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What does the error for "Invalid value for prop `value` on <select>" mean in React?

I have been building a trivia game using the Open Trivia API and I am getting an error that I have been trying to figure out but am stuck on. It says: Invalid value for prop value on tag. Either remove it from the element, or pass a string or number value to keep it in the DOM.
I will include my code pages. Thank you in advance for you help.
App.js page:
import React from 'react'
import './App.css'
import axios from 'axios'
import Questions from './components/questions'
import CategorySelector from './components/CategorySelector'
class App extends React.Component {
constructor () {
super()
this.state = {
categories: [],
selectedCategory: null
}
this.selectedCategory = this.selectedCategory.bind(this)
}
componentDidMount () {
axios
.get('https://opentdb.com/api_category.php')
.then(response => {
console.log(response.data)
this.setState({
categories: response.data.trivia_categories
})
})
}
selectedCategory (category) {
this.setState({ selectedCategory: category })
}
render () {
const { categories, selectedCategory } = this.state
return (
<div className='App'>
<h2>Trivia Game</h2>
{
selectedCategory
? <Questions selectedCategory={selectedCategory} />
: (
<CategorySelector
categories={categories}
onSelect={event => selectedCategory}
selectedCategory={this.selectedCategory}
/>
)
}
</div>
)
}
}
export default App
CategorySelector.js page:
import React from 'react'
class CategorySelector extends React.Component {
render () {
const { categories, selectedCategory, onSelect } = this.props
return (
<div className='CategorySelector'>
<select
value={selectedCategory} onChange={onSelect}
>
<option value=''>-- No category selected --</option>
{categories.map(category => (
<option value={selectedCategory} key={category.id}>{category.name}</option>
))}
</select>
</div>
)
}
}
export default CategorySelector
Question.js page
import React from 'react'
import axios from 'axios'
class Questions extends React.Component {
constructor () {
super()
this.state = {
questions: [],
currentQuestionIndex: 0
}
this.handleNextQuestion = this.handleNextQuestion.bind(this)
}
componentDidMount () {
const qUrl = `https://opentdb.com/api.php?amount=3&category=${this.props.selectedCategory.id}`
axios
.get(qUrl)
.then(response => {
console.log(response.data)
this.setState({
questions: response.data.results
})
})
}
handleNextQuestion () {
this.setState({ currentQuestionIndex: this.state.currentQuestionIndex + 1 })
}
render () {
const { selectedCategory } = this.props
const { questions, currentQuestionIndex } = this.state
const currentQuestion = questions[currentQuestionIndex]
let answers = []
if (currentQuestion) {
answers = currentQuestion.incorrect_answers.concat([currentQuestion.correct_answer])
}
return (
<div className='Questions'>
<h2>{selectedCategory.name} Questions</h2>
{currentQuestion && (
<div>
<h3>{currentQuestion.questions}</h3>
<div>
{answers.map((answer, index) => <p key={index}>{answer}</p>)}
</div>
</div>
)}
{(currentQuestionIndex < questions.length - 1) &&
<button onClick={this.handleNextQuestion}>Next Question</button>}
</div>
)
}
}
export default Questions

While passing the prop, you're referring the state variable as this.selectedCategory
Change this line
selectedCategory={this.selectedCategory}
to
selectedCategory={selectedCategory}

Could not add multiple counters in react

I want to make a counter app with increment, decrement and add counter button. But add counter function is not working. It's showing this error:
Parsing error: Adjacent JSX elements must be wrapped in an enclosing tag. Did you want a JSX fragment <>...?
i have already tried enclosing it in tags,still its not working.
import React,{Component} from 'react';
import './App.css';
export class App extends Component {
state={
count:0,
}
increment=()=>{
this.setState({ count: this.state.count + 1 });
}
decrement=()=>{
this.setState({ count: this.state.count - 1 });
}
addCounter=()=>{
<span><button onClick={this.increment}>+</button></span>
<span>{this.state.count}</span>
<span><button onClick={this.decrement}>-</button></span>
}
render() {
return (
<div className="App">
<button onClick={this.addCounter}>Add Counter</button>
</div>
)
}
}
export default App;
add counter function should add another counter just below the previous counter.

