This.setState() is working for almost all variables in my component, but not all.
When i try to set the state of a state variable the value does not change which is a very strange behavior.
this.setState({ notSaved: size }, () => {
console.log(this.state.notSaved, 'notSaved','size',size);
});
}
Result of the console.log
This is not a problem with the async behavior of the setSate, as you can see in the image below, because even if I console.log the value inside the callback, still unchanged. So this is not a duplicated question.
setState(updater[, callback])
use callback in setState to read new value. setState is async and you are expecting your state to be updated instantly. To make sure your state got updated, you would have to leverage on call back function passed in as second argument.
this.setState((state, props) => {
return { notSaved: size }
}, () => console.log(this.state.notSaved, 'updated notSaved'));
Detailed explanation by Dan on why is setState asynchronous?: https://github.com/facebook/react/issues/11527#issuecomment-360199710
Related
I'm facing with weird bug related to setState. I know that it is asynchronous, thus I added console.log(this.state) in the callback function of setState, but state remains the same, and no changes are reflected on app. Here is code:
if (response && response.data && response.data.data) {
if (this._isMounted) {
const arrangedSenderEmailAddresses = arrangeEmailAddresses(response.data.data);
console.log('update data: ', arrangedSenderEmailAddresses);
this.setState({ emailAddresses: arrangedSenderEmailAddresses }, () => {
console.log('state after update: ', this.state.emailAddresses);
});
}
}
response is object retrieved from backend.
console.log('update data: ', arrangedSenderEmailAddresses) shows new data that should be set on state, but in callback:
console.log('state after update: ', this.state.emailAddresses); shows old state.
Did anyone face with such bug?
Even in the callback it will be using the previous value of state, as you are passing in this.state to the console log, which doesn't get re-evaluated in that callback, but gets given the old state.
From the React docs:
The second parameter to setState() is an optional callback function that will be executed once setState is completed and the component is re-rendered. Generally we recommend using componentDidUpdate() for such logic instead.
First let me put my code here and I'll explain what's happening.
Parent.js
callback = (id) => {
this.setState({des: id});
console.log(this.state.des);
}
//on my render i have this when i call my child component
<Child callback={this.callback}/>
Child.js
handleChange = (event) => {
let des = event.target.value;
console.log(des);
this.props.callback(des);
};
When i console.logon my Child component, it returns the data that i want to pass, but when I do it in calbackon my Parent component, it returns <empty string> and i don't know why that's happening.
The reason this is happening is because setState is an async function. When you are trying to log this.state.des, the state would not have been set yet. If you want to console log your state to see if it has been set as expected, what you want to do is log it in the callback of this.setState (so it logs once we know state is set). Try something like the following in your parent.js :
callback = (id) => {
this.setState({des: id}, () => {
console.log(this.state.des);
});
}
see the React Docs for setState for more details
The call to setState is asynchronous and therefore you might not read the updated state if you are accessing it directly after calling setState. Because of this setState(updater[, callback]) actually exposes a callback which can be used for operations which depend on the state update being done. (This is explained in the react docs for setState.)
In your case, adjusting the callback function like this
callback = (id) => {
this.setState({des: id}, () => {
console.log(this.state.des);
});
}
should do the trick.
If you want to know more about the reasoning behind setState being asynchronous (even if it might be a bit confusing in the beginning, like in your case) you should check out this github issue and especially this comment.
I am checking a condition and if that condition holds true then through function a want to change the state. i tries but i console it is still showing false only. it will be really helpful if you suggest good way to to it,
this.state = {
popUpStatus:false,
}
handlePopUp = () => {
this.setState({
popUpStatus: true,
});
}
if(a > 100){
this.handlePopUp;
console.log(popUpStatus)l // false
}
this.setState() is an asynchronous call. Therefore, You should not check this.state.popUpStat value right after calling this.setState().
The correct way to do so is passing a callback function as a second parameter in this.setState() call like
this.setState({
popUpStatus: true,
},()=>console.log(this.state.popUpStatus))
setState function does not update the state immediately. It just enqueues the changes. popUpStatus will have a different value once the component is re-rendered.
"setState() enqueues changes to the component state and tells React that this component and its children need to be re-rendered with the updated state. Think of setState() as a request rather than an immediate command to update the component....."
You can read more on setState here https://reactjs.org/docs/react-component.html#setstate
To test it, you can log it in your render function, and check how it updates.
You have to display this.state.popUpStatus
Replace console.log(popUpStatus) with console.log(this.state.popUpStatus)
Try to do this:
this.handlePopUp();
console.log(this.state.popUpStatus);
But consider that setState is async function so you have to do this:
this.setState({
popUpStatus: true,
}, ()=> { console.log(this.state.popUpStatus); });
Perhaps with hooks should be better:
useEffect(()=> { setPopUpStatus(a>100); }, [a])
How do I call setState with the previous state plus an additional value in a controlled component?
The typical example is:
Increment() {
this.setState((prevState, props) => ({counter: prevState.counter = props.step})
}
However, how do you update the state based on the prevState and a value from a handler.
