Infinity Loop with promises and state variables in ReactJs - javascript

My react component is rendering multiple times in infinity loop. What am I missing?
Here is my code.
const DocViewer = ({ title, closeModal }) => {
const [docsSimilares, setDocsSimilares] = useState([]);
const baseUrl = '/docs'
async function similares() {
return await axios.get(`${baseUrl}/${title}`).then(data => setDocsSimilares(data.data.body.hits.hits[0]._source.documentos_similares))
}
similares().then(console.log(docsSimilares))
return (
<div class="row">
<div class="column pdf">
<h1>{title}</h1>
<PDFViewer url={sFileNameDemo} />
</div>
<div class="column semelhantes">
<button onClick={closeModal} >Fechar</button>
<p>{docsSimilares.map(doc => (
<div>
<p>{doc}</p>
<img alt={doc} src={doc} width="100%" />
</div>
))}</p>
</div>
</div>
);
}
export default DocViewer

When you call your similares() function you are setting state when calling setDocsSimilares.
By setting state you trigger re-render, when you trigger re-render it calls your similares() function again, that changes the state... You got the idea :)
By simply calling similares() inside of your functional component - it executes on each re-render.
You cannot just simply call similares() the way you do.
I think what you trying to achieve is to get data on first component mount, so you should use useEffect with empty array as second argument like so:
useEffect(() => similares().then(console.log(docsSimilares)),[])

If you are using hooks you need to understand useEffect
useEffect(() => similares().then(console.log(docsSimilares)),[])
The square brackets are where you add dependencies, if no dependencies it will only run on initializing "mounting" it is the same as componentDidMount() so that it will only fire the async request on load.

This code solved my problem:
async function similares() {
const simDocs = await axios.get(`${baseUrl}/${title}`).then(data => (data.data.body.hits.hits[0]._source.documentos_similares))
setDocsSimilares(simDocs)
}
useEffect(() => {
similares().then(data => console.log(data))
},[]);

Related

React, useEfect and axios. infinite loop [duplicate]

