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React State seems to be overwritten / setState seems not to work

I am learning React and I think I am missing something fundamental with updating the state / rendering components.
const allFalse = new Array(data.length)
const allTrue = new Array(data.length)
allFalse.fill(false)
allTrue.fill(true)
const [memoryStatus, setMemoryStatus] = useState(allFalse)
const [baseValue, setBaseValue] = useState(false)
The memory game has 5 cards at this point (just learning here) and depending on the memoryStatus it is determined if one side or other side is shown (true / false).
When clicked on a card I obviously want to change the value of that card in the array. I am doing that with this function:
const handleChange = (position) => {
const newMemoryStatus = memoryStatus.map((item, index) =>
{
if(index === position) {
return !item
}
else return item
}
)
// i really dont understand why this does not change the state
setMemoryStatus[newMemoryStatus]
}
The render part is:
<div className={styles.container}>
{data.map((item, index) => {
return (
<div
key={index}
onClick={() => {handleChange(index)}}
className={styles.card}
>
{!memoryStatus[index] && <Image
src={item.img}
width="100px"
height="100px"
/>}
<span>
<center>
{memoryStatus[index] ? item.latinName : ''}
</center>
</span>
</div>
)})
}
</div>
Just in case it matters my data looks like this:
const data = [
{
name: 'Staande geranium',
latinName: 'Pelargonium zonate',
img: '/../public/1.png'
},
{
name: 'Groot Afrikaantje',
latinName: 'Tagetes Erecta',
img: '/../public/2.png'
},
{
name: 'Vuursalie',
latinName: 'Salvia splendens',
img: '/../public/3.png'
},
{
name: 'Kattenstaart',
latinName: 'Amaranthus caudatus',
img: '/../public/4.png'
},
{
name: 'Waterbegonia',
latinName: 'Begonia semperflorens',
img: '/../public/5.png'
}]
What am I doing wrong ??

setMemoryStatus is a function, thus you should be using parentheses () instead of brackets [] when calling it. The line to call it should be:
setMemoryStatus(newMemoryStatus);

Problem with alert after adding new value to the array in React.js

During the React.js course I'm doing, I was tasked with making a simple fortune-teller app. Theoretically, everything works as planned, but I did the task differently than the tutor. Instead of a simple fortune-telling table, I've created an array of objects, each with its id and 'omen'. The problem arose when after adding a new 'omen' an alert should be displayed that gives the current content of 'omens' in state. Only the previous values appear, without the added value. I will be grateful for the hints. In the original design, this problem does not occur, although it is very similar.
class Draw extends React.Component {
state = {
index: "",
value: "",
omens: [
{ id: 1, omen: "Hard work pays off" },
{ id: 2, omen: "You will be rich" },
{ id: 3, omen: "Be kind to others" },
],
};
handleDrawOmen = () => {
const index = Math.floor(Math.random() * this.state.omens.length + 1);
this.setState({
index: index,
});
};
showOmen = () => {
let omens = this.state.omens;
omens = omens.filter((omen) => omen.id === this.state.index);
return omens.map((omen) => (
<h1 id={omen.id} key={omen.id}>
{omen.omen}
</h1>
));
};
handleInputChange = (e) => {
this.setState({
value: e.target.value,
});
};
handleAddOmen = () => {
if (this.state.value === "") {
return alert("Enter some omen!");
}
const omens = this.state.omens.concat({
id: this.state.omens.length + 1,
omen: this.state.value,
});
this.setState({
omens,
value: "",
});
console.log(this.state.omens);
alert(
`Omen added. Actual omens: ${this.state.omens.map(
(omen) => omen.omen
)}`
);
};
render() {
return (
<div>
<button onClick={this.handleDrawOmen}>Show omen</button>
<br />
<input
placeholder="Write your own omen..."
value={this.state.value}
onChange={this.handleInputChange}
/>
<button onClick={this.handleAddOmen}>Add omen</button>
{this.showOmen()}
</div>
);
}
}
ReactDOM.render(<Draw />, document.getElementById("root"));

The state object is immutable. So you need to create your new array and apply it afterwards:
const omens = [
...this.state.omens,
{
id: this.state.omens.length + 1,
omen: this.state.value,
}
]
also setState is async so you need to wait until it finished:
this.setState({
omens,
value: "",
}, () => {
alert(
`Omen added. Actual omens: ${this.state.omens.map(
(omen) => omen.omen
)}`
)
https://reactjs.org/docs/react-component.html#setstate

