I ran into a need of cleaning a useEffect when component is unmounted but with an access to the current props. Like componentWillUnmount can do by getting this.props.whatever
Here is a small example:
Click "set count" button 5 times and look at your console. You'll see 0 from the console.log in component B, regardless "count" will be 5
https://codesandbox.io/embed/vibrant-feather-v9f6o
How to implement the behavior of getting current props in useEffect cleanup function (5 in my case)?
UPDATE:
Passing count to the dependencies of useEffect won't help because:
Cleanup will be invoked on every new count is passed to the props
The last value will be 4 instead of the desired 5
Was referenced to this question from another one so will do a bit of grave digging for it since there is no accepted answer.
The behaviour you want can never happen because B never renders with a count value of 5, component A will not render component B when count is 5 because it'll render unmounted instead of B, the last value B is rendered with will be 4.
If you want B to log the last value it had for count when it unmounts you can do the following:
Note that effects executes after all components have rendered
const useIsMounted = () => {
const isMounted = React.useRef(false);
React.useEffect(() => {
isMounted.current = true;
return () => (isMounted.current = false);
}, []);
return isMounted;
};
const B = ({ count }) => {
const mounted = useIsMounted();
React.useEffect(() => {
// eslint-disable-next-line react-hooks/exhaustive-deps
return () => !mounted.current && console.log(count);
}, [count, mounted]);
return <div>{count}</div>;
};
const A = () => {
const [count, setCount] = React.useState(0);
const handleClick = React.useCallback(() => {
setCount(prevCount => prevCount + 1);
}, []);
if (count === 5) {
//B will never be rendered with 5
return <div>Unmounted</div>;
}
return (
<React.Fragment>
<B count={count} />
<button onClick={handleClick}>Set count</button>
</React.Fragment>
);
};
ReactDOM.render(<A />, document.getElementById('root'));
<script src="https://cdnjs.cloudflare.com/ajax/libs/react/16.8.4/umd/react.production.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/react-dom/16.8.4/umd/react-dom.production.min.js"></script>
<div id="root"></div>
You need to track count in useEffect:
import React, { useEffect } from "react";
const B = ({ count }) => {
useEffect(() => {
return () => console.log(count);
}, [count]);
return <div>{count}</div>;
};
export default B;
Since you passed empty array as dependency list for useEffect, your effect function only gets registered the first time. At that time, the value of count was 0. And that is the value of count which gets attached to the function inside useEffect. If you want to get hold of the latest count value, you need your useEffect to run on every render. Remove the empty array dependency.
useEffect(() => {
return () => console.log(count);
})
or use count as the dependency. That way, your effect function is created anew every time count changes and a new cleanup function is also created which has access to the latest count value.
useEffect(() => {
return () => console.log(count);
}, [count])
Related
I've got a hooks problem I've been unable to find an answer for. The closest article being this other post.
Essentially I want to have a function that is invoked once per component lifecycle with hooks. I'd normally use useEffect as such:
useEffect(() => {
doSomethingOnce()
setInterval(doSomethingOften, 1000)
}, [])
I'm trying not to disable eslint rules and get a react-hooks/exhaustive-deps warning.
Changing this code to the following creates an issue of my functions being invoked every time the component is rendered... I don't want this.
useEffect(() => {
doSomethingOnce()
setInterval(doSomethingOften, 1000)
}, [doSomethingOnce])
The suggested solution is to wrap my functions in useCallback but what I can't find the answer to is what if doSomethingOnce depends on the state.