Are there ways to simulate componentDidMount in React functional components via hooks?
For the stable version of hooks (React Version 16.8.0+)
For componentDidMount
useEffect(() => {
// Your code here
}, []);
For componentDidUpdate
useEffect(() => {
// Your code here
}, [yourDependency]);
For componentWillUnmount
useEffect(() => {
// componentWillUnmount
return () => {
// Your code here
}
}, [yourDependency]);
So in this situation, you need to pass your dependency into this array. Let's assume you have a state like this
const [count, setCount] = useState(0);
And whenever count increases you want to re-render your function component. Then your useEffect should look like this
useEffect(() => {
// <div>{count}</div>
}, [count]);
This way whenever your count updates your component will re-render. Hopefully this will help a bit.
There is no exact equivalent for componentDidMount in react hooks.
In my experience, react hooks requires a different mindset when developing it and generally speaking you should not compare it to the class methods like componentDidMount.
With that said, there are ways in which you can use hooks to produce a similar effect to componentDidMount.
Solution 1:
useEffect(() => {
console.log("I have been mounted")
}, [])
Solution 2:
const num = 5
useEffect(() => {
console.log("I will only run if my deps change: ", num)
}, [num])
Solution 3 (With function):
useEffect(() => {
const someFunc = () => {
console.log("Function being run after/on mount")
}
someFunc()
}, [])
Solution 4 (useCallback):
const msg = "some message"
const myFunc = useCallback(() => {
console.log(msg)
}, [msg])
useEffect(() => {
myFunc()
}, [myFunc])
Solution 5 (Getting creative):
export default function useDidMountHook(callback) {
const didMount = useRef(null)
useEffect(() => {
if (callback && !didMount.current) {
didMount.current = true
callback()
}
})
}
It is worth noting that solution 5 should only really be used if none of the other solutions work for your use case. If you do decide you need solution 5 then I recommend using this pre-made hook use-did-mount.
Source (With more detail): Using componentDidMount in react hooks
There's no componentDidMount on functional components, but React Hooks provide a way you can emulate the behavior by using the useEffect hook.
Pass an empty array as the second argument to useEffect() to run only the callback on mount only.
Please read the documentation on useEffect.
function ComponentDidMount() {
const [count, setCount] = React.useState(0);
React.useEffect(() => {
console.log('componentDidMount');
}, []);
return (
<div>
<p>componentDidMount: {count} times</p>
<button
onClick={() => {
setCount(count + 1);
}}
>
Click Me
</button>
</div>
);
}
ReactDOM.render(
<div>
<ComponentDidMount />
</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>
useEffect() hook allows us to achieve the functionality of componentDidMount, componentDidUpdate componentWillUnMount functionalities.
Different syntaxes of useEffect() allows to achieve each of the above methods.
i) componentDidMount
useEffect(() => {
//code here
}, []);
ii) componentDidUpdate
useEffect(() => {
//code here
}, [x,y,z]);
//where x,y,z are state variables on whose update, this method should get triggered
iii) componentDidUnmount
useEffect(() => {
//code here
return function() {
//code to be run during unmount phase
}
}, []);
You can check the official react site for more info. Official React Page on Hooks
Although accepted answer works, it is not recommended. When you have more than one state and you use it with useEffect, it will give you warning about adding it to dependency array or not using it at all.
It sometimes causes the problem which might give you unpredictable output. So I suggest that you take a little effort to rewrite your function as class. There are very little changes, and you can have some components as class and some as function. You're not obligated to use only one convention.
Take this for example
function App() {
const [appointments, setAppointments] = useState([]);
const [aptId, setAptId] = useState(1);
useEffect(() => {
fetch('./data.json')
.then(response => response.json())
.then(result => {
const apts = result.map(item => {
item.aptId = aptId;
console.log(aptId);
setAptId(aptId + 1);
return item;
})
setAppointments(apts);
});
}, []);
return(...);
}
and
class App extends Component {
constructor() {
super();
this.state = {
appointments: [],
aptId: 1,
}
}
componentDidMount() {
fetch('./data.json')
.then(response => response.json())
.then(result => {
const apts = result.map(item => {
item.aptId = this.state.aptId;
this.setState({aptId: this.state.aptId + 1});
console.log(this.state.aptId);
return item;
});
this.setState({appointments: apts});
});
}
render(...);
}
This is only for example. so lets not talk about best practices or potential issues with the code. Both of this has same logic but the later only works as expected. You might get componentDidMount functionality with useEffect running for this time, but as your app grows, there are chances that you MAY face some issues. So, rather than rewriting at that phase, it's better to do this at early stage.
Besides, OOP is not that bad, if Procedure-Oriented Programming was enough, we would never have had Object-Oriented Programming. It's painful sometimes, but better (technically. personal issues aside).
import React, { useState, useEffect } from 'react';
function Example() {
const [count, setCount] = useState(0);
// Similar to componentDidMount and componentDidUpdate:
useEffect(() => {
// Update the document title using the browser API
document.title = `You clicked ${count} times`;
});
return (
<div>
<p>You clicked {count} times</p>
<button onClick={() => setCount(count + 1)}>
Click me
</button>
</div>
);
}
Please visit this official docs. Very easy to understand the latest way.
https://reactjs.org/docs/hooks-effect.html
Info about async functions inside the hook:
Effect callbacks are synchronous to prevent race conditions. Put the async function inside:
useEffect(() => {
async function fetchData() {
// You can await here
const response = await MyAPI.getData(someId);
// ...
}
fetchData();
}, [someId]); // Or [] if effect doesn't need props or state
useLayoutEffect hook is the best alternative to ComponentDidMount in React Hooks.
useLayoutEffect hook executes before Rendering UI and useEffect hook executes after rendering UI. Use it depend on your needs.
Sample Code:
import { useLayoutEffect, useEffect } from "react";
