score:0
you may need to add the dependency for count
in useeffect
. currently useeffect
is only called on the mount and is not called after that (i.e when the count value changes).
so it always says 0
because useeffect
is executed only once ( on mount ) and that time the count value is set to 0
. and thus it every time it logs 0 on setinterval
due to closure.
i have updated the code sandbox to find the reason and meaningful logs. here is the sandbox link: https://codesandbox.io/s/admiring-thompson-uz2xe
how to find closure value: you can check the logs and traverse through prototype object to find [[scopes]]
and you will get the values as seen in the below screenshot:
this would work:
react.useeffect(() => {
const intervalid = setinterval(() => {
setcount(count + 1);
console.log(count);
}, 1000);
return () => clearinterval(intervalid);
}, [count]);
you can check this doc: https://reactjs.org/docs/hooks-reference.html#conditionally-firing-an-effect
you can read this as well: you can read this as well: https://overreacted.io/making-setinterval-declarative-with-react-hooks/
hope this helps!
score:0
if you removed second params for useeffect your application will be rendered always if u have some change in state, but this is bad practice. you need in second params choose for wich parametrs you need watch ...
example with choosen params:
react.useeffect(() => {
const intervalid = setinterval(() => {
setcount(count + 1);
console.log(count);
}, 1000);
return () => clearinterval(intervalid);
}[count]);
without
react.useeffect(() => {
const intervalid = setinterval(() => {
setcount(count + 1);
console.log(count);
}, 1000);
return () => clearinterval(intervalid);
});
score:0
because you didn't add count
to useeffect's dependences, then inside the effect count always is 0
.
you should use usereducer
to resolve your problem:
function useeffectbugcounter() {
const [count, dispatchcount] = react.usereducer((state, { type }) => {
switch(type) {
case 'inc':
return state + 1
default:
return state
}
}, 0);
react.useeffect(() => {
const intervalid = setinterval(() => {
dispatchcount({ type: 'inc'})
}, 1000);
return () => clearinterval(intervalid);
}, []);
return <div>the count is: {count}</div>;
}
score:0
you need to pass count instead of blank array in useeffect
function useeffectbugcounter() {
const [count, setcount] = react.usestate(0);
react.useeffect(() => {
const intervalid = setinterval(() => {
setcount(count + 1);
console.log(count);
}, 1000);
return () => clearinterval(intervalid);
},[count]);
return <div>the count is: {count}</div>;
}
score:2
you can use callback for set state to use latest counter value:
setcount(count => (count + 1));
Source: stackoverflow.com
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