Basically extract a Counter component.
Then have your App maintain a list of Counter components.
// Counter Component
export class Counter extends React.Component {
state = {
count:0,
}
increment=()=>{
this.setState({ count: this.state.count + 1 });
}
decrement=()=>{
this.setState({ count: this.state.count - 1 });
}
render() {
return (
<div>
<span><button onClick={this.increment}>+</button></span>
<span>{this.state.count}</span>
<span><button onClick={this.decrement}>-</button></span>
</div>
);
}
}
// App Component
export class App extends React.Component {
state = {
counters: [], // additional state for Counter components
}
addCounter = () => {
this.setState({
counters: [
...this.state.counters,
Counter
]
})
}
render() {
return (
<div className="App">
<button onClick={this.addCounter}>Add Counter</button>
{ this.state.counters.map((Counter, index) => (
<Counter key={index} />)
)}
</div>
)
}
}
Demo

You are just adding more spans and button but refering to the same counter.
state={
i : 0,
count:0,
}
var newCount="counter"+this.state.i;
this.setState({
i : this.state.i+1,
})
this.setState({
count: {
...this.state.count,
newCount: 0
}
});
So with this you add a new counter with a progresive autoincrement number.

I think it is what you want to do.
const { useState } = React;
function App(){
const [counter, setCounter] = useState([]);
function addCounter(){
setCounter(counter.concat({id: counter.length, count: 0}));
}
function increase(id){
setCounter(counter.map(el=>{
if(el.id === id){
el.count++;
}
return el;
}));
}
function decrease(id){
setCounter(counter.map(el=>{
if(el.id === id){
el.count--;
}
return el;
}));
}
return (
<div className="App">
<button onClick={addCounter}>Add Counter</button>
{
counter.map(el=>{
return(
<div key={el.id}>
<span>Counter #{el.id}</span>
<div>
<button onClick={()=>{increase(el.id)}}>+</button>
<span>{el.count}</span>
<button onClick={()=>{decrease(el.id)}}>-</button>
</div>
</div>
)
})
}
</div>
)
}
ReactDOM.render(
<App />, document.getElementById('root')
)
<script crossorigin src="https://unpkg.com/react#16/umd/react.development.js"></script>
<script crossorigin src="https://unpkg.com/react-dom#16/umd/react-dom.development.js"></script>
<div id="root"></div>

Your "counter" needs to be a react component with its own state, what you have there will have each "counter" you add use the same component state from App. You also do not save the returned JSX to then render anywhere.
const Counter = () => {
const [count, setCount] = useState(0);
return (
<div> // <-- React components can return only a single node*
<span>
<button onClick={() => setCount(count + 1)}>+</button>
</span>
<span>{count}</span>
<span>
<button onClick={() => setCount(count - 1)}>-</button>
</span>
</div>
);
};
class App extends Component {
state = {
counters: []
};
addCounter = () => {
this.setState(prevState => ({
counters: [...prevState.counters, <Counter />]
}));
};
render() {
return (
<div>
<button onClick={this.addCounter}>Add Counter</button>
{this.state.counters}
</div>
);
}
}
* React render lifecycle function can render arrays.

You have to add another functional Component instead of adding JSX. DEMO
import React from 'react';
const counter = (props) => {
return (
<div>
<span><button onClick={() => props.increment(props.index)}>+</button></span>
<span>{props.count}</span>
<span><button onClick={() => props.decrement(props.index)}>-</button></span>
</div>
)
}
export default counter;
And your main App component
import React, {Component} from 'react';
import Counter from './Counter';
import './App.css';
export class App extends Component {
state={
counters: []
}
valueChanger = (index, inc) => {
this.setState((prevState) => {
const counters = prevState.counters.slice();
counters[index] += inc;
return {
counters: counters
}
});
}
increment=(index)=>{
this.valueChanger(index, 1);
}
decrement=(index)=>{
this.valueChanger(index, -1);
}
addCounter=()=>{
this.setState((prevState) => {
return { counters: [...prevState.counters, 0] }
});
}
render() {
let counterElems = this.state.counters.map((c, index) => {
return <Counter key={index} index={index} increment={this.increment} decrement={this.decrement} count={c} />
});
return (
<div className="App">
{counterElems}
<button onClick={this.addCounter}>Add Counter</button>
</div>
)
}
}
export default App;