export default function App() {
useEffect(() => {
console.log("useEffect Statements");
}, []);
useLayoutEffect(() => {
console.log("useLayoutEffect Statements");
}, []);
return (
<div>
<h1>Hello Guys</h1>
</div>
);
}
Yes, there is a way to SIMULATE a componentDidMount in a React functional component
DISCLAIMER: The real problem here is that you need to change from "component life cycle mindset" to a "mindset of useEffect"
A React component is still a javascript function, so, if you want something to be executed BEFORE some other thing you must simply need to execute it first from top to bottom, if you think about it a function it's still a funtion like for example:
const myFunction = () => console.log('a')
const mySecondFunction = () => console.log('b)
mySecondFunction()
myFunction()
/* Result:
'b'
'a'
*/
That is really simple isn't it?
const MyComponent = () => {
const someCleverFunction = () => {...}
someCleverFunction() /* there I can execute it BEFORE
the first render (componentWillMount)*/
useEffect(()=> {
someCleverFunction() /* there I can execute it AFTER the first render */
},[]) /*I lie to react saying "hey, there are not external data (dependencies) that needs to be mapped here, trust me, I will leave this in blank.*/
return (
<div>
<h1>Hi!</h1>
</div>
)}
And in this specific case it's true. But what happens if I do something like that:
const MyComponent = () => {
const someCleverFunction = () => {...}
someCleverFunction() /* there I can execute it BEFORE
the first render (componentWillMount)*/
useEffect(()=> {
someCleverFunction() /* there I can execute it AFTER the first render */
},[]) /*I lie to react saying "hey, there are not external data (dependencies) that needs to be maped here, trust me, I will leave this in blank.*/
return (
<div>
<h1>Hi!</h1>
</div>
)}
This "cleverFunction" we are defining it's not the same in every re-render of the component.
This lead to some nasty bugs and, in some cases to unnecessary re-renders of components or infinite re-render loops.
The real problem with that is that a React functional component is a function that "executes itself" several times depending on your state thanks to the useEffect hook (among others).
In short useEffect it's a hook designed specifically to synchronize your data with whatever you are seeing on the screen. If your data changes, your useEffect hook needs to be aware of that, always. That includes your methods, for that it's the array dependencies.
Leaving that undefined leaves you open to hard-to-find bugs.
Because of that it's important to know how this work, and what you can do to get what you want in the "react" way.
const initialState = {
count: 0,
step: 1,
done: false
};
function reducer(state, action) {
const { count, step } = state;
if (action.type === 'doSomething') {
if(state.done === true) return state;
return { ...state, count: state.count + state.step, state.done:true };
} else if (action.type === 'step') {
return { ...state, step: action.step };
} else {
throw new Error();
}
}
const MyComponent = () => {
const [state, dispatch] = useReducer(reducer, initialState);
const { count, step } = state;
useEffect(() => {
dispatch({ type: 'doSomething' });
}, [dispatch]);
return (
<div>
<h1>Hi!</h1>
</div>
)}
useReducer's dispatch method it's static so it means it will be the same method no matter the amount of times your component is re-rendered. So if you want to execute something just once and you want it rigth after the component is mounted, you can do something like the above example. This is a declarative way of do it right.
Source: The Complete Guide to useEffect - By Dan Abramov
That being said if you like to experiment with things and want to know how to do it "the imperative wat" you can use a useRef() with a counter or a boolean to check if that ref stores a defined reference or not, this is an imperative approach and it's recommended to avoid it if you're not familiar with what happen with react behind curtains.
That is because useRef() is a hook that saves the argument passed to it regardless of the amount of renders (I am keeping it simple because it's not the focus of the problem here, you can read this amazing article about useRef ). So it's the best approach to known when the first render of the component happened.
I leave an example showing 3 different ways of synchronise an "outside" effect (like an external function) with the "inner" component state.
You can run this snippet right here to see the logs and understand when these 3 functions are executed.
const { useRef, useState, useEffect, useCallback } = React
// External functions outside react component (like a data fetch)
function renderOnce(count) {
console.log(`renderOnce: I executed ${count} times because my default state is: undefined by default!`);
}
function renderOnFirstReRender(count) {
console.log(`renderOnUpdate: I executed just ${count} times!`);
}
function renderOnEveryUpdate(count) {
console.log(`renderOnEveryUpdate: I executed ${count ? count + 1 : 1} times!`);
}
const MyComponent = () => {
const [count, setCount] = useState(undefined);
const mounted = useRef(0);
// useCallback is used just to avoid warnings in console.log
const renderOnEveryUpdateCallBack = useCallback(count => {
renderOnEveryUpdate(count);
}, []);
if (mounted.current === 0) {
renderOnce(count);
}
if (mounted.current === 1) renderOnFirstReRender(count);
useEffect(() => {
mounted.current = mounted.current + 1;
renderOnEveryUpdateCallBack(count);
}, [count, renderOnEveryUpdateCallBack]);
return (
<div>
<h1>{count}</h1>
<button onClick={() => setCount(prevState => (prevState ? prevState + 1 : 1))}>TouchMe</button>
</div>
);
};
class App extends React.Component {
render() {
return (
<div>
<h1>hI!</h1>
</div>
);
}
}
ReactDOM.createRoot(
document.getElementById("root")
).render(
<MyComponent/>
);
<div id="root"></div>
<script src="https://cdnjs.cloudflare.com/ajax/libs/react/18.1.0/umd/react.development.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/react-dom/18.1.0/umd/react-dom.development.js"></script>
If you execute it you will see something like this:
You want to use useEffect(), which, depending on how you use the function, can act just like componentDidMount().
Eg. you could use a custom loaded state property which is initially set to false, and switch it to true on render, and only fire the effect when this value changes.
Documentation
the exact equivalent hook for componentDidMount() is
useEffect(()=>{},[]);
hope this helpful :)

