This question already has answers here:
Infinite loop in useEffect
(16 answers)
Closed 3 years ago.
I am new to react hooks and as i read in docs useEffect runs on every update of component and on first render also (so it is componentDidMount and componentDidUpdate combined) but when i am fetching data inside useEffect it will re-do it when state updates and so if I set state value after fetching data from server it will create infinite loop. and i want to prevent it and only fetch it for first render like in componentDidMount, how can i implement this?
useEffect() takes a dependencies as an array parameter. This dependency(ies) acts as a decider such that, if one of the dependencies' value change, useEffect is invoked.
It's effectively equivalent to saying:
shouldComponentUpdate(nextProps, nextState) {
// If props or state changes, update the component
if(this.props.someProp != nextProps.someProp || this.state.someState != nextState.someState){
return true;
}
return false;
}
With useEffect() :
useEffect(() => {
//Do stuff here
}, [somePropValue, someStateValue]);
But to say that only do once, specify an empty dependency array(ideal for API calls that happens once on load):
useEffect(() => {
//Do stuff here
}, []);
Related
This question already has answers here:
ReactJs componentDidMount executes twice
(2 answers)
Closed 3 months ago.
I am calling an api using axios inside componentDidMount and the api is being called multiple times.
Here is my code
export default class Listings extends Component{
state = {
listings: null
}
//when the component is mounted to dom first
componentDidMount() {
this.getAllCryptoListings();
}
getAllCryptoListings() {
let response = null;
axios.get('http://localhost:8000/')
.then( (res) => {
response = res;
console.log(response.data.data);
})
.catch( (error) => console.log(error) )
.finally( () => { this.setState({listings: response.data.data}) } )
}
}
Normally I expected the code to run one time only as per the description of the function here componentDidMount.
The documentation says
You may call setState() immediately in componentDidUpdate() but note that it must be wrapped in a condition like in the example above, or you’ll cause an infinite loop. It would also cause an extra re-rendering which, while not visible to the user, can affect the component performance.
How to make the code inside componentDidMount run only once?
componentDidMount() is called twice if you setState in it, you won't be able to disable this.
You can make your call in componentWillMount, but this is UNSAFE so be careful using it! (not recommended)
If this does not work you should check where your component is called. Maybe his parents are re-rendering and so calling the render to verify twice with the initial state.
You can read more about it here
In React version 18, a change was made to strict mode so that components will mount, then unmount, then mount again. This was added to help us all start catching issues that will affect an upcoming feature. In a future version of react, the state will be able to be preserved between unmounts, and as a result, components may mount multiple times.
This question already has answers here:
The useState set method is not reflecting a change immediately
(15 answers)
Closed 5 months ago.
This is my code:
const [address, setAddress] = useState("1");
const updateData = () => {
setAmount("2");
console.log(address);
}
After updateData, why printed 1? I changed it to 2.
Setting the state in React acts like an async function.
Meaning that the when you set the state and put a console.log right after it, it will likely run before the state has actually finished updating.
Which is why we have useEffect, a built-in React hook that activates a callback when one of it's dependencies have changed.
Example:
useEffect(() => {
console.log(address)
// Whatever else we want to do after the state has been updated.
}, [address])
This console.log will run only after the state has finished changing and a render has occurred.
Note: "address" in the example is interchangeable with whatever other state piece you're dealing with.
Check the documentation for more info about this.
I'm using a functional component as a child with props from parent. In the props i have a value array in which i'm getting as empty([]) inside useEffect and after some time, the same is getting rendered in the UI. I'm using useEffect to call a function and set the state only once as below and i want it to be like that. Here value is the props and checkEmpty is the function i'm checking and setting the state, completed. Is there any way to invoke the function inside useEffect once the value array is filled. But i want the useEffect to be invoked only once and needs to change the state completed with out letting it to an infinite loop.
useEffect(() => {
checkEmpty(value)?setCompleted(false):setCompleted(true)
}, [])
You can pass the value as a dependency of useEffect, and the hook only run when the value is changed.
useEffect(() => {
checkEmpty(value) ? setCompleted(false) : setCompleted(true);
}, [value])
So, I'm using hooks to manage the state of a set of forms, set up like so:
const [fieldValues, setFieldValues] = useState({}) // Nothing, at first
When setting the value, the state doesn't update:
const handleSetValues = values => {
const _fieldValues = {
...fieldValues,
...values
}
setFieldValues(_fieldValues)
console.log(fieldValues) // these won't be updated
setTimeout(() => {
console.log(fieldValues) // after ten seconds, it's still not updated
},10000)
}
If I call the function a second time, it'll have updated, but that's not gonna work for me.
I never saw behaviour like this in class components.
Is it meant to... like, not update? Or just update whenever it feels like it? Really confusing behaviour.
setFieldValues(_fieldValues) is an async call, means you won't able to get the result in the very next line of this.
You can use useEffect hook.
useEffect(() => {
// do your work here
}, [fieldValues]);
It seems from your question that you have background of Class components of React, so useEffect is similar to componentDidMount and componentDidUpdate lifecycle methods.
useEffect calls whenever the state in the dependency array (in your case [fieldValues]) changes and you get the updated value in useEffect body.
You can also perform componentWillUnmount work in useEffect as well.
Have a brief guide.
setFieldValues is an asynchronous function, so logging the value below the statement will not have any effect.
Regarding using setTimeout, the function would capture the current value of props being passed to it and hence that would be the value printed to the console. This is true to any JS function, see the snippet below:
function init(val) {
setTimeout(() => {
console.log(val);
}, 1000);
}
let counterVal = 1;
init(counterVal);
counterVal++;
So how can we print the values when the value changes? The easy mechanism is to use a useEffect:
useEffect(() => {
console.log(fieldValues)
}, [fieldValues]);
This question already has answers here:
The useState set method is not reflecting a change immediately
(15 answers)
Closed 2 years ago.
I have various functions in React where I need to change the state with useState then do some action conditional on whether the new state meets some criteria.
This uses a setValues method in useState to set the value of newpassword when handleChange is called with prop="newpassword". The new password is then evaluated with a regex test, and if it is valid the state variable passwordIsValid should be set to true.
const handleChange = prop => event => {
setValues({ ...values, [prop]: event.target.value })
if (prop === 'newpassword' && passwordValidation.test(values.newpassword)) {
setValues({ ...values, passwordisValid: true })
console.log(prop, values.passwordisValid)
} else {
console.log(prop, values.passwordisValid)
}
}
The state is always one step behind tho - and I know this is because useState is async, but I don't know how to use useEffect to check the state? Very new to hooks, could someone help me out?
useState() hook is just a function call. It returns value and function pair. values is just a constant it doesn't have any property binding.
// Think of this line
const [values, setValues] = useState(0);
// As these two lines
const values = 0;
const setValues = someFunction;
When you call setValues react updates value for the next render. Next time component renders, it will call useState again which will return new values.
As a solution you should use event.target.value. You don't want to use it directly though because event.target is nullified after you observe it.
const newValue = event.target.value
// use newValue to setValues etc
Inside any particular render, props and state forever stay the same and Every function inside the component render (including event handlers, effects, timeouts or API calls inside them) captures the props and state of the render call that defined it. For that reason if you try to access values.newPassword in your event handler you will always get the state for that particular render i.e the old password.
Just think of useState as a function that returns the state for that particular render and that state is immutable for that particular render.