Send multiple props across components React

I am trying to send two variables from the Component 'Game' to the Component 'App' but I am unsure how to send more than one prop at a time.
This what I have:
//App Component
class App extends Component {
constructor(props) {
super(props)
this.state = {
score: 0,
}
this.changeScore = this.changeScore.bind(this)
}
changeScore(newScore) {
this.setState(prevState => ({
score: prevState.score + newScore
}))
}
render() {
return(
<div>
<Game onClick={this.changeScore}/>
<Score score={this.state.score}/>
</div>
)
}
}
//Game Componenet
class Game extends Component {
constructor(props) {
super(props)
this.state = {
score: 0,
}
this.handleClick = this.handleClick.bind(this)
}
handleClick() {
console.log('Clicked')
this.props.onClick(this.state.score)
}
render() {
return(
<div>
<button onClick={this.handleClick}> Score Button </button>
</div>
)
}
}
//Score Component
class Score extends Component {
render() {
const score = this.props.score
return(
<div>
<h1>Score: {score}</h1>
</div>
)
}
}
With this I am able to send the prop 'score' from 'Game' to 'App' but I was wondering if it was possible to send more then just the one prop, such as 'score' and a new variable, 'count' with the same button press, to ultimately be able to display both 'score' and 'count' in the 'Score' Componenet.
Thanks.

Sure you can, just update the function you defined in the Parent App component to accept two arguments.
App.js
class App extends Component {
constructor(props) {
super(props)
this.state = {
score: 0,
count: 0
}
this.changeScore = this.changeScore.bind(this)
}
changeScore(newScore, count) {
this.setState(prevState => ({
score: prevState.score + newScore,
count: prevState.count + count
}))
}
render() {
return(
<div>
<Game
onClick={this.changeScore}
score={this.state.score}
count={this.state.count}
/>
<Score score={this.state.score} count={this.state.count}/>
</div>
)
}
}
Game.js //refactored since it doesnt need to use state
const Game = ({ onClick, count, score }) => {
const newScore = score + 10
const newCount = count + 1
return (
<button onClick={() => onClick(newScore, newCount)}>Score</button>
)
}

You can definitely send more than one prop at a time. Here's the example that you've described:
<Score
score={this.state.score}
count={this.state.count}
/>
And in your Score component:
class Score extends Component {
render() {
const score = this.props.score;
const count = this.props.count;
return(
<div>
<h1>Score: {score}</h1>
<h1>Count: {count}</h1>
</div>
)
}
}

Is setState running again and again in this React code?