Unexpected mutation of array in React

I am just learning to program and am writing one of my first applications in React. I am having trouble with an unexpected mutation that I cannot find the roots of. The snippet is part of a functional component and is as follows:
const players = props.gameList[gameIndex].players.map((item, index) => {
const readyPlayer = [];
props.gameList[gameIndex].players.forEach(item => {
readyPlayer.push({
id: item.id,
name: item.name,
ready: item.ready
})
})
console.log(readyPlayer);
readyPlayer[index].test = "test";
console.log(readyPlayer);
return (
<li key={item.id}>
{/* not relevant to the question */}
</li>
)
})
Now the problem is that readyPlayer seems to be mutated before it is supposed to. Both console.log's read the same exact thing. That is the array with the object inside having the test key as "test". forEach does not mutate the original array, and all the key values, that is id, name and ready, are primitives being either boolean or string. I am also not implementing any asynchronous actions here, so why do I get such an output? Any help would be greatly appreciated.
Below is the entire component for reference in its original composition ( here also the test key is replaced with the actual key I was needing, but the problem persists either way.
import React from 'react';
import { Link } from 'react-router-dom';
import PropTypes from 'prop-types';
// import styles from './Lobby.module.css';
const Lobby = ( props ) => {
const gameIndex = props.gameList.findIndex(item => item.id === props.current.id);
const isHost = props.gameList[gameIndex].hostId === props.user.uid;
const players = props.gameList[gameIndex].players.map((item, index) => {
const isPlayer = item.id === props.user.uid;
const withoutPlayer = [...props.gameList[gameIndex].players];
withoutPlayer.splice(index, 1);
const readyPlayer = [];
props.gameList[gameIndex].players.forEach(item => {
readyPlayer.push({
id: item.id,
name: item.name,
ready: item.ready
})
})
const isReady = readyPlayer[index].ready;
console.log(readyPlayer);
console.log(!isReady);
readyPlayer[index].ready = !isReady;
console.log(readyPlayer);
return (
<li key={item.id}>
{isHost && index !== 0 && <button onClick={() => props.updatePlayers(props.gameList[gameIndex].id, withoutPlayer)}>Kick Player</button>}
<p>{item.name}</p>
{isPlayer && <button onClick={() =>props.updatePlayers(props.gameList[gameIndex].id, readyPlayer)}>Ready</button>}
</li>
)
})
let showStart = props.gameList[gameIndex].players.length >= 2;
props.gameList[gameIndex].players.forEach(item => {
if (item.ready === false) {
showStart = false;
}
})
console.log(showStart);
return (
<main>
<div>
{showStart && <Link to="/gameboard" onClick={props.start}>Start Game</Link>}
<Link to="/main-menu">Go back to Main Menu</Link>
</div>
<div>
<h3>Players: {props.gameList[gameIndex].players.length}/4</h3>
{players}
</div>
</main>
);
}
Lobby.propTypes = {
start: PropTypes.func.isRequired,
current: PropTypes.object.isRequired,
gameList: PropTypes.array.isRequired,
updatePlayers: PropTypes.func.isRequired,
user: PropTypes.object.isRequired
}
export default Lobby;
Note: I did manage to make the component actually do what it is supposed, but the aforementioned unexpected mutation persists and is still a mystery to me.

I have created a basic working example using the code snippet you provided. Both console.log statements return a different value here. The first one returns readyPlayer.test as undefined, the second one as "test". Are you certain that the issue happens within your code snippet? Or am I missing something?
(Note: This answer should be a comment, but I am unable to create a code snippet in comments.)
const players = [
{
id: 0,
name: "John",
ready: false,
},
{
id: 1,
name: "Jack",
ready: false,
},
{
id: 2,
name: "Eric",
ready: false
}
];
players.map((player, index) => {
const readyPlayer = [];
players.forEach((item)=> {
readyPlayer.push({
id: item.id,
name: item.name,
ready: item.ready
});
});
// console.log(`${index}:${readyPlayer[index].test}`);
readyPlayer[index].test = "test";
// console.log(`${index}:${readyPlayer[index].test}`);
});
console.log(players)

How should I implement saving state to localStorage?

