i am calling console.log just below the setState but it showing my blank array even state has been changed.
let text = reminderText;
let data = [...this.state.reminderText]
data.push(text);
this.setState({reminderText:data})
console.log(this.state.reminderText);
Note that setState() is asynchronous. That means, when your console log is executed, the state is not updated yet.
If you want to print the updated state, use the callback function as shown below.
this.setState(
{reminderText:data},
() => console.log(this.state.reminderText)
)
React will invoke the callback function soon after the asynchronous state update is complete.
Related
I have added the below piece of code in my react project to set the state with the response of a method. But the state never updated. It is always null. May i know how to set the state here
_onload():any{
this._DataAccessObj = new DataAccess();
let result = this._DataAccessObj.getRequest(this.props.itemId).then((item:IRequest) =>{
console.log("item");
console.log(item);
this.setState ({
Request: item
});
console.log(this.state.Request);
console.log("setstate");
});
return;
}
From the documentation of ReactJS and lifecycle:
Every second the browser calls the tick() method. Inside it, the Clock
component schedules a UI update by calling setState() with an object
containing the current time. Thanks to the setState() call, React
knows the state has changed, and calls the render() method again to
learn what should be on the screen. This time, this.state.date in the
render() method will be different, and so the render output will
include the updated time. React updates the DOM accordingly.
The last part is important: The actual state object will be update later on the line, not directly when the setState method has been called.
On the same page:
React may batch multiple setState() calls into a single update for
performance.
Because this.props and this.state may be updated asynchronously, you
should not rely on their values for calculating the next state.
componentWillMount() {
let dID=this.props.match.params.datumID;
this.setState({datumID: dID});
console.log(dID);
console.log(this.state.datumID);
}
I'm just trying use setState() method.
But the problem is it is not working.
Here is what I get as output:
First console.log() : 66
Second console.log(): null
this.setState accepts a second argument as callback which is fired after the state is successfully changed.
componentWillMount() {
let dID=this.props.match.params.datumID;
console.log(dID);
this.setState({datumID: dID},()=>console.log(this.state.datumID));
}
side note : Move your code to componentDidMount.
componentWillMount() is invoked immediately before mounting occurs. It
is called before render(), therefore setting state in this method
will not trigger a re-render. Avoid introducing any side-effects or
subscriptions in this method.
Think of setState() as a request rather than an immediate command to update the component. For better perceived performance, React may delay it, and then update several components in a single pass. React does not guarantee that the state changes are applied immediately.
If you need to set the state based on the previous state,
read about the updater argument below.
this.setState((prevState, props) => {
return {datumID: props.match.params.datumID};
});
Because SetState() is an asynchronous function, use below code
componentWillMount() {
let dID=this.props.match.params.datumID;
this.setState({datumID: dID},
function(){
console.log(this.state.datumID)});
console.log(dID);
}
and if possible try to avoid compenentWillMount since it would be deprecated soon
setState() being an asynchronous function it is in progress of updating the state and javascript being single-threaded language it will execute the next line of code. So if you want to see the state value set you have to do something like this.
componentDidMount=()=> {
let dID=this.props.match.params.datumID;
this.setState({
datumID: dID
},()=>{
console.log(dID);
console.log(this.state.datumID);
});
}
Also I would recommend you to use componentDidMount as willMount won't work in the future as it is deprecated
this.setState is an asynchronous function that takes time and passes the newly assigned
state value over all of the React's life-cycle functions in order to update a state and trigger the re rendering of a component. Your first console logs out the value that is assigned to set state and the next console logs out the value that is currently residing in the state this.state.datumID. if you need to perform operations after setting state you can pass a callback that is triggered after a state is successfully updated.
this.setState({datumID: dID}, () => console.log(this.state.datumID) ); // this will log the updated value
Another thing I want to point out is componentWillMount will wont work in the future release. If you want to do something after DOM is rendered, use componentDidMount or you can perform pre-render task in the constructor of your Class Component.
I really don't understand the react life cycle. I run console.log in componentDidUpdate() and saw that it ran multiple times
Console showed that componentDidUpdate ran 3 times
Maybe I can give you an extra example when your issue happens since I cannot see your code.
componentDidUpdate(prevProps, prevState) {
const { something } = this.props;
if (prevProps.something !== something) this.apiCall();
console.log('something')
}
when you change your state or props, componentDidUpdate is being invoked, and apiCall function makes http request via fetch or axios, and change the state twice using setState function.
whenever state gets changed, new render() is being invoked and componentDidUpdate follows.
but the condition
if (prevProps.something !== something) this.apiCall();
may not be satisfied anymore, just console logging at this time instead of calling apiCall function which would trigger inifinite loop.
hope it helps.
componentDidUpdate() is invoked immediately after updating occurs.
your problem may be accorded because the state has been changed, props received or parent re-rendred.
if you are new with React, I recommend you to read the following article:
Post-Render with componentDidUpdate()
I want the getKey function to wait for the updated state from friendExist.
Currently getKey is set to false as the initial state however friendExist should set the boolean to "true". But it seems that "this.props.getKey(this.props.exist)" dosen't wait "this.props.friendExist(uid)" to update before it runs the code.
componentDidMount(){
const uid = 'MZdKuFpGmGRntb0nyF4PO0f6kco1';
this.props.friendExist(uid);
//friendExist should update this.props.exist to true
this.props.getKey(this.props.exist)
}
Both friendExist and getKey are actions that dispatch the new state.
Your function componentDidMount is a synchronous function, so it will execute inside code without waiting for any unrelated functions. Unless you change its type to asynchronous, and tell exactly how you want it to wait, this.props.exist will never be true at the time this.props.getKey is executed.
An alternative, simpler solution is taking the update of this.props.exist at componentWillReceiveProps, and execute this.props.getKey in there instead
componentDidMount(){
const uid = 'MZdKuFpGmGRntb0nyF4PO0f6kco1';
this.props.friendExist(uid);
}
componentWillReceiveProps(nextProps, nextState) {
if (nextProps.exist) {
this.props.getKey(nextProps.exist)
}
}
I assume you are using react-redux mapStateToProps() to get the updated state. This behavior is expected as react-redux uses react's setState() internally, which is asynchronous, meaning your state may not map to the props on the same time you dispatch the action.
I suggest creating a function getKeyIfFriendExist() instead which does the dispatching and checking of the latest state inside.
After the first date selection the selected date is not displayed on console and delayed and only set on state on the next selection.
This is the link to my code.
go to sample code
Am i missing something? I expect it to display on console on the first selection.
setState is asynchronous.
If you call setState and immediately use this.state, chances are it will not be updated yet.
If you want to set the state and immediately act on that change, you can pass in a callback function.
So, in your case his code will work:
handleFromChange(from) {
this.setState({ from }, () => console.log(this.state));
}
Docs:
setState() does not immediately mutate this.state but creates a
pending state transition. Accessing this.state after calling this
method can potentially return the existing value. There is no
guarantee of synchronous operation of calls to setState and calls may
be batched for performance gains.