I just want to understand why in the application I have following situation, below is my constructor of class component:
constructor(props) {
super(props);
this.state = {
tableAlerts: props.householdAlerts,
initialAlerts: props.householdAlerts
}
console.log('householdAlerts', props.householdAlerts)
}
in render function I have:
const { householdAlerts } = this.props;
My issue is that in constructor I got empty array, but in render funtion I have the data. Is it possible to get the data in constructor?
This is a very bad pattern when using the class component. You are ignoring any props updates when you copy the value into state. to manage it:
It requires you to manage two sources of data for the same variable: state and props. Thus, you need to add another render each time your prop change by setting it into state (don't forget to test on equality from prev and next values to avoid being in an infinite loop).
You can avoid setting the state each time your props change by using the getderivedstatefromprops lifecycle method.
So the recommendation is: just use the props; do not copy props into state.
To learn more why you shouldn't, I highly recommend this article.
It is not recommended to set your initial component state in the constructor like so because you gonna lose the ability to use { setState } method after to update this property/state.
The best practice is indeed to refer directly to the prop with { this.prop.householdAlerts }, and keep the state usage for local (or in child components} cases.
if anyhow you want to store props in component state for some reason, call it in lifeCycle -
componentDidMount() {
const { tableAlerts, initialAlerts } = this.props;
this.setState({ tableAlerts, initialAlerts });
}
Hagai Harari is right. Nevertheless, your actual problem seems to be that during your initial rendering the array is empty. Can you ensure that the array has some items, when your component is rendered for the first time?
First rendering -> calls constructor
<YourComponent householdAlerts={[]} />
Second rendering -> updates component
<YourComponent householdAlerts={[alert1, alert2, alert3]} />
If you want initial state to have the prop value.Try something like this with 'this' keyword
constructor(props) {
super(props);
this.state = {
tableAlerts: this.props.householdAlerts,
initialAlerts: this.props.householdAlerts
}
console.log('householdAlerts', props.householdAlerts)
}
In React, does setState always assign a new object to this.state?
In other words, when you call:
this.setState({
key1: val1,
key2: val2
});
does it always merge the current state object with the new properties into a new object, queueing an operation which is functionally equivalent to the following?
this.state = {
...this.state,
key1: val1,
key2: val2
};
The reason I'm asking is that I'd like to know whether this.state !== nextState is always guaranteed to be true inside shouldComponentUpdate() if the update was triggered by setState.
Thanks.
The simple answer to your question will be "Yes", but for one update cycle. Let me explain this.
As you may already know React setState does not always immediately update the component. Sometimes React batch or defer the updates until later. As an example, let's assume you have something like follows in your event handler.
onClick() {
this.setState({quantity: state.quantity + 1});
this.setState({quantity: state.quantity + 1});
this.setState({quantity: state.quantity + 1});
}
These state update will be queued and execute all together in one update cycle. In such scenario React only create a new state object for the first setState and that object will mutate for subsequent setState updates. You can clearly see this in the source code here.
However, this is something totally about how React works under the hood and we won't need to worry about that. Because there will be only one shouldComponentUpdate() and componentDidUpdate() for one cycle. By the time we access the state it will be always a new object. So we can safely assume that setState() guaranteed to create a new state object every time. But make sure that you aware of other implications of setState which explains in the official documentation.
I'd like to know whether this.state !== nextState is always guaranteed
to be true inside shouldComponentUpdate().
The answer to your this question will be "No" as also explained in other answers. shouldComponentUpdate() will be called because of either state change, props change or both. So this.state !== nextState won't true if the component has updated only because of props change.
I think you want to know the answer to this question.
Will this.state !== nextState is always guaranteed to be true inside
shouldComponentUpdate() ?
According to react docs shouldComponentUpdate will be called on three instance
state changed
props changed
state and props changed
So if only props are changing, your current state will be equal the last unmodified state.
shouldComponentUpdate() is invoked before rendering when new props or state are being received. Defaults to true. This method is not called for the initial render or when forceUpdate() is used.
The first question that you asked are depends on your style of data management for your state.
React lets you use whatever style of data management you want, including mutation.
And the docs also states that
shallow merge of stateChange into the new state, e.g., to adjust a shopping cart item quantity
But looking at the internal parts of setState beginUpdateQueue method let me doubtful when state gets mutated.
I think it would be wise to use immutability-helper for this part of the code, if you must guarantee that no mutation happens to the data.
I guess this is a simple question but here goes: Is there a way of instructing your component to re-render when the props change? I have my component connected to the redux store which on a certain action updates the state, which then filters down into the props, upon which point I want the app to respond to the changes.
I guess you would use componentDidUpdate()?
Something like:
// ...
componentDidUpdate(prevProps, prevState) {
if (prevProps !== this.props) {
// re-render x <-- not sure how to do this
}
}
Plus what would be the method to re-render the whole component, and what would be the method of re-rendering only the updated props?
Any help appreciated.
