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React trying to assign props to state variable doesn't work

I'm trying to assign props that I get from parent component and assign it to state in child component(because I want to manipulate the props data I assign it to state first).
When I log the state variable it comes out as an empty array but when I make a new variable in render and assign props to it and log it. It does show the data I need. Also, when I just log this.props I can definitely see that props holds the data I need.
I've assigned props to state a couple of times before, so I'm not sure what is so different this time for it not to work.
Parent component where I pass props to child:
<ShowAvailableTimeslots onClick={this.getSelectedTimeslot} allTimeSlots={this.state.AvailabletimeSlots} />
Child component where I try to assign props to state:
class ShowAvailableTimeslots extends Component {
constructor(props) {
super(props)
this.state = {
sliceEnd: 5,
sliceStart:0,
selectedSlotValue: "",
timeSlotArr: this.props.allTimeSlots,
// timeSlotSlice: timeSlotArr.slice(this.state.sliceStart, this.state.sliceEnd)
}
}
handleTimeSlotClick = (timeSlot) => {
this.setState({ selectedSlotValue: timeSlot }, () => {
this.props.onClick(this.state.selectedSlotValue)
console.log('time slot value', timeSlot)
});
}
previousSlots =()=>{
var test;
}
forwordSlots =()=>{
var test;
}
render() {
var timeSlotArrRender = this.props.allTimeSlots;
return (
<React.Fragment>
{console.log("state", this.state.timeSlotArr)} // --> doesn't show data
{console.log("props", this.props)} // --> does show data
{console.log("render variable", timeSlotArrRender )} // --> does show data
<button className="button btn" onClick={() => this.previousSlots()} disabled={this.state.sliceStart === 0}>left</button>
{/* {this.state.timeSlotArr.map(timeSlot => <a className="timeslot btn " key={timeSlot} value={timeSlot} onClick={() => this.handleTimeSlotClick(timeSlot)}>{timeSlot}</a>)
} */}
<button className="button btn">right</button>
</React.Fragment>
)
}
}
export default ShowAvailableTimeslots

the constructor is called when the component life cycle begins.
You are passing the this.state.AvailabletimeSlots from the parent and by then the constructor have already been called and the assignment for timeSlotArr is already done, so
timeSlotArr: this.props.allTimeSlots // will not work
You have to get help of life cycle methods or hooks
componentWillReceiveProps(nextProps){
this.setState({timeSlotArr: nextProps.allTimeSlots })
}
According to new changes you have to use
static getDerivedStateFromProps(nextProps, prevState){
return {
timeSlotArr: nextProps.allTimeSlots
};
}

I have it working just fine. https://jsfiddle.net/85zc4Lxb/
class App extends React.Component {
render() {
return (<Child passing="I am being passed to child" />);
}
}
class Child extends React.Component {
constructor(props) {
super(props)
this.state = {
passedProp: this.props.passing,
}
}
render() {
return (
<React.Fragment>
<button>{this.state.passedProp}</button>
</React.Fragment>
)
}
}
ReactDOM.render(
<App />,
document.getElementById('container')
);

I try to recreate the scenario and it work try saving all your files again and then check
parents component
import React, { Component } from "react";
import TestOne from "./Components/TestOne/TestOne";
export class App extends Component {
state = {
x: "x data",
y: "y data",
};
render() {
return (
<div>
<TestOne x={this.state.x} allTimeSlots={this.state.y}/>
</div>
);
}
}
export default App;
Child component
import React, { Component } from "react";
export class TestOne extends Component {
constructor(props) {
super(props);
this.state = {
sliceEnd: 5,
sliceStart: 0,
selectedSlotValue: "",
timeSlotArr: this.props.x,
};
}
render() {
var timeSlotArrRender = this.props.allTimeSlots;
return (
<React.Fragment>
{console.log("state", this.state.timeSlotArr)}
{console.log("props", this.props)}
{console.log("render variable", timeSlotArrRender)}
<button className="button btn">right</button>
</React.Fragment>
);
}
}
export default TestOne;
Result:

I think you are missing this.setState({})