Avoiding infinite re-rendering in react component

I have a Supabase database called threads with 5 entries, I am trying to make a new React component for every column in the database but I am running into some issues.
The issue that it's giving me is that it's stopping the program to prevent an infinite loop, I'd assume this is because in my code I am updating my useState every time I render my Component which I am aware of, but I'm not sure how I would get around this problem.
Component.js:
import {useState} from "react";
import {supabase} from "../../utils/supabaseClient";
export default function Sidebar() {
const [newThreads, setNewThreads] = useState([]);
// this is where I am kinda stuck
const threadList = [];
(async () => {
let {data: threads, error} = await supabase
.from('threads')
.select('thread_title');
threadList.push(threads); // how do I get the current index of the loop (and limit it to only get 5 entries?)
threadList.forEach(function (value) {
console.log(value)
})
})();
setNewThreads(threadList);
return (
<div className="sidebar">
<div className="sidebar-widget">
<span className="widget-title">New Threads</span>
</div>
<div className="sidebar-widget">
<span className="widget-title">New Members</span>
{(Object.entries(newThreads) || []).map(([key, value]) => {
return (
<div className="widget-cell">
<div className={"widget-cell-image"}/>
<div className="widget-cell-content">
<span className={"widget-cell-title"}>{value}</span>
<div className="widget-cell-values">
<span className={"widget-cell-by-user"}>by harley_swift,</span>
<span className={"widget-cell-time"}>22:02</span>
</div>
</div>
</div>
);
})}
</div>
</div>
)
}
Any help with an explanation would be greatly appreciated!
The way to get around the problem of infinite re-rendering is to use useEffect to fetch your data:
useEffect(() => {
// this is where I am kinda stuck
const threadList = [];
(async () => {
let {
data: threads,
error
} = await supabase
.from('threads')
.select('thread_title');
threadList.push(threads); // how do I get the current index of the loop (and limit it to only get 5 entries?)
threadList.forEach(function(value) {
console.log(value)
})
})();
setNewThreads(threadList);
}, []); // note the empty dependency list
The empty dependency list is one of the ways you know the effect will only run once.
To only add 5 of the items to the list, you can slice the result (or check to see if supabase has a builtin way of doing this):
threadList.push(threads.slice(0, 5));
Finally, note that state changes are asynchronous and more so in your case because you are actually doing a fetch over the network.
If you wanted to get notified when newThreads has been updated, you can use another useEffect like so:
useEffect(() => {
if (newThreads) {
console.warn(newThreads.length);
}
}, [newThreads]);
The if statement is not really needed, but added for completeness
This also applies for when you want to actually render the newThreads, you need to use conditional rendering to do that:
{
(Object.entries(newThreads) || []).map(([key, value]) => {
return (
<div className="widget-cell">
<div className={"widget-cell-image"} />
<div className="widget-cell-content">
<span className={"widget-cell-title"}>{value}</span>
<div className="widget-cell-values">
<span className={"widget-cell-by-user"}>by harley_swift,</span>
<span className={"widget-cell-time"}>22:02</span>
</div>
</div>
</div>
);
}) || <div>Loading...</div>
}

