We Keep Coding

How does React update a component and its children after a state change?

I am watching Paul O Shannessy - Building React From Scratch
And I understand the mounting process very well but I have hard day trying to understand how React update a component and its children
The reconciler controls the update process by this method:
function receiveComponent(component, element) {
let prevElement = component._currentElement;
if (prevElement === element) {
return;
}
component.receiveComponent(element);
}
Component.receiveComponent
receiveComponent(nextElement) {
this.updateComponent(this._currentElement, nextElement);
}
and this is the Component.updateComponent method:
updateComponent(prevElement, nextElement) {
if (prevElement !== nextElement) {
// React would call componentWillReceiveProps here
}
// React would call componentWillUpdate here
// Update instance data
this._currentElement = nextElement;
this.props = nextElement.props;
this.state = this._pendingState;
this._pendingState = null;
let prevRenderedElement = this._renderedComponent._currentElement;
let nextRenderedElement = this.render();
if (shouldUpdateComponent(prevRenderedElement, nextRenderedElement)) {
Reconciler.receiveComponent(this._renderedComponent, nextRenderedElement);
}
}
This is the part of the code that updates the component after state change, and i assume that it should update the children too, but i can't understand how this code achieves that, in the mounting process React instantiate components to dive deeper in the tree but this doesn't happen here, we need to find the first HTML element then we can change our strategy and update that HTML element in another place in the code, and I can't find any way to find any HTML elements this way.
Finding the first HTML is the way to stop this endless recursion and logically this is what I expect from the code, to stop recursion the same way in the mounting process, but in mounting, this demanded component instantiation so we can delegate to the reconciler that will discover that we are dealing with a wrapper instance of an HTML element not a wrapper instance of a custom component then React can place that HTML element in the DOM.
I can't understand how the code works in the update process. this code as I see won't dive deeper in the tree and I think won't update the children and can't let React find the first HTML element so React can update the DOM element, isn't it?
This is the code repo on Github

I created a codesandbox to dig in
Here is the codesandbox I created
and here's a short recording of me opening the debugger and seeing the call stack.
How it works
Starting from where you left off, Component.updateComponent:
updateComponent(prevElement, nextElement) {
//...
if (shouldUpdateComponent(prevRenderedElement, nextRenderedElement)) {
Reconciler.receiveComponent(this._renderedComponent, nextRenderedElement);
//...
in the Component.updateComponent method Reconciler.receiveComponent is called which calls component.receiveComponent(element);
Now, this component refers to this._renderedComponent and is not an instance of Component but of DOMComponentWrapper
and here's the receiveComponent method of DOMComponentWrapper:
receiveComponent(nextElement) {
this.updateComponent(this._currentElement, nextElement);
}
updateComponent(prevElement, nextElement) {
// debugger;
this._currentElement = nextElement;
this._updateDOMProperties(prevElement.props, nextElement.props);
this._updateDOMChildren(prevElement.props, nextElement.props);
}
Then _updateDOMChildren ends up calling the children render method.
here's a call stack from the codesandbox I created to dig in.
How do we end up in DOMComponentWrapper
in the Component's mountComponent method we have:
let renderedComponent = instantiateComponent(renderedElement);
this._renderedComponent = renderedComponent;
and in instantiateComponent we have:
let type = element.type;
let wrapperInstance;
if (typeof type === 'string') {
wrapperInstance = HostComponent.construct(element);
} else if (typeof type === 'function') {
wrapperInstance = new element.type(element.props);
wrapperInstance._construct(element);
} else if (typeof element === 'string' || typeof element === 'number') {
wrapperInstance = HostComponent.constructTextComponent(element);
}
return wrapperInstance;
HostComponent is being injected with DOMComponentWrapper in dilithium.js main file:
HostComponent.inject(DOMComponentWrapper);
HostComponent is only a kind of proxy meant to invert control and allow different Hosts in React.
here's the inject method:
function inject(impl) {
implementation = impl;
}
and the construct method:
function construct(element) {
assert(implementation);
return new implementation(element);
}
When we have no DOMComponentWrapper
If we are updating a chain of Non Host Components like:
const Child = <div>Hello</div>
const Parent = () => <Child />
How does Child get rendered from an update to Parent?
the Parent Component has the following:
_renderedComponent which is an instance of Child(which is also a Component)
renderedComponent has an instance of Child because it gets the type of the "root" Element (the one returned by the render method)
so Reconciler.receiveComponent(this._renderedComponent, nextRenderedElement) will be calling component.receiveComponent(element) of the Child which in turn calls this.updateComponent(this._currentElement, nextElement); (of Child) which calls it's render method (let nextRenderedElement = this.render();)