Below is the minimal code example of what I am trying to achieve:
import "./styles.css";
import { useEffect, useState, useCallback } from "react";
export default function App() {
const [count, setCount] = useState(0);
const getRandomNumber = useCallback(
() => Math.floor(Math.random() * 1000),
[]
);
const startCounterWithARandomNumber = useCallback(() => {
setCount(getRandomNumber());
}, [setCount, getRandomNumber]);
const incrementCounter = useCallback(() => {
setCount(count + 1);
}, [count, setCount]);
useEffect(() => {
startCounterWithARandomNumber();
setInterval(incrementCounter, 1000);
}, [startCounterWithARandomNumber, incrementCounter]);
return (
<div className="App">
<h1>Counter {count}</h1>
</div>
);
}
As you can see from this demo since incrementCounter depends on count it gets recreated. Which in turn re-invokes my useEffect callback that I only wanted to be called the once. The result is that the startCounterWithARandomNumber and incrementCounter get called many more times than I would expect.
Any help would be much appreciated.
Update
I should have pointed out, this is a minimal example of a real-use case where the events are aysncronous.
In my real code the context is a live transcription app. I initially make a fetch call to GET /api/all.json to get the entire transcription then poll GET /api/latest.json every second merging the very latest speech to text from with the current state. I tried to emulate this in my minimal example with a seed to start followed by a polled method call that has a dependency on the current state.
I think you have overcomplicated your code considerably.
Solution
If you want the startCounterWithARandomNumber function to run once to set initial state, then just use a state initialization function.
const getRandomNumber = () => Math.floor(Math.random() * 1000);
const [count, setCount] = useState(getRandomNumber);
As for the effect setting up the interval, you will also want to only run this once when mounting. Move the interval callback that "ticks" and increments the count state into the effect callback so it is no longer a dependency. Use a functional state update to correctly update from the previous state and not the initial state. Don't forget to return a cleanup function from the useEffect hook to clear any running interval timers.
useEffect(() => {
const incrementCounter = () => setCount((c) => c + 1);
const timer = setInterval(incrementCounter, 1000);
return () => clearInterval(timer);
}, []);
Demo
Full code:
import { useEffect, useState } from "react";
export default function App() {
const getRandomNumber = () => Math.floor(Math.random() * 1000);
const [count, setCount] = useState(getRandomNumber);
useEffect(() => {
const incrementCounter = () => setCount((c) => c + 1);
const timer = setInterval(incrementCounter, 1000);
return () => clearInterval(timer);
}, []);
return (
<div className="App">
<h1>Counter {count}</h1>
</div>
);
}
Update
I guess it's still a bit unclear what your real code and use case is doing. It seems you've written your doSomethingOnce and doSomethingOften functions in such a way so as to still have some outer dependency that when used inside an useEffect hook is flagged by the linter.
Based on your counting example here an example that doesn't warn about dependencies.
const getRandomNumber = () => Math.floor(Math.random() * 1000);
const fetch = () =>
new Promise((resolve) => {
setTimeout(() => {
if (Math.random() < 0.1) { // 10% to return new value
resolve(getRandomNumber());
}
}, 3000);
});
function App() {
const [count, setCount] = React.useState(0);
const doSomethingOnce = () => {
console.log('doSomethingOnce');
fetch().then((val) => setCount(val));
};
const doSomethingOften = () => {
console.log('doSomethingOften');
fetch().then((val) => {
console.log('new value, update state')
setCount(val);
});
};
React.useEffect(() => {
doSomethingOnce();
const timer = setInterval(doSomethingOften, 1000);
return () => clearInterval(timer);
}, []);
// This effect is only to update state independently of any state updates from "polling"
React.useEffect(() => {
const tick = () => setCount((c) => c + 1);
const timer = setInterval(tick, 100);
return () => clearInterval(timer);
}, []);
return (
<div className="App">
<h1>Counter {count}</h1>
</div>
);
}
const rootElement = document.getElementById("root");
ReactDOM.render(
<App />,
rootElement
);
<script src="https://cdnjs.cloudflare.com/ajax/libs/react/16.13.1/umd/react.production.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/react-dom/16.13.1/umd/react-dom.production.min.js"></script>
<div id="root"></div>
This is sort of a contrived and "tuned" solution though since the "fetch" only resolves when it's returning a new value. In reality if you are likely polling and completely replacing a chunk of state that the component isn't regularly modifying (at least I hope it isn't as this makes merging/synchronizing more difficult). I can improve this answer if there were a better/clearer view of what your code is actually doing.
