unnecessary overhead).
Your version looks like it works as expected, because the old value that is shown is the one that you expect to see as the new value. But the actual new value is not the one you want.
Example:
import React, { useState, useEffect, useRef } from 'react';
export const MyComponent = function(props){
const [state, setState ] = useState(0);
return <React.Fragment>
state: { state },<br />
with useEffect: { usePreviousDistinctUE( state ) },<br />
w/o useEffect: { usePreviousDistinctR( state ) },<br />
<button onClick={ function(){
setState( state + 1 );
} }>
increment
</button>
</React.Fragment>;
};
const usePreviousDistinctUE = function( value ){
const prevRef = useRef();
useEffect(() => {
prevRef.current = value;
console.log('with useEffect, prev:', prevRef.current, ', current:', value);
}, [value]);
return prevRef.current;
};
const usePreviousDistinctR = function( value ){
const prevRef = useRef();
const curRef = useRef( value );
if( curRef.current !== value ){
prevRef.current = curRef.current;
curRef.current = value;
}
console.log('w/o useEffect, prev:', prevRef.current, ', current:', curRef.current);
return prevRef.current;
};
The values shown on the page are the same, but in the console they are different. That means the value in the useEffect() version is changed, it is only not yet shown on the page.
If you just add another hook that updates anything unrelated (leaving everything else unchanged), then the page (might*) magically show the updated value again, because the page is re-rendered and the previously already changed value is shown. The value is now wrong in your eyes, but it is not changed, only shown:
// ...
with useEffect: { usePreviousDistinctUE( state ) },<br />
w/o useEffect: { usePreviousDistinctR( state ) },<br />
anything updated: { useAnythingUpdating( state ) },<br />
// ...
const useAnythingUpdating = function(state){
const [result, setResult ] = useState(0);
useEffect(() => {
setResult( state );
console.log('anything updated');
});
return result;
};
*But you shouldn't rely on something else triggering a re-render. I'm not even sure this would update as expected under all circumstances.
more Details:
useEffect() is triggered at some time when react decides that the prop must have been changed. The ref is changed then, but react will not 'get informed' about a ref change, so it doesn't find it necessary to re-render the page to show the changed value.
In the example without useEffect() the change happens synchronously.
React doesn't 'know' about this change either, but (if everything else runs as expected) there will be always a re-render when necessary anyway (you will have called that function from another function that is rendered at the end).
(Not informing react about the change is basically the point in using useRef(): sometimes you want just a value, under your own control, without react doing magic things with it.)