React completely copy the actual DOM and create the virtual DOM in javascript. In our application whenever we update any of the data that ends up being rendered in our components, React does not rerender the entire DOM. It only affects the thing that matters. So react actually copies the virtual DOM again. This time it applies the changes to the data that got updated.
It will make the change in the red component and then it will compare this virtual DOM to the old DOM. It will see the different part. Then it will apply the DOM changes only to that different component.
The updating phase starts if props or the state changes. If the data at the top level changes:
If it is passing that data down to its children, all the children are going to be rerendered. If the state of the component at the mid-level gets changed:
This time only its children will get rerendered. React will rerender any part of the tree below that node. Because the data that generates the children components' view actually sits at the parent component(mid-level one). But anything above it, the parent or the siblings will not rerender. because data does not affect them. this concept is called Unidirectional Data Flow.
You can see in action in chrome browser. chose the rendering and then enable the painting flushing option
If you make any change on the page, you will see that updated components will be flashed.
UPDATING PHASE
componentWillReceiveProps method is invoked first in the component lifecycle's updating phase. It is called when a component receives new properties from its parent component. With this method we compare the current component's properties using the this.props object with the next component's properties
using the nextElement.props object. Based on this comparison, we can choose to update the component's state using the this.setState() function, which will NOT trigger
an additional render in this scenario.
Note that no matter how many times you call this.setState() in the componentWillReceiveProps() method, it won't trigger any additional renders of that component. React does an internal optimization where it batches the state updates together.
shouldComponentUpdated dictates if the components should rerender or not. By default, all class components will rerender whenever the props they receive or their state change. this method can prevent the default behavior by returning False. In this method, existing props and state values get compared with the next props and state values and return boolean to let React know whether the component should update or not. this method is for performance optimization. If it returns False componentWillUpdate(), render() and componentDidUpdate() wont get called.
The componentWillUpdate() method is called immediately before React updates the DOM. It gets two arguments: nextProps and nextState. You can use these arguments to prepare for the DOM update. However, you cannot use this.setState() in the componentWillUpdate() method.
After calling the componentWillUpdate() method, React invokes the render() method that performs the DOM update. Then, the componentDidUpdate() method is called.
The componentDidUpdate() method is called immediately after React updates the DOM. It gets these two arguments: prevProps and prevState. We use this method to interact with the updated DOM or perform any post-render operations. For example, in a counter example, counter number is increased in componentDidUpdate.
After componentDidUpdate() is called, the updating cycle ends. A new cycle is started when a component's state is updated or a parent component passes new properties. Or when you call the forceUpdate() method, it triggers a new updating cycle, but skips the shouldComponentUpdate() method (this method is for optimization) on a component that
triggered the update. However, shouldComponentUpdate() is called on all the child components as per the usual updating phase. Try to avoid using the forceUpdate() method as much as possible; this will promote your application's maintainability

Another answer might be the structure of the Fiber tree. During execution, react renders a ReactComponent into an object made out of ReactNodes and props. These ReactNodes are assembled into a FiberNode tree (which might be the in memory representation of the virutal dom?).
In the FiberNode tree, depending on the traversal algorithm (children first, sibling first, etc), React always has a single "next" node to continue. So, React will dive deeper into the tree, and update FiberNodes, as it goes along.
If we take the same example,
function App() {
return <div>
<Parent>
<Child01/>
<Child01/>
</Parent>
<Child03/>
</div>
}
function Parent({children}) {
const [state, setState] = useState(0);
return <div>
<button onClick={x => x+1)>click</button>
<Child02 />
{children}
</div>
}
Which React will transform into this FiberNode tree:
node01 = { type: App, return: null, child: node02, sibling: null }
node02 = { type: 'div', return: node01, child: node03, sibling: null }
node03 = { type: Parent, return: node02, child: node05(?), sibling: node04 }
node04 = { type: Child03, return: node02, child: null, sibling: null }
node05 = { type: Child01, return: node03, child: null, sibling: node06 }
node06 = { type: Child01, return: node03, child: null, sibling: null }
// Parent will spawn its own FiberTree,
node10 = { type: 'div', return: node02, child: node11, sibling: null }
node11 = { type: 'button', return: node10, child: null, sibling: node12 }
node12 = { type: Child02, return: node10, child: null, sibling: node05 }
I might have missed something (ie. node03's child might be node10), but the idea is this - React always have a single node (the 'next' node) to render when it traverses the fiber tree.