Unmounted components still being rendered first

I'm new to React, just a question on componentWillUnmount() and render() lifecycle method. Below is some code:
...
export default class App extends Component {
constructor(props) {
super(props);
this.state = { showMessage: true }
}
handleChange = () => { this.setState({ showMessage: !this.state.showMessage });
}
render(){
<div>
<input type="checkbox" checked={ this.state.showMessage } onChange={ this.handleChange } />
<label>Show</label>
</div>
{ this.state.showMessage && <Message message="Hello" /> }
}
}
export class Message extends Component {
...
componentWillUnmount() {
console.log("Unmount Message Component");
}
render(){
console.log(`Render Message Component `);
return (
<div>{this.props.message}</div/
)
}
}
I trigger the unmounting phase of the Message componentby unchecking the checkbox , so I have those in the console output:
Render Message Component
Unmount Message Component
so my question is:
It is not efficient because the current Message component is going to be destroyed since I don't need it once I uncheck the box. But Message component's render() was still being called, which is unnecessary since we don't really care what content it contains, is it a way to avoid the call of re-render method and just get componentWillUnmount() to be called? I was thinking to use shouldComponentUpdate(), but I can stop render() method to be called, but that will also stop componentWillUnmount() to be called too

When you unmount a component render is not executed - only componentWillUnmount is called. The Render Message Component log is caused by initial render when Message is visible:
class App extends React.Component {
constructor(props) {
super(props);
this.state = { showMessage: true }
console.log("initial render:");
}
handleChange = () => { this.setState({ showMessage: !this.state.showMessage });
}
render(){
this.state.showMessage || console.log("unmounting:");
return <div>
<input type="checkbox" checked={ this.state.showMessage } onChange={ this.handleChange } />
<label>Show</label>
{ this.state.showMessage && <Message message="Hello" /> }
</div>
}
}
class Message extends React.Component {
componentWillUnmount() {
console.log("Unmount Message Component");
}
render(){
console.log(`Render Message Component `);
return (
<div>{this.props.message}</div>
)
}
}
ReactDOM.render(<App/>, document.getElementById("root"))
<script src="https://cdnjs.cloudflare.com/ajax/libs/react/16.6.3/umd/react.development.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/react-dom/16.6.3/umd/react-dom.development.js"></script>
<div id="root"></div>

Running method of component child from this.props.children array

import React from "react";
import ReactDOM from "react-dom";
class NestedComponent extends React.Component {
constructor(props) {
super(props);
this.childMethod = this.childMethod.bind(this);
}
childMethod() {
alert("Child method one ran");
}
render() {
return <div>NestedComponent</div>;
}
}
class NestedComponentTwo extends React.Component {
constructor(props) {
super(props);
this.childMethod = this.childMethod.bind(this);
}
childMethod() {
alert("Child method two ran");
}
render() {
return <div>NestedComponentTwo</div>;
}
}
class WrappingComponent extends React.Component {
constructor(props) {
super(props);
this.runMethod = this.runMethod.bind(this);
}
runMethod() {
let child = this.props.children[0];
/** Always returns as undefined */
//if (typeof child.childMethod == "function") {
// child.childMethod();
//}
/**
* EDIT: Close, however the this binding seems to not be working. I can however provide the childs props to the childMethod and work with that.
*/
if(typeof child.type.prototype.childMethod == "funciton"){
child.type.prototype.childMethod();
}
}
render() {
return (
<div>
{this.props.children}
<button onClick={this.runMethod}>run</button>
</div>
);
}
}
const App = ({}) => {
return (
<div>
<WrappingComponent>
<NestedComponent />
<NestedComponentTwo />
</WrappingComponent>
</div>
);
};
if (document.getElementById("example")) {
ReactDOM.render(<App />, document.getElementById("example"));
}
So the goal is to have optional methods attached to a nested component that can execute from the wrapping component, almost like an event emmiter. For some reason though, the method that exists on the child component claims not to exist. However whenever I log the child component pulled from the array of the this.props.children the prototype has the method listed.
Am I missing a special way to access methods of children components through a methods variable perhaps?