I dont know the usecase you want to solve so keeping minimal code changes to your code.
A very simple solution would be to increment counter as
const incrementCounter = useCallback(() => {
setCount(prevCount => prevCount+1);
}, [setCount]);
This way the increment counter does not rely on count.
Forked from your sandbox.
https://codesandbox.io/s/fragrant-architecture-t58bs
I ran into a situation where I set an interval timer from inside useEffect. I can access component variables and state inside the useEffect, and the interval timer runs as expected. However, the timer callback doesn't have access to the component variables / state. Normally, I would expect this to be an issue with "this". However, I do not believe "this" is the the case here. No puns were intended. I have included a simple example below:
import React, { useEffect, useState } from 'react';
const App = () => {
const [count, setCount] = useState(0);
const [intervalSet, setIntervalSet] = useState(false);
useEffect(() => {
if (!intervalSet) {
setInterval(() => {
console.log(`count=${count}`);
setCount(count + 1);
}, 1000);
setIntervalSet(true);
}
}, [count, intervalSet]);
return <div></div>;
};
export default App;
The console outputs only count=0 each second. I know that there's a way to pass a function to the setCount which updates current state and that works in this trivial example. However, that was not the point I was trying to make. The real code is much more complex than what I showed here. My real code looks at current state objects that are being managed by async thunk actions. Also, I am aware that I didn't include the cleanup function for when the component dismounts. I didn't need that for this simple example.
The first time you run the useEffect the intervalSet variable is set to true and your interval function is created using the current value (0).
On subsequent runs of the useEffect it does not recreate the interval due to the intervalSet check and continues to run the existing interval where count is the original value (0).
You are making this more complicated than it needs to be.
The useState set function can take a function which is passed the current value of the state and returns the new value, i.e. setCount(currentValue => newValue);
An interval should always be cleared when the component is unmounted otherwise you will get issues when it attempts to set the state and the state no longer exists.
import React, { useEffect, useState } from 'react';
const App = () => {
// State to hold count.
const [count, setCount] = useState(0);
// Use effect to create and clean up the interval
// (should only run once with current dependencies)
useEffect(() => {
// Create interval get the interval ID so it can be cleared later.
const intervalId = setInterval(() => {
// use the function based set state to avoid needing count as a dependency in the useEffect.
// this stops the need to code logic around stoping and recreating the interval.
setCount(currentCount => {
console.log(`count=${currentCount}`);
return currentCount + 1;
});
}, 1000);
// Create function to clean up the interval when the component unmounts.
return () => {
if (intervalId) {
clearInterval(intervalId);
}
}
}, [setCount]);
return <div></div>;
};
export default App;
You can run the code and see this working below.
const App = () => {
// State to hold count.
const [count, setCount] = React.useState(0);
// Use effect to create and clean up the interval
// (should only run once with current dependencies)
React.useEffect(() => {
// Create interval get the interval ID so it can be cleared later.
const intervalId = setInterval(() => {
// use the function based set state to avoid needing count as a dependency in the useEffect.
// this stops the need to code logic around stoping and recreating the interval.
setCount(currentCount => {
console.log(`count=${currentCount}`);
return currentCount + 1;
});
}, 1000);
// Create function to clean up the interval when the component unmounts.
return () => {
if (intervalId) {
clearInterval(intervalId);
}
}
}, [setCount]);
return <div></div>;
};
ReactDOM.render(<App />, document.getElementById('app'))
<script src="https://cdnjs.cloudflare.com/ajax/libs/react/17.0.1/umd/react.production.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/react-dom/17.0.1/umd/react-dom.production.min.js"></script>
<div id="app"></div>
If you need a more complex implementation as mention in your comment on another answer, you should try using a ref perhaps. For example, this is a custom interval hook I use in my projects. You can see there is an effect that updates callback if it changes.