For example (and I know this example is wrong, since you cannot pass value into the setState callback):
HandleOnAddItem(evt, { value }) {
this.setState((prevState, props, value) -> ({items: value, ...prevState.items})
}
If you are passing that value in event handler function, then it will be available inside setState without passing into updater function.
Write it like this:
HandleOnAddItem(evt, { value }) {
this.setState((prevState, props) => ({items: value, ...prevState.items}))
}
If you are updating only one state property that doesn't dependent on previous state, as i think in your case, then directly you can write:
HandleOnAddItem(evt, { value }) {
this.setState({ items: value })
}
Check this answer for more details: How do JavaScript closures work?
Example:
//setTimeout will call the method after 2sec, similar to setState
let abc = a => {setTimeout(() => console.log('a', a), 2000)};
abc(10);
Because of javascript closures, inner functions have access to the variables of outer functions, a value passed as parameter on to the handler function is available in the setState callback. You can use it like
HandleOnAddItem(evt, { value }) {
this.setState((prevState, props) -> ({items: value, ...prevState.items})
}
When a react component state changes, the render method is called. Hence for any state change, an action can be performed in the render methods body. Is there a particular use case for the setState callback then?
Yes there is, since setState works in an asynchronous way. That means after calling setState the this.state variable is not immediately changed. so if you want to perform an action immediately after setting state on a state variable and then return a result, a callback will be useful
Consider the example below
....
changeTitle: function changeTitle (event) {
this.setState({ title: event.target.value });
this.validateTitle();
},
validateTitle: function validateTitle () {
if (this.state.title.length === 0) {
this.setState({ titleError: "Title can't be blank" });
}
},
....
The above code may not work as expected since the title variable may not have mutated before validation is performed on it. Now you may wonder that we can perform the validation in the render() function itself but it would be better and a cleaner way if we can handle this in the changeTitle function itself since that would make your code more organised and understandable
In this case callback is useful
....
changeTitle: function changeTitle (event) {
this.setState({ title: event.target.value }, function() {
this.validateTitle();
});
},
validateTitle: function validateTitle () {
if (this.state.title.length === 0) {
this.setState({ titleError: "Title can't be blank" });
}
},
....
Another example will be when you want to dispatch and action when the state changed. you will want to do it in a callback and not the render() as it will be called everytime rerendering occurs and hence many such scenarios are possible where you will need callback.
Another case is a API Call
A case may arise when you need to make an API call based on a particular state change, if you do that in the render method, it will be called on every render onState change or because some Prop passed down to the Child Component changed.
In this case you would want to use a setState callback to pass the updated state value to the API call
....
changeTitle: function (event) {
this.setState({ title: event.target.value }, () => this.APICallFunction());
},
APICallFunction: function () {
// Call API with the updated value
}
....
this.setState({
name:'value'
},() => {
console.log(this.state.name);
});
The 1. usecase which comes into my mind, is an api call, which should't go into the render, because it will run for each state change. And the API call should be only performed on special state change, and not on every render.
changeSearchParams = (params) => {
this.setState({ params }, this.performSearch)
}
performSearch = () => {
API.search(this.state.params, (result) => {
this.setState({ result })
});
}
Hence for any state change, an action can be performed in the render methods body.
Very bad practice, because the render-method should be pure, it means no actions, state changes, api calls, should be performed, just composite your view and return it. Actions should be performed on some events only. Render is not an event, but componentDidMount for example.
Consider setState call
this.setState({ counter: this.state.counter + 1 })
IDEA
setState may be called in async function
So you cannot rely on this. If the above call was made inside a async function this will refer to state of component at that point of time but we expected this to refer to property inside state at time setState calling or beginning of async task. And as task was async call thus that property may have changed in time being. Thus it is unreliable to use this keyword to refer to some property of state thus we use callback function whose arguments are previousState and props which means when async task was done and it was time to update state using setState call prevState will refer to state now when setState has not started yet. Ensuring reliability that nextState would not be corrupted.
Wrong Code: would lead to corruption of data
this.setState(
{counter:this.state.counter+1}
);
Correct Code with setState having call back function:
this.setState(
(prevState,props)=>{
return {counter:prevState.counter+1};
}
);
Thus whenever we need to update our current state to next state based on value possed by property just now and all this is happening in async fashion it is good idea to use setState as callback function.
I have tried to explain it in codepen here CODE PEN
Sometimes we need a code block where we need to perform some operation right after setState where we are sure the state is being updated. That is where setState callback comes into play
For example, there was a scenario where I needed to enable a modal for 2 customers out of 20 customers, for the customers where we enabled it, there was a set of time taking API calls, so it looked like this
async componentDidMount() {
const appConfig = getCustomerConfig();
this.setState({enableModal: appConfig?.enableFeatures?.paymentModal }, async
()=>{
if(this.state.enableModal){
//make some API call for data needed in poput
}
});
}
enableModal boolean was required in UI blocks in the render function as well, that's why I did setState here, otherwise, could've just checked condition once and either called API set or not.