I think React not re-render parent component first instead of that, React re-render child component first.
Example: A (parent) -> B (child) -> C (child of B)
When A update state C (re-render) -> B -> A

Hey Consider using a Tree data structure for your need, ReactJs follows a unidirectional manner of Updating the state i.e. As soon as the there is a Change in the parent state then all the children which are passed on the props that are residing in the Parent Component are updated once and for all!
Consider using something known as Depth First Search as an algo option which will find you the Node that connects to the parent and once you reach that node , you check for the state and if there is a deviation from the state variables that are shared by the parent you can update them!
Note : This may all seem a bit theoretical but if you could do something remotely close to this thing you will have created a way to update components just how react does!

I found out experimentally that React will only re-render elements if it have to, which is always, except for {children} and React.memo().
Using children correctly, together with batched dom updates makes a very efficient and smooth user experience.
consider this case:
function App() {
return <div>
<Parent>
<Child01/>
<Child01/>
</Parent>
<Child03/>
</div>
}
function Parent({children}) {
const [state, setState] = useState(0);
return <div>
<button onClick={x => x+1)>click</button>
<Child02 />
{children}
</div>
}
when clicking on the button, you will get the following:
- button click
- setState(...), add Parent to dirty list
- start re-rendering all dirty nodes
- Parent rerenders
- Child02 rerenders
- DONE
Note that
Parent (app) and sibling (Child03) nodes will not get re-rendered, or you'll end up with a re-render recursion.
Parent is re-rendered because its state has changed, so its output has to be recalculated.
{children} have not been affected by this change, so it stays the same. (unless a context is involved, but that's a different mechanism).
finally, <Child02 /> has been marked dirty, because that part of the virtual dom has been touched. While it's trivial for us to see it was not effected, the only way React could verify it is by comparing props, which is not done by default!
the only way to prevent Child02 from rendering is wrapping it with React.memo, which might be slower than just re-rendring it.

How does the splice work in Function Component(hooks) in React? [duplicate]

I understand that React tutorials and documentation warn in no uncertain terms that state should not be directly mutated and that everything should go through setState.
I would like to understand why, exactly, I can't just directly change state and then (in the same function) call this.setState({}) just to trigger the render.
E.g.: The below code seems to work just fine:
const React = require('react');
const App = React.createClass({
getInitialState: function() {
return {
some: {
rather: {
deeply: {
embedded: {
stuff: 1,
},
},
},
},
},
};
updateCounter: function () {
this.state.some.rather.deeply.embedded.stuff++;
this.setState({}); // just to trigger the render ...
},
render: function() {
return (
<div>
Counter value: {this.state.some.rather.deeply.embedded.stuff}
<br></br>
<button onClick={this.updateCounter}>Increment</button>
</div>
);
},
});
export default App;
I am all for following conventions but I would like to enhance my further understanding of how ReactJS actually works and what can go wrong or is it sub-optimal with the above code.
The notes under the this.setState documentation basically identify two gotchas:
That if you mutate state directly and then subsequently call this.setState this may replace (overwrite?) the mutation you made. I don't see how this can happen in the above code.
That setState may mutate this.state effectively in an asynchronous / deferred way and so when accessing this.state right after calling this.setState you are not guaranteed to access the final mutated state. I get that, by this is not an issue if this.setState is the last call of the update function.