This ensures you always have the most recent state values and you don't need to use the custom updater function syntax like setCount(count => count + 1).
const useInterval = (callback, delay) => {
const savedCallback = useRef()
useEffect(() => {
savedCallback.current = callback
}, [callback])
useEffect(() => {
if (delay !== null) {
const id = setInterval(() => savedCallback.current(), delay)
return () => clearInterval(id)
}
}, [delay])
}
// Usage
const App = () => {
useInterval(() => {
// do something every second
}, 1000)
return (...)
}
This is a very flexible option you could use. However, this hook assumes you want to start your interval when the component mounts. Your code example leads me to believe you want this to start based on the state change of the intervalSet boolean. You could update the custom interval hook, or implement this in your component.
It would look like this in your example:
const useInterval = (callback, delay, initialStart = true) => {
const [start, setStart] = React.useState(initialStart)
const savedCallback = React.useRef()
React.useEffect(() => {
savedCallback.current = callback
}, [callback])
React.useEffect(() => {
if (start && delay !== null) {
const id = setInterval(() => savedCallback.current(), delay)
return () => clearInterval(id)
}
}, [delay, start])
// this function ensures our state is read-only
const startInterval = () => {
setStart(true)
}
return [start, startInterval]
}
const App = () => {
const [countOne, setCountOne] = React.useState(0);
const [countTwo, setCountTwo] = React.useState(0);
const incrementCountOne = () => {
setCountOne(countOne + 1)
}
const incrementCountTwo = () => {
setCountTwo(countTwo + 1)
}
// Starts on component mount by default
useInterval(incrementCountOne, 1000)
// Starts when you call `startIntervalTwo(true)`
const [intervalTwoStarted, startIntervalTwo] = useInterval(incrementCountTwo, 1000, false)
return (
<div>
<p>started: {countOne}</p>
<p>{intervalTwoStarted ? 'started' : <button onClick={startIntervalTwo}>start</button>}: {countTwo}</p>
</div>
);
};
ReactDOM.render(<App />, document.getElementById('app'))
<script src="https://cdnjs.cloudflare.com/ajax/libs/react/17.0.1/umd/react.production.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/react-dom/17.0.1/umd/react-dom.production.min.js"></script>
<div id="app"></div>
The problem is the interval is created only once and keeps pointing to the same state value. What I would suggest - move firing the interval to separate useEffect, so it starts when the component mounts. Store interval in a variable so you are able to restart it or clear. Lastly - clear it with every unmount.
const App = () => {
const [count, setCount] = React.useState(0);
const [intervalSet, setIntervalSet] = React.useState(false);
React.useEffect(() => {
setIntervalSet(true);
}, []);
React.useEffect(() => {
const interval = intervalSet ? setInterval(() => {
setCount((c) => {
console.log(c);
return c + 1;
});
}, 1000) : null;
return () => clearInterval(interval);
}, [intervalSet]);
return null;
};
ReactDOM.render(<App />, document.getElementById("root"));
<script src="https://unpkg.com/react#16/umd/react.development.js"></script>
<script src="https://unpkg.com/react-dom#16/umd/react-dom.development.js"></script>
<div id="root"></div>
I have a problem trying to change a state in react using setInterval.
Why does the function "alert" always show a "null" message?
import React, { useEffect, useState } from 'react';
const Playlist = (props) => {
const [test, setTest] = useState("");
useEffect(() => {
setInterval(load, 3000)
}, []);
const load = async () => {
alert(test)
setTest("TEST");
}
return (
<div>
{test}
</div>
)
}
export default Playlist
Issue :
is callback passed into setInterval's closure and it accesses the test variable in the first render.