Returning a component from a map function

I want to create a function that maps throw a firestore collection and return a Component for each document in the collection I have tried to use the code below
<div className="posts">
{
db.collection("posts")
.get()
.then((snapshot) => {
snapshot.docs.map((doc)=>(
<PostContect img={doc.data().image} Admin={doc.data().admin} Date={"January 14, 2019"} CommentsNo={"2"} Title={doc.data().title} Body={doc.data().title} />
))})}
</div>
but this error shows:
Error: Objects are not valid as a React child (found: [object Promise]). If you meant to render a collection of children, use an array instead.ode
JSX is not the place to make Ajax calls. Instead make the call inside componentDidMount/useEffect and set the data received inside state. As soon as state is updated with received data, React will automatically re-render the component and display the expected content. Try something as follows:
const [snapshots, setSnapshots] = useState();
useEffect(()=> {
db.collection("posts")
.get()
.then((snapshot) => {
setSnapshots(snapshot.docs)
}
)
}, []);
render
<div className="posts">
{snapshots && snapshots.map((doc)=>(
<PostContect img={doc.data().image} Admin={doc.data().admin} Date={"January 14, 2019"} CommentsNo={"2"} Title={doc.data().title} Body={doc.data().title} />
)
}
</div>
Since you haven't given a hint as to whether you're using hooks or not, here is how you should deal with asynchronous stuff and setting states in React (using hooks) :-
function MyComponent () {
const [snapshot,setSnapshot] = useState();
useEffect(()=>{
async function getPosts(){
let snapshot = await db.collection("posts").get();
setSnapshot(snapshot);
}
getPosts();
},[])
return
(<div className="posts">
snapshot?.docs?.map((doc)=>(
<PostContect img={doc.data().image} Admin={doc.data().admin} Date={"January 14, 2019"} CommentsNo={"2"} Title={doc.data().title} Body={doc.data().title} />)
</div>)
}
What you are doing is returning Promise object as the error states. That async stuff is taken care either inside a useEffect,event handler or a custom-hook. There could be more ways but this is a general point to start.
As the error message states Promise is not a valid React child. To solve that issue you can introduce a state which keeps the result of db.collection('posts').get() method. Then iterating through on that array with your .map() which will be a valid React child.
See a possible solution for your scenario:
// in the root of you component, introduce a state
const [posts, setPosts] = useState([])
// here in useEffect you can do your API call and update the state
useEffect(() => {
db.collection("posts")
.get()
.then((snapshot) => {
setPosts(snapshot.docs)
})
}, [])
// ... rest of your component's code
return (
<div className="posts">
{
posts && posts.map((doc, i) => (
<div key={i}>
doc.data().title
</div>
// here you can use <PostContect /> component
// either doc.data().title or doc.title to get the value
))
}
</div>
)
Suggested reads:
Using the Effect Hook
Using the State Hook
The code you wrote returns a promise, which can't be rendered by react. You can use conditional rendering to return something else while the data is being fetched.
Check this question for more info

React hooks not set state at first time

i have a problem with hooks.
i'm using react-hooks, i have a button which at onClick getting data from api and setState with it.
Problem is:
when i click to button first time i get response from api but can't set it to state. When click to button second time i can setState. Why it happened ?
here is my component look like:
function App() {
const [a, setA] = useState(null);
const fetchData = () => {
let data = {
id: 1
}
axios.post(baseUrl, data)
.then(res => {
if (res.data) {
setA(res.data)
}
console.log(a)
})
}
return (
<div className="App">
<div>
<button onClick={fetchData}></button>
</div>
</div>
);
}
export default App;
i tried to using fetchData function like that:
function fetchData() {
let data = {
id: 1
}
axios.post(baseUrl, data)
.then(res => {
if (res.data) {
setA(res.data)
}
console.log(a)
})
}
but it's not helped too
a is a const. It's impossible to change it, so there's no way your console.log statement at the end of fetchData could ever log out something different than the value that a had when fetchData was created.
So that's not what setA does. The point of setA isn't to change the value of the const, but to cause the component to rerender. On that new render, a new set of variables will be created, and the new a will get the new value. Move your console.log out to the body of your component, and you'll see it rerender with the new value.
In short: Your code appears to be already working, you just have the wrong logging.
If your scope is to fetch data, use this:
const [data, setData] = useState("");
useEffect(async () => {
const result = await axios(
'here will be your api',
);
setData(result.data);
});
Now your response will be stored in data variable.
I would not use an effect for it, effects are useful if the props or state changes and can thereby substitute lifecycle methods like componentDidMount, componentDidUpdate, componentWillUnmount, etc.. But in your case these props haven't changed yet (you want to change your state though). Btw, be aware that #Asking's approach will fetch data on EVERY rerender (props or state change). If you want to use useEffect, be sure to add the second parameter to tell React when to update.
Normally, your code should work, I haven't tested but looks legit. Have you used developer tools for react to check if the state/hook has changed? Because if you say it did not work because of the console.log printing: Have in mind that setA() is an async function. The state was most likely not yet changed when you try to print it. If you haven't react developer tools (which I highly recommend) you can check the real state by adding this code in the function:
useEffect(() => console.log(a), [a]);
I have a few real improvements to your code though:
function App() {
const [a, setA] = useState(null);
const fetchData = useCallback(async () => {
let data = {
id: 1
}
const res = await axios.post(baseUrl, data);
setA(res.data);
}, []);
return (
<div className="App">
<div>
<button onClick={fetchData}></button>
</div>
</div>
);
}
export default App;
By adding useCallback you ensure that the function is memorized and not declared again and again on every rerender.
#Nicholas Tower has already answered your question. But if you are looking for code
function App() {
const [a, setA] = useState(null);
const fetchData = () => {
let data = {
id: 1
}
axios.post(baseUrl, data)
.then(res => {
if (res.data) {
setA(res.data)
}
})
}
console.log(a)
return (
<div className="App">
<div>
<button onClick={fetchData}></button>
</div>
</div>
);
}
export default App;
just place log before return (. This should do