This answer is to provide enough information to not change/mutate the state directly in React.
React follows Unidirectional Data Flow. Meaning, the data flow inside react should and will be expected to be in a circular path.
React's Data flow without flux
To make React work like this, developers made React similar to functional programming. The rule of thumb of functional programming is immutability. Let me explain it loud and clear.
How does the unidirectional flow works?
states are a data store which contains the data of a component.
The view of a component renders based on the state.
When the view needs to change something on the screen, that value should be supplied from the store.
To make this happen, React provides setState() function which takes in an object of new states and does a compare and merge(similar to object.assign()) over the previous state and adds the new state to the state data store.
Whenever the data in the state store changes, react will trigger an re-render with the new state which the view consumes and shows it on the screen.
This cycle will continue throughout the component's lifetime.
If you see the above steps, it clearly shows a lot of things are happening behind when you change the state. So, when you mutate the state directly and call setState() with an empty object. The previous state will be polluted with your mutation. Due to which, the shallow compare and merge of two states will be disturbed or won't happen, because you'll have only one state now. This will disrupt all the React's Lifecycle Methods.
As a result, your app will behave abnormal or even crash. Most of the times, it won't affect your app because all the apps which we use for testing this are pretty small.
And another downside of mutation of Objects and Arrays in JavaScript is, when you assign an object or an array, you're just making a reference of that object or that array. When you mutate them, all the reference to that object or that array will be affected. React handles this in a intelligent way in the background and simply give us an API to make it work.
Most common errors done when handling states in React
// original state
this.state = {
a: [1,2,3,4,5]
}
// changing the state in react
// need to add '6' in the array
// bad approach
const b = this.state.a.push(6)
this.setState({
a: b
})
In the above example, this.state.a.push(6) will mutate the state directly. Assigning it to another variable and calling setState is same as what's shown below. As we mutated the state anyway, there's no point assigning it to another variable and calling setState with that variable.
// same as
this.state.a.push(6)
this.setState({})
Many people do this. This is so wrong. This breaks the beauty of React and is bad programming practice.
So, what's the best way to handle states in React? Let me explain.
When you need to change 'something' in the existing state, first get a copy of that 'something' from the current state.
// original state
this.state = {
a: [1,2,3,4,5]
}
// changing the state in react
// need to add '6' in the array
// create a copy of this.state.a
// you can use ES6's destructuring or loadash's _.clone()
const currentStateCopy = [...this.state.a]
Now, mutating currentStateCopy won't mutate the original state. Do operations over currentStateCopy and set it as the new state using setState().
currentStateCopy.push(6)
this.setState({
a: currentStateCopy
})
This is beautiful, right?
By doing this, all the references of this.state.a won't get affected until we use setState. This gives you control over your code and this'll help you write elegant test and make you confident about the performance of the code in production.
To answer your question,
Why can't I directly modify a component's state?
Well, you can. But, you need to face the following consequences.
When you scale, you'll be writing unmanageable code.
You'll lose control of state across components.
Instead of using React, you'll be writing custom codes over React.
Immutability is not a necessity because JavaScript is single threaded, but it's a good to follow practices which will help you in the long run.
PS. I've written about 10000 lines of mutable React JS code. If it breaks now, I don't know where to look into because all the values are mutated somewhere. When I realized this, I started writing immutable code. Trust me! That's the best thing you can do it to a product or an app.

The React docs for setState have this to say:
NEVER mutate this.state directly, as calling setState() afterwards may replace the mutation you made. Treat this.state as if it were immutable.
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.
setState() will always trigger a re-render unless conditional rendering logic is implemented in shouldComponentUpdate(). If mutable objects are being used and the logic cannot be implemented in shouldComponentUpdate(), calling setState() only when the new state differs from the previous state will avoid unnecessary re-renders.
Basically, if you modify this.state directly, you create a situation where those modifications might get overwritten.
Related to your extended questions 1) and 2), setState() is not immediate. It queues a state transition based on what it thinks is going on which may not include the direct changes to this.state. Since it's queued rather than applied immediately, it's entirely possible that something is modified in between such that your direct changes get overwritten.
If nothing else, you might be better off just considering that not directly modifying this.state can be seen as good practice. You may know personally that your code interacts with React in such a way that these over-writes or other issues can't happen but you're creating a situation where other developers or future updates can suddenly find themselves with weird or subtle issues.

the simplest answer to "
Why can't I directly modify a component's state:
is all about Updating phase.
when we update the state of a component all it's children are going to be rendered as well. or our entire component tree rendered.
but when i say our entire component tree is rendered that doesn’t mean that the entire DOM is updated.
when a component is rendered we basically get a react element, so that is updating our virtual dom.
React will then look at the virtual DOM, it also has a copy of the old virtual DOM, that is why we shouldn’t update the state directly, so we can have two different object references in memory, we have the old virtual DOM as well as the new virtual DOM.
then react will figure out what is changed and based on that it will update the real DOM accordingly .
hope it helps.

