Develop Reference

Does this.setState really make component re-render?

something about this.setState in class components is confusing me. in react docs, i read that this.setState may be asynchronous in react docs. im okay with this and i tested it with logging the updated value after this.setState. but it is really confusing me: when we call this.setState, react calls render method of the class to re-render the ui. but this.setState is asynchronous and it means that first render method will be called and ui will be re-rendered by react, then the value in the state will be changed! so how does component show us the updated value? i dont... am i thinking right?
thanks for helping.

this.setState is asynchronous and it means that first render method will be called and ui will be re-rendered by react, then the value in the state will be changed! so how does component show us the updated value? i dont... am i thinking right?
I think you missed something.setState is asynchronous which means that the actions are batched together for performance gains . But why ? since it's not a a WebAPI or server call?
This is because setState alters the state and causes rerendering. This can be an expensive operation and making it synchronous might leave the browser to behave unexpectedly.

Where to make API hits ComonentDidMount or in ComonentDidUpdate [React.JS]

In an interview, an interviewer asked me that where should you make API-hits in a simple react application? Meaning in which life-cycle method in a Class-Component. I knew the answer to be ComponentDidMount - because it is the first life-cycle method where we get complete rendered dom meaning dom is now ready!
Then he asked, but why NOT in comonentDidUpdate?
Now, I told him what I had read somewhere, I don't know the exact answer of this -- except ComponentDidMount runs first, so make it there.
Now, can someone tell me if my answer was correct? Or should we make API-hits in ComponentDidUpdate()?
I am confused. Kindly, someone explain with reasoning? Thanks in Advance!

It depends on when you want to call the API:
If an API call is done only once then do componentDidMount
If after render based on some state, you want to fetch data again then do it in componentDidUpdate
EDIT:
Same scenarios can be handled within functional components using useEffect hook as follows:
1- Only runs the first time when the components render same as componentDidMount:
useEffect(() => {
// Run only once when the component renders
}, []); // Pass empty array as dependencies
2- Run every time when component renders either by props change or by local state change same as componentDidUpdate without comparing previous and current props:
useEffect(() => {
// Run every time the component re-renders including the first time
}); // Do NOT pass array dependencies
3- Run only when particular props change, same as componentDidUpdate but with props comparison:
useEffect(() => {
// Run only when the component prop1 and prop2 changes
}, [prop1, prop2]); // Pass props as array dependencies
Reference: Using the Effect Hook

Lets take an example scenario.
You have a profile page and it has a text box which allows you to update tags.
You do a fetch for the whole profile in the componentDidMount to get all the details and show the content.
And then componentDidUpdate will have to be used for something like the update on tags, lets say you do a fetch to get tags based on the user input for every 3 letters the user type. then you use componenDidUpdate to check the state and do the call.
If you think of the same in functional components we'll have to use useEffect.
useEffect(()=>{},[]);
See the array of dependecies, if you pass an empty array it would act similar to componentDidMount.
And the componentDidUpdate
useEffect(()=>{},[tagText]);
Here the effect will run only when a change it done to the tagText, but the componenDidUpdate would be different as you will have to compare the previous state and decide whether the text is updated.

According the Official React Documentation (Link):
componentDidMount
componentDidMount() is invoked immediately after a component is mounted (inserted into the tree). Initialization that requires DOM nodes should go here. If you need to load data from a remote endpoint, this is a good place to instantiate the network request.
This method is a good place to set up any subscriptions. If you do
that, don’t forget to unsubscribe in componentWillUnmount().
componentDidUpdate()
componentDidUpdate() is invoked immediately after updating occurs.
This method is not called for the initial render.
Use this as an opportunity to operate on the DOM when the component
has been updated. This is also a good place to do network requests as
long as you compare the current props to previous props (e.g. a
network request may not be necessary if the props have not changed).
Check out this link for a complete big picture. This will be handy while learning react.

React componentWillUpdate getting called twice

Iam new to React and i came across with this doc. It says that:
Either way, it is unsafe to use componentWillUpdate for this purpose
in async mode, because the external callback might get called multiple
times for a single update
How is it possible for the componentWillUpdate to be called multiple times? It doesnt explain it.
Thank you

Any change that is supposed to trigger a render will first go through a componentWillUpdate lifecycle. The change change can be a parent re-render causing the child to re-render, a change in components props, or a change in state.
However from v16.3.0 this lifecycle method is deprecated and it is encouraged that any sideeffect be handled in componentDidUpdate which will be triggered after render method.

On async mode, your component update/render may be postponed so react can deliver some hi-priority stuff. This means willUpdate will be called each time react starts working on your component, but it might not finish the full update, hence calling willUpdate every time it starts working on this component but only calls didUpdate once, after this process is finished.