I have created three react components and I don't know why I am getting an infinite network request and this warning: index.js:1375 Warning: Cannot update during an existing state transition (such as within render). Render methods should be a pure function of props and state.
in MenuCategory (at App.js:19)
in App (at src/​index.js:5)
also a network request in MenuItems.js is getting called in a loop. I think it is due to setState but I don't know where is the error.
And here is my code :
import React from "react";
import MenuCategory from "./components/MenuCategory";
import MenuItems from "./components/MenuItems";
class App extends React.Component {
constructor(props) {
super(props);
this.state = { shortName: "" };
}
handleProps = ss => {
if (this.state.shortName === "") {
this.setState({ shortName: ss });
}
// console.log(ss, ".../PP");
};
render() {
return (
<div className="App">
<MenuCategory callback={this.handleProps} />
<MenuItems shortNameProp={this.state.shortName} />
</div>
);
}
}
export default App;
import React from "react";
class MenuCategory extends React.Component {
constructor(props) {
super(props);
this.state = { category: "", selectedCat: "" };
}
async UNSAFE_componentWillMount() {
const url = "http://stream-restaurant-menu-svc.herokuapp.com/category";
await fetch(url)
.then(data => data.json())
.then(element => {
this.setState({ category: element });
});
}
menuCat = () => {
let cat = this.state.category;
// console.log(cat, "...Cat", this.state.selectedCat, "...Cat");
if (this.state.selectedCat !== "") {
this.props.callback(this.state.selectedCat);
}
return cat.map(items => {
return (
<li
key={items.short_name}
onClick={() => this.setState({ selectedCat: items.short_name })}
>
{items.name}
</li>
);
});
};
render() {
return <div>{this.state.category.length > 0 ? this.menuCat() : null}</div>;
}
}
export default MenuCategory;
import React from "react";
class MenuItems extends React.Component {
constructor(props) {
super(props);
this.state = { catItems: "", items: "" };
}
renderItems = () => {
let shortName = this.props.shortNameProp;
if (shortName !== "") {
const url =
"https://stream-restaurant-menu-svc.herokuapp.com/item?category=" +
shortName;
fetch(url)
.then(data => data.json())
.then(element => {
this.setState({ items: element });
});
}
if (this.state.items !== "") {
let selectedMenu = this.state.items;
console.log(selectedMenu);
return selectedMenu.map(item => {
return <div key={item.name}> {item.name}</div>;
});
}
};
render() {
return <div>{this.renderItems()}</div>;
}
}
export default MenuItems;

Let's call App a parent and MenuCategory a child.
Let's denote a function call as the '->' sign.
There is an infinite loop formed like that:
child.render -> child.menuCat -> child.props.callback -> parent.handleProps -> parent.setState -> parent.render -> child.render.

can anyone tell me why i am unable to delete an item from the todo list?

Im having issues with react todo list.
When submitted the list item appears fine.
I should then be able to delete this be clicking on the item,however nothing happens. As soon as i try to add another item the pages refreshes and all items are removed?
console log just shows
[object object]
App
import React, { Component } from 'react';
import './App.css';
import TodoItem from './TodoItem';
class App extends Component {
constructor(props) {
super(props);
this.state = {
items: []
};
this.addItems = this.addItems.bind(this);
this.deleteItem = this.deleteItem.bind(this);
}
addItems(e) {
if (this._inputElement !== '') {
let newItem = {
text: this._inputElement.value,
key: Date.now()
};
this.setState(prevState => {
return {
items: prevState.items.concat(newItem)
};
});
}
this._inputElement.value = '';
e.preventDefault();
}
deleteItem(key) {
console.log('key is' + key);
console.log('itesm as' + this.state.items);
var filteredItems = this.state.items.filter(function(item) {
return item.key !== key;
});
this.setState = {
items: filteredItems
};
}
render() {
return (
<div className="app">
<h2> things to do</h2>
<div className="form-inline">
<div className="header">
<form onSubmit={this.addItems}>
<input
ref={a => (this._inputElement = a)}
placeholder="enter task"
/>
<button type="submit">add</button>
</form>
</div>
</div>
<TodoItem entries={this.state.items} delete={this.deleteItem} />
</div>
);
}
}
export default App;
todoItem
import React, { Component } from 'react';
//search bar
class TodoItem extends Component {
constructor(props) {
super(props);
this.createTasks = this.createTasks.bind(this);
}
createTasks(item) {
return (
<li onClick={() => this.delete(item.key)} key={item.key}>
{item.text}
</li>
);
}
delete(key) {
console.log('key is ' + key);
this.props.delete(key);
}
render() {
let todoEntries = this.props.entries;
let listItems = todoEntries.map(this.createTasks);
return <ul className="theList">{listItems}</ul>;
}
}
export default TodoItem;

You are assigning to setState instead of using it as a method that it is
Change
this.setState = {
items: filteredItems
};
to
this.setState({
items: filteredItems
});
And that is also the reason it will reload the app, as you have overwritten the setState method you should be getting an error that setState is not a function and it would crash the app.