You can call this.forceUpdate() in the method. Another way is this.setState(this.state).
You can use
componentWillReceiveProps (props){
this.doSomething(props) // To some function.
this.setState({data: props}) // This will update your component.
}
If your props is something that needs to change the state you will get an infinity loop.
Check out Reacts life cycle here by the way, might help aswell.
This question already has answers here:
Why does calling react setState method not mutate the state immediately?
(9 answers)
The useState set method is not reflecting a change immediately
(15 answers)
Closed 8 months ago.
I would like to ask why my state is not changing when I do an onClick event. I've search a while ago that I need to bind the onClick function in constructor but still the state is not updating.
Here's my code:
import React from 'react';
import Grid from 'react-bootstrap/lib/Grid';
import Row from 'react-bootstrap/lib/Row';
import Col from 'react-bootstrap/lib/Col';
import BoardAddModal from 'components/board/BoardAddModal.jsx';
import style from 'styles/boarditem.css';
class BoardAdd extends React.Component {
constructor(props) {
super(props);
this.state = {
boardAddModalShow: false
};
this.openAddBoardModal = this.openAddBoardModal.bind(this);
}
openAddBoardModal() {
this.setState({ boardAddModalShow: true }); // set boardAddModalShow to true
/* After setting a new state it still returns a false value */
console.log(this.state.boardAddModalShow);
}
render() {
return (
<Col lg={3}>
<a href="javascript:;"
className={style.boardItemAdd}
onClick={this.openAddBoardModal}>
<div className={[style.boardItemContainer,
style.boardItemGray].join(' ')}>
Create New Board
</div>
</a>
</Col>
);
}
}
export default BoardAdd
Your state needs some time to mutate, and since console.log(this.state.boardAddModalShow) executes before the state mutates, you get the previous value as output. So you need to write the console in the callback to the setState function
openAddBoardModal() {
this.setState({ boardAddModalShow: true }, function () {
console.log(this.state.boardAddModalShow);
});
}
setState is asynchronous. It means you can’t call it on one line and assume the state has changed on the next.
According to React 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.
Why would they make setState async
This is because setState alters the state and causes rerendering. This
can be an expensive operation and making it synchronous might leave
the browser unresponsive.
Thus the setState calls are asynchronous as well as batched for better
UI experience and performance.
Fortunately setState() takes a callback. And this is where we get updated state.
Consider this example.
this.setState({ name: "myname" }, () => {
//callback
console.log(this.state.name) // myname
});
So When callback fires, this.state is the updated state.
You can get mutated/updated data in callback.
For anyone trying to do this with hooks, you need useEffect.
function App() {
const [x, setX] = useState(5)
const [y, setY] = useState(15)
console.log("Element is rendered:", x, y)
// setting y does not trigger the effect
// the second argument is an array of dependencies
useEffect(() => console.log("re-render because x changed:", x), [x])
function handleXClick() {
console.log("x before setting:", x)
setX(10)
console.log("x in *line* after setting:", x)
}
return <>
<div> x is {x}. </div>
<button onClick={handleXClick}> set x to 10</button>
<div> y is {y}. </div>
<button onClick={() => setY(20)}> set y to 20</button>
</>
}
Output:
Element is rendered: 5 15
re-render because x changed: 5
(press x button)
x before setting: 5
x in *line* after setting: 5
Element is rendered: 10 15
re-render because x changed: 10
(press y button)
Element is rendered: 10 20
Live version
Since setSatate is a asynchronous function so you need to console the state as a callback like this.
openAddBoardModal(){
this.setState({ boardAddModalShow: true }, () => {
console.log(this.state.boardAddModalShow)
});
}
setState() does not always immediately update the component. It may batch or defer the update until later. This makes reading this.state right after calling setState() a potential pitfall. Instead, use componentDidUpdate or a setState callback (setState(updater, callback)), either of which are guaranteed to fire after the update has been applied. If you need to set the state based on the previous state, read about the updater argument below.
setState() will always lead to a re-render unless shouldComponentUpdate() returns false. If mutable objects are being used and conditional rendering logic cannot be implemented in shouldComponentUpdate(), calling setState() only when the new state differs from the previous state will avoid unnecessary re-renders.
The first argument is an updater function with the signature:
(state, props) => stateChange
state is a reference to the component state at the time the change is being applied. It should not be directly mutated. Instead, changes should be represented by building a new object based on the input from state and props. For instance, suppose we wanted to increment a value in state by props.step:
this.setState((state, props) => {
return {counter: state.counter + props.step};
});
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.
Check this for more information.
In your case you have sent a request to update the state. It takes time for React to respond. If you try to immediately console.log the state, you will get the old value.
The above solutions don't work for useState hooks.
