State not updating when using React state hook within setInterval
useEffect
function is evaluated only once on component mount when empty input list is provided.
An alternative to setInterval
is to set new interval with setTimeout
each time the state is updated:
const [time, setTime] = React.useState(0);
React.useEffect(() => {
const timer = setTimeout(() => {
setTime(time + 1);
}, 1000);
return () => {
clearTimeout(timer);
};
}, [time]);
The performance impact of setTimeout
is insignificant and can be generally ignored. Unless the component is time-sensitive to the point where newly set timeouts cause undesirable effects, both setInterval
and setTimeout
approaches are acceptable.
As others have pointed out, the problem is that useState
is only called once (as deps = []
) to set up the interval:
React.useEffect(() => {
const timer = window.setInterval(() => {
setTime(time + 1);
}, 1000);
return () => window.clearInterval(timer);
}, []);
Then, every time setInterval
ticks, it will actually call setTime(time + 1)
, but time
will always hold the value it had initially when the setInterval
callback (closure) was defined.
You can use the alternative form of useState
's setter and provide a callback rather than the actual value you want to set (just like with setState
):
setTime(prevTime => prevTime + 1);
But I would encourage you to create your own useInterval
hook so that you can DRY and simplify your code by using setInterval
declaratively, as Dan Abramov suggests here in Making setInterval Declarative with React Hooks:
function useInterval(callback, delay) {
const intervalRef = React.useRef();
const callbackRef = React.useRef(callback);
// Remember the latest callback:
//
// Without this, if you change the callback, when setInterval ticks again, it
// will still call your old callback.
//
// If you add `callback` to useEffect's deps, it will work fine but the
// interval will be reset.
React.useEffect(() => {
callbackRef.current = callback;
}, [callback]);
// Set up the interval:
React.useEffect(() => {
if (typeof delay === 'number') {
intervalRef.current = window.setInterval(() => callbackRef.current(), delay);
// Clear interval if the components is unmounted or the delay changes:
return () => window.clearInterval(intervalRef.current);
}
}, [delay]);
// Returns a ref to the interval ID in case you want to clear it manually:
return intervalRef;
}
const Clock = () => {
const [time, setTime] = React.useState(0);
const [isPaused, setPaused] = React.useState(false);
const intervalRef = useInterval(() => {
if (time < 10) {
setTime(time + 1);
} else {
window.clearInterval(intervalRef.current);
}
}, isPaused ? null : 1000);
return (<React.Fragment>
<button onClick={ () => setPaused(prevIsPaused => !prevIsPaused) } disabled={ time === 10 }>
{ isPaused ? 'RESUME ⏳' : 'PAUSE ð§' }
</button>
<p>{ time.toString().padStart(2, '0') }/10 sec.</p>
<p>setInterval { time === 10 ? 'stopped.' : 'running...' }</p>
</React.Fragment>);
}
ReactDOM.render(<Clock />, document.querySelector('#app'));
body,
button {
font-family: monospace;
}
body, p {
margin: 0;
}
p + p {
margin-top: 8px;
}
#app {
display: flex;
flex-direction: column;
align-items: center;
min-height: 100vh;
}
button {
margin: 32px 0;
padding: 8px;
border: 2px solid black;
background: transparent;
cursor: pointer;
border-radius: 2px;
}
<script src="https://unpkg.com/[email protected]/umd/react.development.js"></script>
<script src="https://unpkg.com/[email protected]/umd/react-dom.development.js"></script>
<div id="app"></div>
Apart from producing simpler and cleaner code, this allows you to pause (and clear) the interval automatically by simply passing delay = null
and also returns the interval ID, in case you want to cancel it yourself manually (that's not covered in Dan's posts).
Actually, this could also be improved so that it doesn't restart the delay
when unpaused, but I guess for most uses cases this is good enough.
If you are looking for a similar answer for setTimeout
rather than setInterval
, check this out: https://stackoverflow.com/a/59274757/3723993.
You can also find declarative version of setTimeout
and setInterval
, useTimeout
and useInterval
, a few additional hooks written in TypeScript in https://www.npmjs.com/package/@swyg/corre.
The reason is because the callback passed into setInterval
's closure only accesses the time
variable in the first render, it doesn't have access to the new time
value in the subsequent render because the useEffect()
is not invoked the second time.
time
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.
Bonus: Alternative Approaches
Dan Abramov, goes in-depth into the topic about using
setInterval
with hooks in his blog post and provides alternative ways around this issue. Highly recommend reading it!
function Clock() {
const [time, setTime] = React.useState(0);
React.useEffect(() => {
const timer = window.setInterval(() => {
setTime(prevTime => prevTime + 1); // <-- Change this line!
}, 1000);
return () => {
window.clearInterval(timer);
};
}, []);
return (
<div>Seconds: {time}</div>
);
}
ReactDOM.render(<Clock />, document.querySelector('#app'));
<script src="https://unpkg.com/[email protected]/umd/react.development.js"></script>
<script src="https://unpkg.com/[email protected]/umd/react-dom.development.js"></script>
<div id="app"></div>