How to force a functional React component to render?

I have a function component, and I want to force it to re-render.
How can I do so?
Since there's no instance this, I cannot call this.forceUpdate().
🎉 You can now, using React hooks
Using react hooks, you can now call useState() in your function component.
useState() will return an array of 2 things:
A value, representing the current state.
Its setter. Use it to update the value.
Updating the value by its setter will force your function component to re-render,
just like forceUpdate does:
import React, { useState } from 'react';
//create your forceUpdate hook
function useForceUpdate(){
const [value, setValue] = useState(0); // integer state
return () => setValue(value => value + 1); // update state to force render
// A function that increment 👆🏻 the previous state like here
// is better than directly setting `setValue(value + 1)`
}
function MyComponent() {
// call your hook here
const forceUpdate = useForceUpdate();
return (
<div>
{/*Clicking on the button will force to re-render like force update does */}
<button onClick={forceUpdate}>
Click to re-render
</button>
</div>
);
}
You can find a demo here.
The component above uses a custom hook function (useForceUpdate) which uses the react state hook useState. It increments the component's state's value and thus tells React to re-render the component.
EDIT
In an old version of this answer, the snippet used a boolean value, and toggled it in forceUpdate(). Now that I've edited my answer, the snippet use a number rather than a boolean.
Why ? (you would ask me)
Because once it happened to me that my forceUpdate() was called twice subsequently from 2 different events, and thus it was reseting the boolean value at its original state, and the component never rendered.
This is because in the useState's setter (setValue here), React compare the previous state with the new one, and render only if the state is different.
Update react v16.8 (16 Feb 2019 realease)
Since react 16.8 released with hooks, function components have the ability to hold persistent state. With that ability you can now mimic a forceUpdate:
function App() {
const [, updateState] = React.useState();
const forceUpdate = React.useCallback(() => updateState({}), []);
console.log("render");
return (
<div>
<button onClick={forceUpdate}>Force Render</button>
</div>
);
}
const rootElement = document.getElementById("root");
ReactDOM.render(<App />, rootElement);
<script src="https://cdnjs.cloudflare.com/ajax/libs/react/16.8.1/umd/react.production.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/react-dom/16.8.1/umd/react-dom.production.min.js"></script>
<div id="root"/>
Note that this approach should be re-considered and in most cases when you need to force an update you probably doing something wrong.
Before react 16.8.0
No you can't, State-Less function components are just normal functions that returns jsx, you don't have any access to the React life cycle methods as you are not extending from the React.Component.
Think of function-component as the render method part of the class components.
Official FAQ now recommends this way if you really need to do it:
const [ignored, forceUpdate] = useReducer(x => x + 1, 0);
function handleClick() {
forceUpdate();
}
Simplest way 👌
if you want to force a re-render, add a dummy state you can change to initiate a re-render.
const [rerender, setRerender] = useState(false);
...
setRerender(!rerender); //whenever you want to re-render
And this will ensure a re-render, And you can call setRerender(!rerender) anywhere, whenever you want :)
I used a third party library called
use-force-update
to force render my react functional components. Worked like charm.
Just use import the package in your project and use like this.
import useForceUpdate from 'use-force-update';
const MyButton = () => {
const forceUpdate = useForceUpdate();
const handleClick = () => {
alert('I will re-render now.');
forceUpdate();
};
return <button onClick={handleClick} />;
};
Best approach - no excess variables re-created on each render:
const forceUpdateReducer = (i) => i + 1
export const useForceUpdate = () => {
const [, forceUpdate] = useReducer(forceUpdateReducer, 0)
return forceUpdate
}
Usage:
const forceUpdate = useForceUpdate()
forceUpdate()
If you already have a state inside the function component and you don't want to alter it and requires a re-render you could fake a state update which will, in turn, re-render the component