CODE:
var React = require('react');
var Recipe = require('./Recipe.jsx');
var AddRecipe = require('./AddRecipe.jsx');
var EditRecipe = require('./EditRecipe.jsx');
var RecipeBox = React.createClass({
getInitialState: function () {
return {
recipesArray: [],
adding: false,
editing: false,
currentIndex: 0
};
},
handleClick: function () {
this.setState({
adding: true
});
},
handleEditClick: function(index) {
this.setState({
editing: true,
currentIndex: index
});
},
handleDeleteClick: function(index) {
var newRecipesArray = this.state.recipesArray;
newRecipesArray.splice(index-1,1);
this.setState({
recipesArray: newRecipesArray
});
},
handleClose: function() {
this.setState({
adding: false,
editing: false
});
},
handleAdd: function(newRecipe) {
this.setState({
recipesArray: this.state.recipesArray.concat(newRecipe)
});
},
handleEdit: function(newRecipe, index) {
var newRecipesArray = this.state.recipesArray;
newRecipesArray[index-1] = newRecipe;
this.setState({
recipesArray: newRecipesArray
});
},
render: function() {
var i = 0;
var that = this;
var recipes = this.state.recipesArray.map(function(item) {
i++
return (
<div key={"div"+i} className="table">
<Recipe key={i} name={item.name} ingredients={item.ingredients} />
<button key ={"edit"+i} onClick={() => { that.handleEditClick(i)}} className="btn edit btn-primary">Edit</button>
<button key ={"delete"+i} onClick={() => { that.handleDeleteClick(i)}} className="btn delete btn-danger">Delete</button>
</div>
);
});
return (
<div>
<h1>React.js Recipe Box</h1>
<button className="btn btn-primary" onClick={this.handleClick}>Add Recipe</button>
<table>
<tbody>
<tr>
<th>RECIPES</th>
</tr>
</tbody>
</table>
{recipes}
{ this.state.adding ? <AddRecipe handleClose={this.handleClose} handleAdd={this.handleAdd} /> : null }
{ this.state.editing ? <EditRecipe currentIndex = {this.state.currentIndex} handleClose={this.handleClose} handleEdit={this.handleEdit}/> : null }
</div>
);
},
});
module.exports = RecipeBox;
QUESTION:
How should I implement saving state to localStorage ?
What would be the most elegant implementation ?
Currently learning React and looking to write clean and elegant code.

Whenever an update to state is fired, it will trigger the lifecycle method of componentDidUpdate. You can hook into that method in order to save the state of the component.
componentDidUpdate() {
window.localStorage.setItem('state', JSON.stringify(this.state));
}
Depending on your use case, you should be able to load it back up on componentDidMount.
componentDidMount() {
// there is a chance the item does not exist
// or the json fails to parse
try {
const state = window.localStorage.getItem('state');
this.setState({ ...JSON.parse(state) });
} catch (e) {}
}
I would warn you, you probably want a solution more like redux with a localStorage adapter for a "full-fledged" solution. This one is pretty frail in a few different ways.

I would take a look at plugins that make localstorage easier (not browser specific). An example would be this:
https://github.com/artberri/jquery-html5storage
The page above has all the information you need to get started. If that one doesn't work then I would continue to search. There are plenty out there. There may be newer ones that use React as well. The jQuery plugins have worked for me when I was learning/doing Angular.

ReactJS not able to reference state created

http://jsfiddle.net/adamchenwei/3rt0930z/20/
I just trying to create an example to learn how state works in a list.
What I intent to do is to allow a particular value that got repeated in a list, to change, in ALL items in the list, by using state. For example, in this case, I want to change all the list item's name to 'lalala' when I run changeName of onClick.
However I have this warning (issue at fiddle version 11, resolved at version 15)
Any help on resolving it to achieve purpose above?
Actual Code
var items = [
{ name: 'Believe In Allah', link: 'https://www.quran.com' },
{ name: 'Prayer', link: 'https://www.quran.com' },
{ name: 'Zakat', link: 'https://www.quran.com' },
{ name: 'Fasting', link: 'https://www.quran.com' },
{ name: 'Hajj', link: 'https://www.quran.com' },
];
var ItemModule = React.createClass({
getInitialState: function() {
return { newName: this.props.name }
},
changeName() {
console.log('changed name');
this.setState({ newName: 'lalala' });
},
render() {
//<!-- <a className='button' href={this.props.link}>{this.props.name}</a> -->
return (
<li onClick={this.changeName}>
{this.state.newName}
</li>
);
}
});
var RepeatModule = React.createClass({
getInitialState: function() {
return { items: [] }
},
render: function() {
var listItems = this.props.items.map(function(item) {
return (
<div>
<ItemModule
key={item.name}
name={item.name} />
</div>
);
});
return (
<div className='pure-menu'>
<h3>Islam Pillars</h3>
<ul>
{listItems}
</ul>
</div>
);
}
});
ReactDOM.render(<RepeatModule items={items} />,
document.getElementById('react-content'));
-UPDATE-
fiddle version 16
updated fidle, now there is issue with key, also, the onClick did not update the value for all the list item. Is there something wrong I did?
-UPDATE-
fiddle version 20
Now the only issue is change all the list item's name to 'lalala' when I run changeName of onClick.

remove the parenthesis from
onClick={this.changeName()},
so
onClick={this.changeName}
you want to call the function onClick, but you are calling it on render that way

I think you meant to do onClick={this.changeName}
In the way you have it you are calling the changeName function on render instead of on click.