One can use the below code
setState((prevState) => {
console.log(boardAddModalShow)
// call functions
// fetch state using prevState and update
return { ...prevState, boardAddModalShow: true }
});
This callback is really messy. Just use async await instead:
async openAddBoardModal(){
await this.setState({ boardAddModalShow: true });
console.log(this.state.boardAddModalShow);
}
If you want to track the state is updating or not then the another way of doing the same thing is
_stateUpdated(){
console.log(this.state. boardAddModalShow);
}
openAddBoardModal(){
this.setState(
{boardAddModalShow: true},
this._stateUpdated.bind(this)
);
}
This way you can call the method "_stateUpdated" every time you try to update the state for debugging.
Although there are many good answers, if someone lands on this page searching for alternative to useState for implementing UI components like Navigation drawers which should be opened or closed based on user input, this answer would be helpful.
Though useState seems handy approach, the state is not set immediately and thus, your website or app looks laggy... And if your page is large enough, react is going to take long time to compute what all should be updated upon state change...
My suggestion is to use refs and directly manipulate the DOM when you want UI to change immediately in response to user action.
Using state for this purspose is really a bad idea in case of react.
setState() is asynchronous. The best way to verify if the state is updating would be in the componentDidUpdate() and not to put a console.log(this.state.boardAddModalShow) after this.setState({ boardAddModalShow: true }) .
according to React Docs
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
According to React Docs
React does not guarantee that the state changes are applied immediately.
This makes reading this.state right after calling setState() a potential pitfall and can potentially return the existing value due to async nature .
Instead, use componentDidUpdate or a setState callback that is executed right after setState operation is successful.Generally we recommend using componentDidUpdate() for such logic instead.
Example:
import React from "react";
import ReactDOM from "react-dom";
import "./styles.css";
class App extends React.Component {
constructor() {
super();
this.state = {
counter: 1
};
}
componentDidUpdate() {
console.log("componentDidUpdate fired");
console.log("STATE", this.state);
}
updateState = () => {
this.setState(
(state, props) => {
return { counter: state.counter + 1 };
});
};
render() {
return (
<div className="App">
<h1>Hello CodeSandbox</h1>
<h2>Start editing to see some magic happen!</h2>
<button onClick={this.updateState}>Update State</button>
</div>
);
}
}
const rootElement = document.getElementById("root");
ReactDOM.render(<App />, rootElement);
this.setState({
isMonthFee: !this.state.isMonthFee,
}, () => {
console.log(this.state.isMonthFee);
})
when i was running the code and checking my output at console it showing the that it is undefined.
After i search around and find something that worked for me.
componentDidUpdate(){}
I added this method in my code after constructor().
check out the life cycle of react native workflow.
https://images.app.goo.gl/BVRAi4ea2P4LchqJ8
Yes because setState is an asynchronous function. The best way to set state right after you write set state is by using Object.assign like this:
For eg you want to set a property isValid to true, do it like this
Object.assign(this.state, { isValid: true })
You can access updated state just after writing this line.
I'm new to React and am trying to update the state of a parent component from the child everytime an onChange action happens. The onchange action comes from an input box that when letters are typed it updates the state of searchInputVal with the value of what has been typed. I have a parent <App/> component with the following properties and states here:
updateSampleFilteredState(filteredSamples) {
this.setState({
samples: filteredSamples
});
},
getInitialState () {
return {
samples:allSamples,
searchInputVal:""
}}
I pass the properties and states down to a child component here:
updateNewSampleState(filteredSamples){
return (
this.props.updateSampleFilteredState(filteredSamples)
)
}
render() {
const filteredSamples = this.props.samples.filter(sample => {
return sample.sampleFamily.toLowerCase().indexOf(this.props.searchInputVal.toLowerCase()) !== -1;
});
this.updateNewSampleState(filteredSamples);
return <div className="samples-container-inner-styling">
{
filteredSamples.map((sample) => {
return (...
Before I added the line this.updateNewSampleState(filteredSamples); the child component would render out the filtering just fine but obviously not update the state of sample with the new filtered state. When I the line this.updateNewSampleState(filteredSamples); to execute the function in the component to set the new state I get a list of re-occuring errors that eventually make my app crash. The errors say something about an anti pattern. I'm not sure how else to update the state?
You should't be updating the state from the render function, and you are facing the reason why that's a bad way to do things. Every time you call the setState the component re-renders, so if you call it inside the render function it will be called again and so on... You should ask yourself why are you calling that function there. I guess you could just do it in the onChange function you are using for the input.
As already mentioned by #César, setting the state in the renderer doesn't make sense, since setting the state triggers a rerender of the component, so you basically get something like an infinite render loop.
Given that you are computing filteredSamples only from the props, you could compute that state in the constructor:
The constructor is the right place to initialize state.
However, note the following when deriving state from props in the constructor:
It's okay to initialize state based on props if you know what you're doing. [...]
Beware of this pattern, as it effectively "forks" the props and can lead to bugs. Instead of syncing props to state, you often want to lift the state up.
If you "fork" props by using them for state, you might also want to implement componentWillReceiveProps(nextProps) to keep the state up-to-date with them. But lifting state up is often easier and less bug-prone.