Use componentWillMount or componentDidMount lifecycle functions for async request in React

I am reading up on react lifecycle and am getting a little confused. Some recommend using componentWillMount to make ajax calls:
https://hashnode.com/post/why-is-it-a-bad-idea-to-call-setstate-immediately-after-componentdidmount-in-react-cim5vz8kn01flek53aqa22mby
Calling setState in componentDidMount will trigger another render()
call and it can lead to layout thrashing.
and in other places it says not to put ajax calls in the componentWillMount:
https://medium.com/#baphemot/understanding-reactjs-component-life-cycle-823a640b3e8d
...this function might end up being called multiple times before the
initial render is called so might result in triggering multiple
side-effects. Due to this fact it is not recommended to use this
function for any side-effect causing operations.
Which is correct?

The React docs recommend on using componentDidMount for making network Requests
componentDidMount() is invoked immediately after a component is
mounted. Initialization that requires DOM nodes should go here. If you
need to load data from a remote endpoint, this is a good place to
instantiate the network request.
Calling setState() in this method will trigger an extra rendering, but
it is guaranteed to flush during the same tick. This guarantees that
even though the render() will be called twice in this case, the user
won’t see the intermediate state.
As per the case for componentWillMount:
EDIT:
This lifecycle is deprecated since v16.3.0 of react and is no longer encouraged for usage.However its renamed to UNSAFE_componentWillUpdate and is expected to work till at least v17 of react
Before v16.3.0
An asynchronous call to fetch data will not return before the render happens. This means the component will render with empty data at least once.
There is no way to “pause” rendering to wait for data to arrive. You cannot return a promise from componentWillMount or wrangle in a setTimeout somehow. The right way to handle this is to setup the component’s initial state so that it’s valid for rendering.
To Sum it up
In practice, componentDidMount is the best place to put calls to fetch data, for two reasons:
Using DidMount makes it clear that data won’t be loaded until after
the initial render. This reminds you to set up initial state
properly, so you don’t end up with undefined state that causes
errors.
If you ever need to render your app on the server, componentWillMount will actually be
called twice – once on the server, and again on the client – which is
probably not what you want. Putting the data loading code in
componentDidMount will ensure that data is only fetched from the
client.

componentDidMount is the recommended lifecycle method to make Ajax calls as described in their docs

ComponentDidMount is the place.
But if you have time try to look at Redux and make the requests in actions, as your application grow it will help a lot to manage the app state.
;)

Render after componentWillReceiveProps and before componentDidUpdate

Here's my problem:
I'm new at reactjs and I'm trying to make this app using SWAPI (swapi.co). For now I need to list characters and some info about them. The problem is I have this component called SelectedCharacter that returns some info about a character that was selected in a div.
The moment a character is passed through props to this component, I get a response via xmlhttp and the info is displayed. The thing is that I want to put a "Loading..." message while the data is fetched. This is how I was trying to figure it out:
I set up the componentWillReceiveProps function, where I test if I'll need to load stuff and the componentDidUpdate, where I fetch the data from this api and update the status.
I know, from react life cycle, that a render is called between componentWillReceiveProps and componentDidUpdate, and it indeed is.
I expected, then, that if I did this:
render() {
if (criteria) {
return <div>Loading...</div>
}
}
The thing is: even if this criteria is true (I tested it using console.log()), the message doesn't show until the next re-render. Am I doing anything too wrong here? If it helps, my code is at github.com/piubellofelipe/StarWars, the problem is at the selected_characters.js, in the src paste.
Thanks

I've been looking at your code, trying to work this out for you and I don't have any concrete answers for you, but I've noticed a few things that may be making things a bit unpredictable.
1. Calling forceUpdate
componentWillReceiveProps(){
this.setState({loading:true})
this.forceUpdate();
}
Calling setState will trigger a render, so the call the forceUpdate is not required here. This means there are more renders occurring than you might expect.
I believe this may be the cause of your issue for a pretty complicated reason. From the setState docs
... setState() is also asynchronous, and multiple calls during the same cycle may be batched together.
And from the forceUpdate docs
Calling forceUpdate() will cause render() to be called on the component...
My theory is that the call render triggered by setState, asynchronously setting loading to true, is being delayed and the one from forceUpdate is sneaking in first, while loading is still false.
2. Updating props
this.props.selected.moviesList = moviesListNames;
Components should never, ever, update their own props. Usually, this would be stored in state instead.
For more details on this, read this section of the docs and this answer.
3. Importing axios, but not using it
import axios from 'axios'
This one isn't really an issue (other than an unused import), and might just be preparation for where you're heading. axios gives a much nicer developer experience than XMLHttpRequest (in my opinion) for http requests and will clean up the fetchData function a lot, which will make troubleshooting easier.
I hope this helps a bit, and you're enjoying React. Feel free to follow up on any of these points in the comments or as new questions.