Solution :
You can clearInterval and setInterval as soon as there is any changes in your state
const { useState , useEffect , useCallback } = React;
const App = () => {
const [test, setTest] = useState("");
useEffect(() => {
const intrvl = setInterval(load, 3000);
return () => clearInterval(intrvl);
}, [test]);
const load = () => {
alert(test)
setTest("TEST");
}
return (
<div>
{test}
</div>
)
}
ReactDOM.render(<App />, document.getElementById('react-root'));
<script src="https://cdnjs.cloudflare.com/ajax/libs/react/16.8.4/umd/react.production.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/react-dom/16.8.4/umd/react-dom.production.min.js"></script>
<div id="react-root"></div>
The problem is that useEffect captures the test from the first render which is equal to empty string. We never re-apply the effect so the closure in setInterval always references the test from the first render.
NOTE: setInterval does not describe a process in time — once you set the interval, you can’t change anything about it except clearing it.
When we pass an empty [] array of dependencies to useEffect, it only runs the useEffect on mount and cleanup on unmount of the component.
One way is to add test to the array of dependencies so anytime there is an update on test, the useEffect will be called.
useEffect(() => {
const intervalId = setInterval(load, 3000);
return () => clearInterval(intervalId);
}, [test]);
But since we are using the test inside useEffect, we can just remove the empty [] array of dependencies so the useEffect will be called every time we are setting the state (component is re-rendered) and the new setInterval will be created with accessing to the latest test state like below
useEffect(() => {
const intervalId = setInterval(load, 3000);
return () => clearInterval(intervalId);
});
Another possible way is to update the state inside the interval's closure like
useEffect(() => {
const id = setInterval(() => {
setTest("TEST");
}, 3000);
return () => clearInterval(id);
});
const App = () => {
const [test, setTest] = useState("");
useEffect(() => {
const intervalId = setInterval(load, 3000);
return () => clearInterval(intervalId);
});
const load = () => {
alert(test)
setTest("TEST");
}
return (
<div>
{test}
</div>
)
}
Example of interval with setting count so you can see the state change
I've been trying to build an React app with multiple alerts that disappear after a set amount of time. Sample: https://codesandbox.io/s/multiple-alert-countdown-294lc
import React, { useState, useEffect } from "react";
import ReactDOM from "react-dom";
import "./styles.css";
function TimeoutAlert({ id, message, deleteAlert }) {
const onClick = () => deleteAlert(id);
useEffect(() => {
const timer = setTimeout(onClick, 2000);
return () => clearTimeout(timer);
});
return (
<p>
<button onClick={onClick}>
{message} {id}
</button>
</p>
);
}
let _ID = 0;
function App() {
const [alerts, setAlerts] = useState([]);
const addAlert = message => setAlerts([...alerts, { id: _ID++, message }]);
const deleteAlert = id => setAlerts(alerts.filter(m => m.id !== id));
console.log({ alerts });
return (
<div className="App">
<button onClick={() => addAlert("test ")}>Add Alertz</button>
<br />
{alerts.map(m => (
<TimeoutAlert key={m.id} {...m} deleteAlert={deleteAlert} />
))}
</div>
);
}
const rootElement = document.getElementById("root");
ReactDOM.render(<App />, rootElement);
The problem is if I create multiple alerts, it disappears in the incorrect order. For example, test 0, test 1, test 2 should disappear starting with test 0, test 1, etc but instead test 1 disappears first and test 0 disappears last.
I keep seeing references to useRefs but my implementations don't resolve this bug.
With #ehab's input, I believe I was able to head down the right direction. I received further warnings in my code about adding dependencies but the additional dependencies would cause my code to act buggy. Eventually I figured out how to use refs. I converted it into a custom hook.
function useTimeout(callback, ms) {
const savedCallBack = useRef();
// Remember the latest callback
useEffect(() => {
savedCallBack.current = callback;
}, [callback]);
// Set up timeout
useEffect(() => {
if (ms !== 0) {
const timer = setTimeout(savedCallBack.current, ms);
return () => clearTimeout(timer);
}
}, [ms]);
}
You have two things wrong with your code,
1) the way you use effect means that this function will get called each time the component is rendered, however obviously depending on your use case, you want this function to be called once, so change it to
useEffect(() => {
const timer = setTimeout(onClick, 2000);
return () => clearTimeout(timer);
}, []);
adding the empty array as a second parameter, means that your effect does not depend on any parameter, and so it should only be called once.