It surprises me that non of the current answers talk about pure/memo components (React.PureComponent or React.memo). These components only re-render when a change in one of the props is detected.
Say you mutate state directly and pass, not the value, but the over coupling object to the component below. This object still has the same reference as the previous object, meaning that pure/memo components won't re-render, even though you mutated one of the properties.
Since you don't always know what type of component you are working with when importing them from libraries, this is yet another reason to stick to the non-mutating rule.
Here is an example of this behaviour in action (using R.evolve to simplify creating a copy and updating nested content):
class App extends React.Component {
state = { some: { rather: { deeply: { nested: { stuff: 1 } } } } };
mutatingIncrement = () => {
this.state.some.rather.deeply.nested.stuff++;
this.setState({});
}
nonMutatingIncrement = () => {
this.setState(R.evolve(
{ some: { rather: { deeply: { nested: { stuff: n => n + 1 } } } } }
));
}
render() {
return (
<div>
Normal Component: <CounterDisplay {...this.state} />
<br />
Pure Component: <PureCounterDisplay {...this.state} />
<br />
<button onClick={this.mutatingIncrement}>mutating increment</button>
<button onClick={this.nonMutatingIncrement}>non-mutating increment</button>
</div>
);
}
}
const CounterDisplay = (props) => (
<React.Fragment>
Counter value: {props.some.rather.deeply.nested.stuff}
</React.Fragment>
);
const PureCounterDisplay = React.memo(CounterDisplay);
ReactDOM.render(<App />, document.querySelector("#root"));
<script src="https://unpkg.com/react#17/umd/react.production.min.js"></script>
<script src="https://unpkg.com/react-dom#17/umd/react-dom.production.min.js"></script>
<script src="https://unpkg.com/ramda#0/dist/ramda.min.js"></script>
<div id="root"></div>

To avoid every time to create a copy of this.state.element you can use update with $set or $push or many others from immutability-helper
e.g.:
import update from 'immutability-helper';
const newData = update(myData, {
x: {y: {z: {$set: 7}}},
a: {b: {$push: [9]}}
});

setState trigger re rendering of the components.when we want to update state again and again we must need to setState otherwise it doesn't work correctly.

My current understanding is based on this and this answers:
IF you do not use shouldComponentUpdate or any other lifecycle methods (like componentWillReceiveProps, componentWillUpdate, and componentDidUpdate) where you compare the old and new props/state
THEN
It is fine to mutate state and then call setState(), otherwise it is not fine.

componentWillReceiveProps vs getDerivedStateFromProps

What exactly componentWillReceiveProps and getDerivedStateFromProps are subtle question for me. Because, I just came across to an issue while using getDerivedStateFromProps:
// Component
state = {
myState: []
}
// Using this method works fine:
componentWillReceiveProps(nextProps) {
this.setState({
myState: nextProps.myPropsState
})
}
// But using this method will cause the checkboxes to be readonly:
static getDerivedStateFromProps(nextProps,prevProps) {
const { myPropsState: myState } = nextProps
return {
myState
}
}
// And here's checkbox
<input type="checkbox" id={`someid`}
onChange={(e) => this.handleMethod(e, comp.myState)}
checked={myState.indexOf(comp.myState) > -1} />
React version: 16.4.1

getDerivedStateFromProps is not a direct alternative to componentWillReceiveProps, purely because of the fact that its called after every update, whether its the change in state or change in props or re-render of parent.
However whatever is the case, simply returning the state from getDerivedStateFromProps is not the right way, you need to compare the state and props before returning the value. Else with every update the state is getting reset to props and the cycle continues
As per the docs
getDerivedStateFromProps is invoked right before calling the render
method, both on the initial mount and on subsequent updates. It should
return an object to update the state, or null to update nothing.
This method exists for rare use cases where the state depends on
changes in props over time. For example, it might be handy for
implementing a <Transition> component that compares its previous and
next children to decide which of them to animate in and out.
Deriving state leads to verbose code and makes your components
difficult to think about. Make sure you’re familiar with simpler
alternatives:
If you need to perform a side effect (for example, data fetching
or an animation) in response to a change in props, use
componentDidUpdate lifecycle instead.
If you want to re-compute some data only when a prop changes, use
a memoization helper instead.
If you want to “reset” some state when a prop changes, consider
either making a component fully controlled or fully uncontrolled
with a key instead.
P.S. Note that the arguments to getDerivedStateFromProps are props and state and not nextProps and prevProps
To get into more details,
In order to make changes based on props change, we need to store prevPropsState in state, in order to detect changes. A typical implementation would look like
static getDerivedStateFromProps(props, state) {
// Note we need to store prevPropsState to detect changes.
if (
props.myPropsState !== state.prevPropsState
) {
return {
prevPropsState: state.myState,
myState: props.myPropsState
};
}
return null;
}

