1. element vs Component
Before: element format
ReactDOM.render(<App />, appDOM) const app = ( <div> <Title>{counter}th cat meme</Title> <CatForm handleFormSubmit={handleFormSubmit} /> <MainCard img="https://cataas.com/cat/595f280b557291a9750ebf65/says/JavaScript" /> <Favorites /> </div> ); const appDOM = document.querySelector('#app'); ReactDOM.render(app, appDOM) After: Component format
If you want to add useState or functions inside
app, it has to be changed from javascript element to React Component. This would frequently happen when you are changing codes to lift state up.The last code should be changed like this :
ReactDOM.render(<App />, appDOM)
If you write App only, like you're calling the name of element, not Component, this error message would pop up:
Warning: Functions are not valid as a React child. This may happen if you return a Component instead of from render. Or maybe you meant to call this function rather than return it.
const App = () { const [counter, setCounter] = React.useState(100); function handleFormSubmit() { // function } return( <div> <Title>{counter}th cat meme</Title> <CatForm handleFormSubmit={handleFormSubmit} /> <MainCard img="https://cataas.com/cat/595f280b557291a9750ebf65/says/JavaScript" /> <Favorites /> </div> ) }; const appDOM = document.querySelector('#app'); ReactDOM.render(<App />, appDOM) 2. Each child in a list should have a unique key prop.
Warning: Each child in a list should have a unique "key" prop.
Check the render method of
Favorites. See https://reactjs.org/link/warning-keys for more information.
at CatItem (:70:45)
at Favorites
at div
at App (:99:31)
Before:
<ul className="favorites"> { cats.map(cat => <CatItem img={cat} />) } </ul> After:
<ul className="favorites"> { cats.map(cat => <CatItem img={cat} key={cat} />) } </ul> 3. How to add a new item to a list that is created by useState
Use es6 Spread Operator
function Favorites() { const [favorites, setFavorites] = React.useState([CAT1, CAT2]) function handleHeartClick() { console.log("Clicked Heart."); // add cat to the list setFavorites([...favorites, CAT3]) } } 
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