Your delete alert depends on the value that was captured when the function was created, this is problematic since at that time, you don't have all the alerts in the array, change it to
const deleteAlert = id => setAlerts(alerts => alerts.filter(m => m.id !== id));
here is your sample working after i forked it
https://codesandbox.io/s/multiple-alert-countdown-02c2h
well your problem is you remount on every re-render, so basically u reset your timers for all components at time of rendering.
just to make it clear try adding {Date.now()} inside your Alert components
<button onClick={onClick}>
{message} {id} {Date.now()}
</button>
you will notice the reset everytime
so to achieve this in functional components you need to use React.memo
example to make your code work i would do:
const TimeoutAlert = React.memo( ({ id, message, deleteAlert }) => {
const onClick = () => deleteAlert(id);
useEffect(() => {
const timer = setTimeout(onClick, 2000);
return () => clearTimeout(timer);
});
return (
<p>
<button onClick={onClick}>
{message} {id}
</button>
</p>
);
},(oldProps, newProps)=>oldProps.id === newProps.id) // memoization condition
2nd fix your useEffect to not run cleanup function on every render
useEffect(() => {
document.title = `You clicked ${count} times`;
}, [count]); // Only re-run the effect if count changes
finally something that is about taste, but really do you need to destruct the {...m} object ? i would pass it as a proper prop to avoid creating new object every time !
Both answers kind of miss a few points with the question, so after a little while of frustration figuring this out, this is the approach I came to:
Have a hook that manages an array of "alerts"
Each "Alert" component manages its own destruction
However, because the functions change with every render, timers will get reset each prop change, which is undesirable to say the least.
It also adds another lay of complexity if you're trying to respect eslint exhaustive deps rule, which you should because otherwise you'll have issues with state responsiveness. Other piece of advice, if you are going down the route of using "useCallback", you are looking in the wrong place.
In my case I'm using "Overlays" that time out, but you can imagine them as alerts etc.
Typescript:
// useOverlayManager.tsx
export default () => {
const [overlays, setOverlays] = useState<IOverlay[]>([]);
const addOverlay = (overlay: IOverlay) => setOverlays([...overlays, overlay]);
const deleteOverlay = (id: number) =>
setOverlays(overlays.filter((m) => m.id !== id));
return { overlays, addOverlay, deleteOverlay };
};
// OverlayIItem.tsx
interface IOverlayItem {
overlay: IOverlay;
deleteOverlay(id: number): void;
}
export default (props: IOverlayItem) => {
const { deleteOverlay, overlay } = props;
const { id } = overlay;
const [alive, setAlive] = useState(true);
useEffect(() => {
const timer = setTimeout(() => setAlive(false), 2000);
return () => {
clearTimeout(timer);
};
}, []);
useEffect(() => {
if (!alive) {
deleteOverlay(id);
}
}, [alive, deleteOverlay, id]);
return <Text>{id}</Text>;
};
Then where the components are rendered:
const { addOverlay, deleteOverlay, overlays } = useOverlayManger();
const [overlayInd, setOverlayInd] = useState(0);
const addOverlayTest = () => {
addOverlay({ id: overlayInd});
setOverlayInd(overlayInd + 1);
};
return {overlays.map((overlay) => (
<OverlayItem
deleteOverlay={deleteOverlay}
overlay={overlay}
key={overlay.id}
/>
))};
Basically: Each "overlay" has a unique ID. Each "overlay" component manages its own destruction, the overlay communicates back to the overlayManger via prop function, and then eslint exhaustive-deps is kept happy by setting an "alive" state property in the overlay component that, when changed to false, will call for its own destruction.