const [items,setItems] = useState({
name:'Your Name',
status: 'Idle'
})
const reRender = () =>{
setItems((state) => [...state])
}
this will keep the state as it was and will make react into thinking the state has been updated
This can be done without explicitly using hooks provided you add a prop to your component and a state to the stateless component's parent component:
const ParentComponent = props => {
const [updateNow, setUpdateNow] = useState(true)
const updateFunc = () => {
setUpdateNow(!updateNow)
}
const MyComponent = props => {
return (<div> .... </div>)
}
const MyButtonComponent = props => {
return (<div> <input type="button" onClick={props.updateFunc} />.... </div>)
}
return (
<div>
<MyComponent updateMe={updateNow} />
<MyButtonComponent updateFunc={updateFunc}/>
</div>
)
}
The accepted answer is good.
Just to make it easier to understand.
Example component:
export default function MyComponent(props) {
const [updateView, setUpdateView] = useState(0);
return (
<>
<span style={{ display: "none" }}>{updateView}</span>
</>
);
}
To force re-rendering call the code below:
setUpdateView((updateView) => ++updateView);
None of these gave me a satisfactory answer so in the end I got what I wanted with the key prop, useRef and some random id generator like shortid.
Basically, I wanted some chat application to play itself out the first time someone opens the app. So, I needed full control over when and what the answers are updated with the ease of async await.
Example code:
function sleep(ms) {
return new Promise(resolve => setTimeout(resolve, ms));
}
// ... your JSX functional component, import shortid somewhere
const [render, rerender] = useState(shortid.generate())
const messageList = useRef([
new Message({id: 1, message: "Hi, let's get started!"})
])
useEffect(()=>{
async function _ () {
await sleep(500)
messageList.current.push(new Message({id: 1, message: "What's your name?"}))
// ... more stuff
// now trigger the update
rerender(shortid.generate())
}
_()
}, [])
// only the component with the right render key will update itself, the others will stay as is and won't rerender.
return <div key={render}>{messageList.current}</div>
In fact this also allowed me to roll something like a chat message with a rolling .
const waitChat = async (ms) => {
let text = "."
for (let i = 0; i < ms; i += 200) {
if (messageList.current[messageList.current.length - 1].id === 100) {
messageList.current = messageList.current.filter(({id}) => id !== 100)
}
messageList.current.push(new Message({
id: 100,
message: text
}))
if (text.length === 3) {
text = "."
} else {
text += "."
}
rerender(shortid.generate())
await sleep(200)
}
if (messageList.current[messageList.current.length - 1].id === 100) {
messageList.current = messageList.current.filter(({id}) => id !== 100)
}
}
If you are using functional components with version < 16.8. One workaround would be to directly call the same function like
import React from 'react';
function MyComponent() {
const forceUpdate = MyComponent();
return (
<div>
<button onClick={forceUpdate}>
Click to re-render
</button>
</div>
);
}
But this will break if you were passing some prop to it. In my case i just passed the same props which I received to rerender function.
For me just updating the state didn't work. I am using a library with components and it looks like I can't force the component to update.
My approach is extending the ones above with conditional rendering. In my case, I want to resize my component when a value is changed.
//hook to force updating the component on specific change
const useUpdateOnChange = (change: unknown): boolean => {
const [update, setUpdate] = useState(false);
useEffect(() => {
setUpdate(!update);
}, [change]);
useEffect(() => {
if (!update) setUpdate(true);
}, [update]);
return update;
};
const MyComponent = () => {
const [myState, setMyState] = useState();
const update = useUpdateOnChange(myState);
...
return (
<div>
... ...
{update && <LibraryComponent />}
</div>
);
};
You need to pass the value you want to track for change. The hook returns boolean which should be used for conditional rendering.
When the change value triggers the useEffect update goes to false which hides the component. After that the second useEffect is triggered and update goes true which makes the component visible again and this results in updating (resizing in my case).

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