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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.

React changing state in Parent , does render calls for all its children and sub-children as iteration?

i am setting the language name in my local storage , when it changes from a dropdown in topbar , i want the whole current view to be re-rendered and words translated to the selected language. my layout is like this
render(){
return (
<MainContainer>
<TopBar/>
<SideBar/>
<RouteInsideSwitch/>
</MainContainer>
)
}
in render of components ,the words to be translated basically calls a function that returns the correct word based on the local storage language name.
i change the language and i set the state in maincontainer for selected langauge and set it in local storage. however i dont want to move that state from Maincontainer to all my components. also dont want to store it in redux because then all the possible containers have to listen to it and then pass it to their children as props.
what currently happens is that saving state in mainContainer without passing it to any children , the children does re-render but only the immediate ones , if there are more children in those children and so on , it does not re-render because i m not passing the state throughout the chain.
open to any suggestion based on different pattern for language changing. but my question is that is there any way to re-render the current open view (all components in dom).

If your concern is that you have a number of "possible containers" which all need to handle the state change, perhaps consider creating a higher order component that includes the common language rendering logic (your RouteInsideSwitch leads me to believe this may the issue). In that way, you can avoid duplicating that logic across a ton of "possible" components that all require the functionality of dynamic language rendering and will avoid the need to dial a bunch of components into a redux store, assuming they are in the same hierarchy.
const DynamicLanguageComp = RenderComponent => {
return class extends Component {
constructor(props) {
super(props)
//additional state setup if needed
}
changeLangFunc = () => { /* handle change */ }
render() {
return <RenderComponent handleLanguageChange={this.changeLangFunc} {...this.props} {...this.state} />
}
}
}
If you would like to avoid a re-render on certain intermediate components that may be receiving props by way of state change you can implement the lifecycle method shouldComponentUpdate(), which by default returns true. You can make a comparison of nextProps to your current props, and return false if a re-render is undesired despite new props.

Child component constructor called multiple times

I have a parent component which is a flat list which contains a header HeaderComponent. This HeaderComponent is a custom component that I have created which contains 2 child components of its own. Whenever i refresh the list, I am passing a boolean to the HeaderComponent as props which get passed onto its own children, I am doing this so I can check if each component needs to fetch new data or not. The problem is that whenever the parent refreshes and sets a new state the constructors of the child components get called everytime. Shouldn't the constructor be called only the first time the parent initializes and then all further calls involve calling the shouldComponentUpdate method of the children in order to see if it needs an update or not.
Parent component
_renderHeader = () => {
return <HeaderComponent Items={this.state.Data} refresh={this.state.refresh}/>;
};
render() {
console.log("TAG_RENDER render called " + this.state.refresh);
return (
<FlatList
refreshing={this.state.refresh}
onRefresh={() => {
console.log("onRefresh");
this.setState({
refresh: true
}, () => {
this._fetchData();
});
}}
......
ListHeaderComponent={() => this._renderHeader()}
.......
/>
);
}
Header Component
export default class HeaderComponent extends React.Component {
constructor(props) {
super(props);
console.debug("HeaderComponent");
}
render() {
return (
<MainHeader Items={this.props.Items}/>
<SubHeader refresh={this.props.refresh}/>
);
}
}
The constructor of MainHeader and Subheader gets called whenever the parent component refreshes. Does this mean that it is creating new child components each time it refreshes because I can see the render of the children also being called multiple times.

Control your index.js file. If you see <React.StrictMode>, you should change to <>. This is solved my problem.
It should be like:
ReactDOM.render(
<>
<App/>
</>,
document.getElementById('root')
);

As correctly stated in the one of the answers , removing the strict mode fixes the issue. Coming to why it does that, its because the strict mode intentionally calls the 'render' method twice in order to detect potential problems.
React works in two phases:render and commit. Render phase checks and determines the new changes to be applied. And commit phase applies it.
Render phase lifecycle includes methods like : constructor, UNSAFE_componentWillMount,UNSAFE_componentWillReceiveProps, ...,render and few more.
The render phase is time consuming and is often broken into pieces to free up the browser. Render phase might be called multiple times before the commit phase(usually very fast).
Since the render phase methods are called more than once, its important that none of those method have any problems or side effects.
Thus just in order to highlight the possible side effects to make them easy to spot, react explicitly double invoke the render phase methods.
You can read more about this on :https://reactjs.org/docs/strict-mode.html#detecting-unexpected-side-effects :)

Strict mode can’t automatically detect side effects for you, but it can help you spot them by making them a little more deterministic. This is done by intentionally double-invoking the following functions:
Class component constructor, render, and shouldComponentUpdate
methods
Class component static getDerivedStateFromProps method
Function component bodies
State updater functions (the first argument to setState)
Functions passed to useState, useMemo, or useReducer
https://reactjs.org/docs/strict-mode.html
As stated in the site,
Note:
This only applies to development mode. Lifecycles will not be double-invoked in production mode.