Found the variable I can use to access it. If anyone has any more insight into this, or reasons why what I am doing is poor practice please let me know.
Editing the question where this is needed, but the item below is accessing the function of the child:
child.type.prototype.childMethod
Does not appear to maintain the this binding. Passing props down does work however.

You should manage all of this logic in the top level component (the App component)
class NestedComponent extends React.Component {
constructor(props) {
super(props);
this.childMethod = this.childMethod.bind(this);
}
childMethod() {
alert("Child method one ran");
}
render() {
return <div>NestedComponent</div>;
}
}
class NestedComponentTwo extends React.Component {
constructor(props) {
super(props);
this.childMethod = this.childMethod.bind(this);
}
childMethod() {
alert("Child method two ran");
}
render() {
return <div>NestedComponentTwo</div>;
}
}
class WrappingComponent extends React.Component {
render() {
return (
<div>
{this.props.children}
<button onClick={this.props.onClick}>run</button>
</div>
);
}
}
class App extends React.Component {
constructor(props) {
super(props);
this.runMethod = this.runMethod.bind(this);
}
runMethod() {
if (this.nestedComponent) {
this.nestedComponent.childMethod();
}
}
render() {
return (
<div>
<WrappingComponent onClick={this.runMethod}>
<NestedComponent ref={el => this.nestedComponent = el} />
<NestedComponentTwo />
</WrappingComponent>
</div>
);
}
};
if (document.getElementById("example")) {
ReactDOM.render(<App />, document.getElementById("example"));
}
<script src="https://cdnjs.cloudflare.com/ajax/libs/react/15.1.0/react.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/react/15.1.0/react-dom.min.js"></script>
<div id="example"></div>
Moreover ref with string attribute is deprecated https://reactjs.org/docs/refs-and-the-dom.html#legacy-api-string-refs

Change parent component state from child component

I know that the question with this title has already been asked few times before but the problem is that I couldn't get an appropriate answer. So as I am new to reactJS and trying to create login logout form.
What I want to do is to pass or change a state of parent component from a child component through an event handler(When a user clicks on logout button). Below are the two Components:
First One:
class Home extends React.Component {
constructor(props){
super(props);
this.state = {login : false};
}
login(){
// this method updates the login.state : true
}
render() {
return (
<div>
{this.state.login ? (<ChatBox userNick="fad" />) : (<LoginScreen onSubmit={this.login} />) }
</div>
);
}
}
And Second:
class ChatBox extends React.Component{
logout(){
// Expecting or trying to update parent.state.login : false
// via this method as well
}
render() {
return (
<div className="chat-box">
<button onClick={this.logout} > Logout </button>
<h3>Hi, {this.props.userNick} </h3>
</div>
);
}
}
I have simplified these component to come on point.
What's going here?
Home Component is the main parent component. Initially the state.login is false and in this situation LoginScreen Components shows up. Now, when user login through LoginScreen Component state.login updates to true, it's time to show for ChatBox Component.
Now you can see that ChatBox Component contains a button which calls a method logout to logout user. What I want is to update once again the state.login to false in Home Component When user click on the Logout Button.
I don't know how to do it, It will be appreciate if you help me.
Thanks in advance.

Do it in the same way as you are doing for Login, pass a function as a prop and call it on logout, see updates below.
const LoginScreen = () => (<div>Login Screen</div>);
class Home extends React.Component {
constructor(props){
super(props);
this.state = {login : true};
this.logout = this.logout.bind(this);
}
login(){
// this method updates the login.state : true
}
logout() {
// this method updates the login.state : false
this.setState({ login: false });
}
render() {
return (
<div>
{this.state.login ? (<ChatBox userNick="fad" onLogout={this.logout} />) : (<LoginScreen onSubmit={this.login} />) }
</div>
);
}
}
class ChatBox extends React.Component{
constructor(props) {
super(props)
// This makes sure `this` keeps pointing on this instance when logout is called from the outside.
this.logout = this.logout.bind(this);
}
logout(){
// Call the onLogout property.
this.props.onLogout();
}
render() {
return (
<div className="chat-box">
<button onClick={this.logout} > Logout </button>
<h3>Hi, {this.props.userNick} </h3>
</div>
);
}
}
ReactDOM.render(<Home />, document.querySelector('#main'));
<script src="https://cdnjs.cloudflare.com/ajax/libs/react/15.1.0/react.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/react/15.1.0/react-dom.min.js"></script>
<div id="main"></div>