Finally, I resolved my issue. It was a painful debugging:
// Child Component
// instead of this
// this.props.onMyDisptach([...myPropsState])
// dispatching true value since myPropsState contains only numbers
this.props.onMyDispatch([...myPropsState, true])
This is because, I have two conditions: 1) on checkbox change (component) 2) on reset button pressed (child component)
I was needing to reset the states when reset button is pressed. So, while dispatching state to the props for reset button, I used a boolean value to know it's a change from the reset. You may use anything you like but need to track that.
Now, here in the component, I found some hints to the differences between componentWillReceiveProps and getDerivedStateFromProps after debugging the console output.
// Component
static getDerivedStateFromProps(props, state) {
const { myPropsState: myState } = props
// if reset button is pressed
const true_myState = myState.some(id=>id===true)
// need to remove true value in the store
const filtered_myState = myState.filter(id=>id!==true)
if(true_myState) {
// we need to dispatch the changes to apply on its child component
// before we return the correct state
props.onMyDispatch([...filtered_myState])
return {
myState: filtered_myState
}
}
// obviously, we need to return null if no condition matches
return null
}
Here's what I found the results of the console output:
getDerivedStateFromProps logs immediately whenever props changes
componentWillReceiveProps logs only after child propagates props changes
getDerivedStateFromProps doesn't respond to the props changes ( I meant for the dispatch changes as in the example code)
componentWillReceiveProps responds to the props changes
Thus, we needed to supply the changes to child component while using getDerivedStateFromProps.
The process of pasting true value in the state I require because getDerivedStateFromProps handle all the changes unlike componentWillReceiveProps handles only the child component dispatches the changes to the props.
By the way, you may use custom property to check if it is changed and update the value if getDerivedStateFromProps but for some reason I have to tweak this technique.
There might be some confusion on my wording but I hope you'll get it.

From the react docs:
Note that this method is fired on every render, regardless of the cause. This is in contrast to UNSAFE_componentWillReceiveProps, which only fires when the parent causes a re-render and not as a result of a local setState.
You are effectively overriding your state with the current props every time after calling setState(). So when you check a box (e) => this.handleMethod(e, comp.myState) is called which is assume calls setState() to update the checked state of the checkbox. But after that getDerivedStateFromProps() will be called (before render) that reverts that change. This is why unconditionally updating state from props is considered an anti-pattern.

React State management

I am very new to React. On going through the React tutorial, I understand the need to lift the state to the parent component which ensures that the children components can stay in sync and can feed off the major chunk of data residing in the state of the parent. But then , with changing data, won't the setState in the parent fire off the re-rendering of all the children components, decreasing UI performance ? Without using flux or redux, which is the best way to position the state in a react application ?

When you change a component's state, it triggers a re-render for that component.
The re-render will create a new set of virtual elements (returned from the render function), which React uses to represent the new state of the DOM. However, unless there are differences between the virtual DOM and the real DOM, nothing will be changed.
The fact that React's virtual elements are simply lightweight object representations of actual HTML elements makes it fast enough that in most cases, you don't even need to think about the performance cost of calling render for child components.
If, however, you do find yourself with a child component that has a particularly expensive render function—you can prevent it from always re-rendering by implementing shouldComponentUpdate. This allows you to specify with fine grained control, which changes in props or state will actually trigger a component to update.
The other approach is to build a custom state management solution which explicitly ties your components together.
let state = { count: 0 };
let listenForState = null;
function ComponentA() {
let onClick = () => {
state.count += 1;
if (listenForState) listenForState(state);
};
return <button onClick={onClick}>+</button>;
}
class ComponentB extends React.Component {
constructor() {
super();
this.state = state;
}
componentWillMount() {
listenForState(state => this.setState(state));
}
render() {
return <span>{this.state.count}</span>;
}
}
However this quite quickly gets out of hand, and there aren't many situations where it would be better than just lifting state up to a shared parent.
If you find that your state driven child components are nested too deeply in the stateful parent, then it's time to look at Redux instead.

