This question already has answers here:
Why useEffect running twice and how to handle it well in React?
(2 answers)
Closed 7 months ago.
I am making a simple get request in React like the following:
function App() {
const [req, setReq] = useState();
const getBasic = () => {
axios
.get(
"https://financialmodelingprep.com/api/v4/price-target?symbol=AAPL&apikey=********"
)
.then((result) => setReq(result));
};
useEffect(() => {
getBasic();
}, []);
console.log(req);
return (
<div>
</div>
);
}
The above code logs the same thing four times in the console.
Was not sure if moving the console.log(req) into the return would do anything different, like:
return (
<div>
{console.log(req)}
</div>
);
It did not however and had the same result.
I tried to do the same code but with async await syntax:
function App() {
const [req, setReq] = useState();
const getBasic = async () => {
const getReq = await axios.get(
"https://financialmodelingprep.com/api/v4/price-target?symbol=AAPL&apikey=**********"
);
setReq(getReq);
};
useEffect(() => {
getBasic();
}, []);
console.log(req);
return (
<div>
</div>
);
}
Still the same, getting four seperate console.log(req)'s in console.
The last thing I tried was using a onClick button instead of useEffect to make a request
return(
<div>
<button onClick={getBasic} />
</div>
)
this results into two logs per click instead of four with useEffect.
I do not understand if in these instances I am making two or four requests or if I am still only making one request but it is logging multiple times, I would like to know why this is happening and what I should do to fix it, even though I can do the rest of my project with it working this way it appears I am not doing it optimally either way.
Disable Strict Mode by deleting Strict Mode tags in index.js
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 :)
This question already has answers here:
react useEffect comparing objects
(7 answers)
Closed 2 years ago.
I have an app that checks the user id on startup and loads the list of todo items based on the user logged in. I have the useEffect change only when data changes, but I have setData in the body of useEffect() meaning data changes and it re-runs infinitum.
However if I change [data] to [] in the second parameter, then it renders once BUT I have to refresh the page everytime I add a todo item for it to render rather than it render automatically. How can I have it render automatically without looping infinitely?
const [data, setData] = useState([])
useEffect(() => {
UserService.getUserById(localStorage.getItem("userId")).then(res => {
if (res.data !== null) {
setData(res.data.todos)
}
})
}, [data])
You can add a condition in the call back function that checks if a certain condition is met, e.g. if data is empty. If it is empty, then fetch data, otherwise do nothing. This will prevent the infinite loop from happening.
const getData = useEffect(()=>{
const fetchData = () => {
UserService.getUserById(localStorage.getItem("userId"))
.then(res => {
if (res.data !== null) {
setData(res.data.todos)
}
})
.catch(error => {
// do something with error
})
}
if (data.length === 0)
fetchData()
},[data]);
Alternatively, you use an empty dependency array so that the callback function in the useEffect is called once.
useCallback Hook can be used with slight modifications in your code.
You will need to import useCallback from "react" first.
import {useCallback} from "react";
And then use this useCallback around our getData function. (Have modified the answer a bit)
const getData = useCallback(()=>{
UserService.getUserById(localStorage.getItem("userId")).then(res => {
if (res.data !== null) {
setData(res.data.todos)
}
})
},[data]);
useEffect(() => {
getData();
}, [data])
This React Hook will make sure that the getData() function is only created when the second argument data changes.
In your code UserService.getUserById(localStorage.getItem("userId")) return a promise and it get data one time so you just have to call getUserById one time at the time of load by using [] and if you want to call it again make a function and use it wherever on refresh function or on adding todos item or update or delete function. Otherwise you have to use observable or useCallBack hook
You need to pass the reset param to prevent loop. once callback trigger reset value false. so that execution not running again until reset the value
Codesanbox
export default function App() {
let i = 1;
const [data, setData] = useState([]);
const [reset, setReset] = useState(true);
useEffect(() => {
if (reset) {
setTimeout(() => {
//callback
setReset(false);
setData(Math.random());
}, 1000);
}
}, [data]);
return (
<div className="App">
<h1>{data}</h1>
<button
onClick={() => {
setReset(true);
setData("");
}}
>
Click this and see the data render again. i just reset the data to empty
</button>
<h2>Start editing to see some magic happen!</h2>
</div>
);
}
Use a condition to stop the loop by setting a condition to stop it. You can check if a certain value is set or check if there are any values sent at all.
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
This question already has answers here:
Where can I make API call with hooks in react?
(5 answers)
Closed 2 years ago.
I'm looking for feedback on this customer React hook. I'm wondering:
Does this look like a proper use of custom React hooks?
Is there a better way to switch between different API endpoints based upon the prop that is passed in? I'm looking to do something like:
<MovieGrid typeOfMovies={"popular"} />
and
<MovieGrid typeOfMovies={"upcoming"} />
Do you have any feedback or recommendations on anything you see. Thank you!
The code I've provided does indeed work. But since hooks a relatively new I don't feel totally confident I'm using them right.
Here's what I've got:
import React, { useState, useEffect } from "react";
function useFetchMovies(typeOfMovies) {
const [movieData, setMovieData] = useState([]);
const movieEndpointURL = () => {
if (typeOfMovies === "upcoming") {
return `https://api.themoviedb.org/3/movie/upcoming?api_key={API_KEY}&language=en-US&page=1®ion=US`;
} else if (typeOfMovies === "popular") {
return `https://api.themoviedb.org/3/movie/popular?api_key={API_KEY}&language=en-US&page=1®ion=US`;
}
};
const fetchMovieData = async () => {
try {
const res = await fetch(movieEndpointURL());
const movies = await res.json();
setMovieData(movies.results);
console.log(movies.results);
} catch (err) {
console.log(err);
}
};
useEffect(() => {
fetchMovieData();
}, []);
return [movieData, setMovieData];
}
export { useFetchMovies };
Your useFetchMovies seems to be correct expect the part that when typeOfMovies changes new data will not be fetched because when the useEffect first runs on mount of component it will refer to the fetchMoviesData that was initially created along with its closure and when the useFetchMovies hook is called again a new function is created which isn't referenced by the useEffect.
In order to make it word correctly you should pass typeOfMovies as the second argument to useEffect like
useEffect(() => {
fetchMovieData();
}, [typeOfMovies]);