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.

Why can't I update props in react.js?

Why do we have both state and props? Why don't we just have one source of data? I'd like to update a component's props and have it re-render itself and all of its children. Seems simple but I can't figure out how to let a component update its own or its parent's props.
Thanks for any help.

The React philosophy is that props should be immutable and top-down. This means that a parent can send whatever prop values it likes to a child, but the child cannot modify its own props. What you do is react to the incoming props and then, if you want to, modify your child's state based on incoming props.
So you don't ever update your own props, or a parent's props. Ever. You only ever update your own state, and react to prop values you are given by parent.
If you want to have an action occur on a child which modifies something on the state, then what you do is pass a callback to the child which it can execute upon the given action. This callback can then modify the parent's state, which in turns can then send different props to the child on re-render.

To answer the question of why
In React, props flow downward, from parent to child.
This means that when we call ReactDOM.render, React can render the root node, pass down any props, and then forget about that node. It's done with. It's already rendered.
This happens at each component, we render it, then move on down the tree, depth-first.
If a component could mutate its props, we would be changing an object that is accessible to the parent node, even after the parent node had already rendered. This could cause all sorts of strange behaviour, for example, a user.name might have one value in one part of the app, and a different value in a different part, and it might update itself the next time a render is triggered.
To give a fictional example:
// App renders a user.name and a profile
const App = (props) =>
React.createElement('div', null, [
props.user.name,
React.createElement(Profile, props)
])
// Profile changes the user.name and renders it
// Now App has the wrong DOM.
const Profile = ({user}) => {
user.name = "Voldemort" // Uh oh!
return React.createElement('div', null, user.name);
}
// Render the App and give it props
ReactDOM.render(
React.createElement(App, {user: {name: "Hermione"}}),
document.getElementById('app'))
);
We render app. It outputs "Hermione" to the Shadow DOM. We render the Profile, it outputs "Voldemort". The App is now wrong. It should say "Voldemort" because user.name is "Voldemort", but we already output "Hermione", and it's too late to change it.
The value will be different in different parts of the app.
Modifying Props would be two-way-binding
Mutating props would be a form of two-way binding. We would be modifying values that might be relied on by another component higher up the tree.
Angular 1 had this, you could change any data anytime from wherever you were. In order to work, it needed a cyclical $digest. Basically, it would loop around and around, re-rendering the DOM, until all the data had finished propagating. This was part of the reason why Angular 1 was so slow.

In React, state and props serve different goals: state allows a component to maintain some changing values, while props are the mecanism to propagate those values to children.
Children are not allowed to alter by themselves the values they get via props just because React designers find it easier to maintain an application built this way. Their point is that when only one component is allowed to update some piece of state, it is easier to discover who altered it, and find the root of bugs.

the Component itself changes its state, and changes not its own, but the children's props.
<Parent>
<Child name={ this.state.childName } />
</Parent>
Parent can change its own state and change the child name, but it will change the props for his children.
edit1:
for calling events from the child to its parent, you should pass in the child an event handler like so:
var Child = React.createClass({
render: function() {
return (<button onClick={ this.props.onClick }>Hey</button>);
}
});
var Parent = React.createClass({
onChildClick: console.log.bind(console), // will print the event..
render: function() {
return (<Child onClick={ this.onChildClick } />);
}
});
React.renderComponent(<Parent />, document.body);
in this code, when you'll click on the Child's button, it will pass the event to its parent.
the purpose of passing the events is decoupling the components. maybe in your app you need this specific action, but in another app you'll have, you'll use it differently.

My solution was fairly different but some people might run into it. On the Chrome Dev tools, it kept saying that my props were read-only and when I tried passing them down even further, I would get an error. Now, the reason why is because I wasn't invoking a render() method. I was instead calling my component like this:
const Navigation = () =>{
return (
<div className="left-navigation">
<ul>
<Link to='/dashboard'><li>Home</li></Link>
<Link to='/create-seedz'><li>Create Seedz</li></Link>
<Link to='/create-promotion'><li>Create Promotion</li></Link>
<Link to='/setting'><li>Setting</li></Link>
<SignOutButton />
</ul>
</div>
);
}
I added a render method and it solved my issue of being able to pass props down:
class Navigation extends Component{
render(){
return (
<div className="left-navigation">
<ul>
<Link to='/dashboard'><li>Home</li></Link>
<Link to='/create-seedz'><li>Create Seedz</li></Link>
<Link to='/create-promotion'><li>Create Promotion</li></Link>
<Link to='/setting'><li>Setting</li></Link>
<SignOutButton user={this.props.user} signedOut={this.props.signedOut} authed={this.props.authed}/>
</ul>
</div>
);
}
}
Hopefully this helps someone.

Contrary to the answers provided here, you actually can update props directly, if you don't mind defying the pedantic circlejerk about "the React way." In React.js, find the following lines of code:
Object.freeze(element.props);
Object.freeze(element);
and comment them out. Voila, mutable props!