You can pass an event from the Parent component to the Child component that handles the change of the state, like so:
class App extends React.Component {
constructor() {
super();
this.state = { isLoggedIn: false };
}
_handleLogin() {
this.setState({ isLoggedIn: true });
}
_handleLogout() {
this.setState({ isLoggedIn: false });
}
render() {
const { isLoggedIn } = this.state;
return (
<div>
{
isLoggedIn ?
<ChatBox logoutEvent={this._handleLogout.bind(this)} />
:
<Login loginEvent={this._handleLogin.bind(this)} />
}
</div>
);
}
}
const Login = ({ loginEvent }) => (
<button type="button" onClick={loginEvent}>Login</button>
);
const ChatBox = ({ logoutEvent }) => (
<div>
<h1>This is the Chat Box!</h1>
<button type="button" onClick={logoutEvent}>Logout</button>
</div>
);
ReactDOM.render(
<App />,
document.getElementById('container')
);
Here's the fiddle

Why doesn't this child component rerender?

I'm experimenting with ReactJS and I'm trying to understand how child component rendering is triggered. In ReactJS, if I set up an example like this:
var externalCounterVar = 10
class Counter extends React.Component {
constructor(props){
super(props);
this.state = props;
}
render() {
console.log('rendering counter')
return (
<div> {externalCounterVar} </div>
)
}
}
class Main extends React.Component {
constructor(props){
super(props);
}
handleClick() {
externalCounterVar += 1;
}
rerender(){
this.render();
}
render() {
console.log('rendering');
return (
<div>
<button onClick={this.rerender.bind(this)} />
<Counter counter={externalCounterVar} />
</div>
)
}
}
ReactDOM.render(<Main />, document.getElementById('root'));
I'm not sure I understand why when you "rerender" it calls the render method of Main but not Counter? It seems like it should call both render methods since it's rendering Main and Counter is a child of Main.
So when rerender is called, 'rendering' will print but 'rendering counter' will not.

It looks like you're overlooking one of the main benefits of using React, namely how state works.
You never, ever need to call this.render within a React component
You should never set state dynamically, ie: this.state = ...
You should always use this.setState to set your state.
Rewritten, your code should look something like the following:
const externalCounterVar = 10
class Counter extends React.Component {
render() {
console.log('rendering counter')
return (
<div> {this.props.counter} </div>
)
}
}
class Main extends React.Component {
state = {
counter: externalCounterVar
}
handleClick() {
this.setState({counter: this.state.counter + 1});
}
render() {
console.log('rendering');
return (
<div>
<button onClick={this.handleClick.bind(this)} />
<Counter counter={this.state.counter} />
</div>
)
}
}
By calling this.setState, React automatically knows it needs to rerender your component, and as a result, all child components will also be rerendered.
Hope this helps!

In this case you don't have to use rerender method, also with purpose re-render all child components you need update state with method setState. And also accordingly to this you have to "move state up".
Here my example:
class Counter extends React.Component {
render() {
console.log('rendering counter');
return (<div> {this.props.counter} </div>);
}
}
class Main extends React.Component {
constructor(props){
super(props);
this.state = {counter: props.counter};
this.handleClick = this.handleClick.bind(this);
}
handleClick() {
this.setState(prevState => ({counter: ++prevState.counter}));
}
render() {
console.log('rendering');
return (
<div>
<button onClick={this.handleClick} />
<Counter counter={this.state.counter} />
</div>
);
}
}
var externalCounterVar = 10;
ReactDOM.render(
<Main counter={externalCounterVar} />,
document.getElementById('root')
);

In some situations you can use this.forceUpdate() to call re-render.
But, if you can not do this, do not do.
https://facebook.github.io/react/docs/react-component.html#forceupdate