This question already has answers here:
State not updating when using React state hook within setInterval
(14 answers)
Closed 4 years ago.
The code is here: https://codesandbox.io/s/nw4jym4n0
export default ({ name }: Props) => {
const [counter, setCounter] = useState(0);
useEffect(() => {
const interval = setInterval(() => {
setCounter(counter + 1);
}, 1000);
return () => {
clearInterval(interval);
};
});
return <h1>{counter}</h1>;
};
The problem is each setCounter trigger re-rendering so the interval got reset and re-created. This might looks fine since the state(counter) keeps incrementing, however it could freeze when combining with other hooks.
What's the correct way to do this? In class component it's simple with a instance variable holding the interval.
You could give an empty array as second argument to useEffect so that the function is only run once after the initial render. Because of how closures work, this will make the counter variable always reference the initial value. You could then use the function version of setCounter instead to always get the correct value.
Example
const { useState, useEffect } = React;
function App() {
const [counter, setCounter] = useState(0);
useEffect(() => {
const interval = setInterval(() => {
setCounter(counter => counter + 1);
}, 1000);
return () => {
clearInterval(interval);
};
}, []);
return <h1>{counter}</h1>;
};
ReactDOM.render(
<App />,
document.getElementById('root')
);
<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="root"></div>
A more versatile approach would be to create a new custom hook that stores the function in a ref and only creates a new interval if the delay should change, like Dan Abramov does in his great blog post "Making setInterval Declarative with React Hooks".
Example
const { useState, useEffect, useRef } = React;
function useInterval(callback, delay) {
const savedCallback = useRef();
// Remember the latest callback.
useEffect(() => {
savedCallback.current = callback;
}, [callback]);
// Set up the interval.
useEffect(() => {
let id = setInterval(() => {
savedCallback.current();
}, delay);
return () => clearInterval(id);
}, [delay]);
}
function App() {
const [counter, setCounter] = useState(0);
useInterval(() => {
setCounter(counter + 1);
}, 1000);
return <h1>{counter}</h1>;
};
ReactDOM.render(
<App />,
document.getElementById('root')
);
<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="root"></div>
As another answer by #YangshunTay already shows, it's possible to make it useEffect callback run only once and work similarly to componentDidMount. In this case it's necessary to use state updater function due to the limitations imposed by function scopes, otherwise updated counter won't be available inside setInterval callback.
The alternative is to make useEffect callback run on each counter update. In this case setInterval should be replaced with setTimeout, and updates should be limited to counter updates:
export default ({ name }: Props) => {
const [counter, setCounter] = useState(0);
useEffect(() => {
const timeout = setTimeout(() => {
setCounter(counter + 1);
}, 1000);
return () => {
clearTimeout(timeout);
};
}, [counter]);
return <h1>{counter}</h1>;
};
The correct way to do this would be to run the effect only once. Since you only need to run the effect once because during mounting, you can pass in an empty array as a second argument to achieve.
However, you will need to change setCounter to use the previous value of counter. The reason is because the callback passed into setInterval's closure only accesses the counter variable in the first render, it doesn't have access to the new counter value in the subsequent render because the useEffect() is not invoked the second time; counter always has the value of 0 within the setInterval callback.
Like the setState you are familiar with, state hooks have two forms: one where it takes in the updated state, and the callback form which the current state is passed in. You should use the second form and read the latest state value within the setState callback to ensure that you have the latest state value before incrementing it.
function Counter() {
const [counter, setCounter] = React.useState(0);
React.useEffect(() => {
const timer = setInterval(() => {
setCounter(prevCount => prevCount + 1); // <-- Change this line!
}, 1000);
return () => {
clearInterval(timer);
};
}, []); // Pass in empty array to run effect only once!
return (
<div>Count: {counter}</div>
);
}
ReactDOM.render(<Counter />, 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>