Yes, your mostly right. But you dont need to pass the state of the component to the children, you r passing only props, its not actually the same, because changing of props doesnt need to rerender your component.
There is amazing method in the component lifecycle, called shouldComponentUpdate, default its return true if only shallow equality of states objects are false (if refs are different - thats why mutate of state will not rerender component). And you can also implement this method by yourself, comparing your props to check if you need an update or not. There is a scheme how would it work.
And also, react will not rerender element if the virtual DOM was not changed. So this way you can rerender component if only his props is changed, but if you need to change props of its children - you must rerender current.
Basicly, because of these there is an advice to pull the state of application from top to the leafs component (which returns most of html). You can read about optimization more here.
And must to say :
Your code must become a spaghetti
If you will have a lot of components, which have a lot of events, based on state, so if you need some complex architecture, use Redux or some other global state manager.

If you are looking only for a State Management library, check Duix (https://www.npmjs.com/package/duix).
Redux solves a lot of problems (adding complexity). If you only want to improve the State Management, just take a look at Duix.
PS.: I created that library

When to use state and when props?

I am studying the principles of react.
According to some reviews, some people says is better to keep your component stateless, what does it mean?
But other people says, that if you need to update your component, then you should learn how to set your state to the proper state.
I saw this.props / this.setProps and this.state / this.setState and I am confuse with that.
Something I am trying to figure is, how can I update a component by itself and not from a parent component? should I use props or state in this case?
I already read some docs about props and state, what I don't have clear, is: when to use one or another ?

Props vs. state comes down to "who owns this data?"
If data is managed by one component, but another component needs access to that data, you'd pass the data from the one component to the other component via props.
If a component manages the data itself, it should use state and setState to manage it.
So the answer to
how can I update a component by itself and not from a parent component? should I use props or state in this case?
is to use state.
Props should be considered immutable and should never be changed via mutation. setProps is only useful on a top-level component and generally should not be used at all. If a component passes another component a property, and the first component wants the second to be able to change it, it should also pass it a function property that the second component can call to ask the first component to update its state. For example:
var ComponentA = React.createClass({
getInitialState: function() {
return { count: 0 };
},
render: function() {
return <Clicker count={this.state.count} incrementCount={this.increment} />;
},
increment: function() {
this.setState({count: this.state.count + 1});
}
});
// Notice that Clicker is stateless! It's only job is to
// (1) render its `count` prop, and (2) call its
// `incrementCount` prop when the button is clicked.
var Clicker = React.createClass({
render: function() {
// clicker knows nothing about *how* to update the count
// only that it got passed a function that will do it for it
return (
<div>
Count: {this.props.count}
<button onClick={this.props.incrementCount}>+1</button>
</div>
);
}
});
(Working example: https://jsbin.com/rakate/edit?html,js,output)
For and object-oriented programming analogy, think of a class/object: state would be the properties you put on the class; the class is free to update those as it sees fit. Props would be like arguments to methods; you should never mutate arguments passed to you.
Keeping a component "stateless" means that it doesn't have any state, and all its rendering is based on its props. Of course, there has to be state somewhere or else your app won't do anything! So this guideline is basically saying to keep as many components as possible stateless, and only manage the state in as few top-level components as possible.
Keeping components stateless makes them easier to understand, reuse, and test.
See A brief interlude: props vs state in the React docs for more information.

Use state when you know the variable value is going to affect the view. This is particularly critical in react, because whenever the state variable changes there is a rerender(though this is optimized with the virtual DOM, you should minimize it if you can), but not when a prop is changed (You can force this, but not really needed).
You can use props for holding all other variables, which you think can be passed into the component during the component creation.
If you have want to make a multi-select dropdown called MyDropdown for example
state = {
show: true,
selected:[],
suggestions:this.props.suggestionArr.filter((i)=>{
return this.state.suggestions.indexOf(i)<0;
})
}
props={
eventNamespace:'mydropdown',
prefix : 'm_',
suggestionArr:[],
onItemSelect:aCallbackFn
}
As you can see, the objects in the state variable are going to affect the view some way or the other.
The objects in the props are mostly objects which should remain the same throughout the component life cycle. So these objects can be callback functions, strings used to namespace events or other holders.
So if you do want to update the component by itself, you need to have to look into how componentWillRecieveProps ,componentWillUpdate, componentDidUpdate and componentShouldUpdate works. More or less, this depends on the requirement and you can use these lifecycle methods to ensure that the rendering is within the component and not in the parent.