I am beginner in react and I am learning about lifecycle hooks. I came across some articles stating don't call setState synchronously inside componentDidMount. But we can setState in then block of promise.
The reason mentioned was setState will cause re-render of the component, which will affect performance.
My question is, even if I write setState inside then block of promise, re-render will be there anyways, also setState is asynchronous so it won't be called immediately. So why its not recommended to call setState inside componentDidMount synchronously, And how does writing inside then block of promise solves the problem if there are any.
PS: I have researching on SO for quite a while, but couldn't find answers that says don't call setState synchronously, I found some related answers, but didn't answer my question, and I couldn't get them because they were totally related to some scenario.
React setState calls are asynchronous. React decides when to do it efficiently. For example, multiple setState calls are batched together into one single update. The way to get around this is using setStates's 2nd parameter which is a callback that will execute once setState is completed and the component re-rendered.
handleThis = () => {
this.setState({someVar: someVal}, funcToCallimmediately )
}
To test, console.log the state variable just after setState() and write another console.log inside the "funcToCallimmediately" function.
đź“ť Take note that if you need to set the state based on the previous state you should use another method. First, just know that setState's 1st parameter can be an object and can also be a function. So you do it like this...
this.setState((state, props) => {
return {counter: state.counter + props.step};
});
To understand more, read more about setState() here. You'll get what you need.
Related
I have used the following pattern in previous ReactJS projects - A LOT!
componentDidMount() {
var promises = [];
var promise1 = this.AsyncFunc1().then(res => {
this.setState({some_state1: res.data.results});
}).catch(error => {
//deal with error
});
var promise2 = this.AsyncFunc2().then(res => {
this.setState({some_state2: res.data.results});
}).catch(error => {
//deal with error
});
promises.push(promise1);
promises.push(promise2);
Promise.all(promises).then(() => {
// Use the state variables
}).catch(error => {
// deal with error
});
}
I understand that state is set asynchronously and not available right away - I avoided accessing the state immediately following the setting of that state in the then statements of the async functions - but I have consistently accessed the state variable values in the promise resolution section in previous projects - but in this new project I'm working on it is failing off and on because the state variable values aren't consistently available. I've resorted to setting temp variables to the res.data.results values and using those in the Promise.all section.
My previous projects have been in production for 2+ years without issues (that I'm aware of) - do I need to go back and rework that code? Did I make a bad assumption that the state variable values would be available in the Promise.all section?
I appreciate the help!
Yes, you made an incorrect assumption; your promise might be called before the state is available. From setState docs, emphasis mine:
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. This is the primary method you use to update the user interface in response to event handlers and server responses.
...
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.
There's also a FAQ entry about your kind of case, emphasis again mine:
Why is setState giving me the wrong value?
In React, both this.props and this.state represent the rendered values, i.e. what’s currently on the screen.
Calls to setState are asynchronous - don’t rely on this.state to reflect the new value immediately after calling setState. Pass an updater function instead of an object if you need to compute values based on the current state (see below for details).
As in the second quote, the state variables won't be updated until after the render happens, and even though you're not immediately reading state after calling setState, it's still likely that your Promise.all will fire before the this.state is updated—and React absolutely reserves the right to do so. Auditing your previous use of this pattern is probably a good idea.
Some options for you if you really want the render to happen first:
Use componentDidUpdate and check that both state variables are set, which the FAQ recommends.
Use the second setState parameter, updater, which will call you back after the setState takes effect. This is not recommended compared to componentDidUpdate.
With hooks, use useEffect. You can pass your some_state1 and some_state2 in the second argument to useEffect to only take effect when the values change.
Within your AsyncFunc then handlers, wrap your code in flushSync. The docs heavily recommend against this option: flushSync is not as specific as the other options above, but for a stopgap measure on old code it might be effective for you.
If you don't need the UI to update before your Promise.all() handler runs, you can save the resolved values to local variables, or you can make sure to return the values from your AsyncFunc then handlers. If your // deal with error handlers provide fallback values, you can return those from your catch handlers there too; conversely, if you don't want your Promise.all() handler to run if either of those calls fail, you should ensure that they re-throw the exceptions from within the catch block so the promises you save stay rejected.
(Of course, you could also move both of your setState calls to the Promise.all handler so you don't even try to call setState before both values are there, but I presume you want the partial UI update that comes from each individual AsyncFunc.)
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.
Trying to understand the workings of setState() in React. My question is this.
If I have a function where one of the lines is setState(), will code lines AFTER this line still be run before the rerendering? For instance, in the code bellow,
foo(value) {
this.setState({stateValue: value});
console.log("Stuff after setState");
}
In that code, is the console.log guaranteed to run? It runs when I test it, but I'm uncertain if this is just because React hasn't had time to re-render yet. Can I expect code after this.setState() to run?
setState is asynchronous function and just like any asynchronous function is being passed on to the event loop, setState also is passed on to the event loop and any code after it will execute seemlessly.
Once the setState method has completed executing, it will trigger the render method, which is when React will work on the document render.
Let's see the setState documentation
Think of setState() as a request rather than an immediate command to update the component
React does not guarantee that the state changes are applied immediately
Think of it as setTimeout(function test() {...}, 0); a message will be added to the queue, the test function will not be fired until all the stack is empty. You can find more information on the EventLoop documentation
I see that setState is async. So if I were to call:
this.setState({ variable: true });
and immediately call:
this.setState({ variable: false });
before render is called, am I guaranteed that 'variable' will be false when React is finished processing? In other words, are the async operations sync? Will render be called twice, or will 'variable' be overwritten and render called once with variable=false?
From the react docs for setState:
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.
So your logic should not rely on setState execution time. If you like to control render you should consider using shouldComponentUpdate(nextProps, nextState).
Use shouldComponentUpdate() to let React know if a component’s
output is not affected by the current change in state or props. The
default behavior is to re-render on every state change, and in the
vast majority of cases you should rely on the default behavior.
It is best not to rely upon this behavior. It will only work sometimes and not others.
To reliably set multiple state properties, gather all the updates and set them in a single call:
const changes = {};
if (some logic) { changes.variable = true; }
if (some more logic) { changes.variable = false; }
this.setState(changes);
Great question! Made me curious as well, so I whipped up this JSFiddle demonstrating this behavior.
React calls render synchronously after your event handler.
Meaning that because your are doing two updates in the same function, it won't re-render until after the function completes. That means that in each render, this.state.variable will be false.
https://facebook.github.io/react/docs/react-component.html#setstate
I have seem the official doc says :
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.
why they recommend to use componentDidUpdate instead of callback when we need to use the latest state?
componentDidUpdate is also invoked immediately after every updating occurs. At this time, latest state is updated.
Putting all checking state logics in componentDidUpdate is easier for managing than many callback with every setState.
By the way if you use redux, we will not do setState in your components.
More about redux
Note: componentDidUpdate is not called for the initial render.
Learn more Component life cycle https://facebook.github.io/react/docs/react-component.html#componentdidupdate