Why it doesn't work?
It's because the Home component is never used, even if it's included in the following snippet:
const activeSearch = () => {
if (searchString.length > 0) {
<Home searchResults={searchString} />;
}
};
The activeSearch function has a couple problems:
- it is used as an event handler though it uses JSX (outside the render phase)
- it doesn't return the JSX (would still fail inside the render phase)
JSX should only be used within the render phase of React's lifecycle. Any event handler exists outside this phase, so any JSX it might use won't end up in the final tree.
The data dictates what to render
That said, the solution is to use the state in order to know what to render during the render phase.
function Header() {
const [searchString, setString] = useState('');
const [showResults, setShowResults] = useState(false);
const onChangHandler = (e) => {
// to avoid fetching results for every character change.
setShowResults(false);
setString(e.target.value);
};
const activeSearch = () => setShowResults(searchString.length > 0);
return (
<div>
<input
value={searchString}
onChange={(e) => onChangHandler(e)}
/>
<button onClick={activeSearch}>Search</button>
{showResults && <Home query={searchString} />}
</div>
);
}
useEffect to trigger effects based on changing props
And then, the Home component can trigger a new search request to some service through useEffect.
function Home({ query }) {
const [results, setResults] = useState(null);
useEffect(() => {
let discardResult = false;
fetchResults(query).then((response) => !discardResult && setResults(response));
// This returned function will run before the query changes and on unmount.
return () => {
// Prevents a race-condition where the results from a previous slow
// request could override the loading state or the latest results from
// a faster request.
discardResult = true;
// Reset the results state whenever the query changes.
setResults(null);
}
}, [query]);
return results ? (
<ul>{results.map((result) => <li>{result}</li>))}</ul>
) : `Loading...`;
}
It's true that it's not optimal to sync some state with props through useEffect like the article highlights:
useEffect(() => {
setInternalState(externalState);
}, [externalState]);
...but in our case, we're not syncing state, we're literally triggering an effect (fetching results), the very reason why useEffect even exists.
const { useState, useEffect } = React;
const FAKE_DELAY = 5; // seconds
function Home({ query }) {
const [results, setResults] = useState(null);
useEffect(() => {
let queryChanged = false;
console.log('Fetch search results for', query);
setTimeout(() => {
if (queryChanged) {
console.log('Query changed since last fetch, results discarded for', query);
return;
}
setResults(['example', 'result', 'for', query])
}, FAKE_DELAY * 1000);
return () => {
// Prevent race-condition
queryChanged = true;
setResults(null);
};
}, [query]);
return (
<div>
{results ? (
<ul>
{results.map((result) => (
<li>{result}</li>
))}
</ul>
) : `Loading... (${FAKE_DELAY} seconds)`}
</div>
);
}
function Header() {
const [searchString, setString] = useState('');
const [showResults, setShowResults] = useState(false);
const onChangHandler = (e) => {
// to avoid fetching results for every character change.
setShowResults(false);
setString(e.target.value);
};
const activeSearch = () => setShowResults(searchString.length > 0);
return (
<div>
<input
placeholder='Search here'
value={searchString}
onChange={(e) => onChangHandler(e)}
/>
<button onClick={activeSearch}>Search</button>
{showResults && <Home query={searchString} />}
</div>
);
}
ReactDOM.render(<Header />, document.querySelector("#app"));
<script src="https://cdnjs.cloudflare.com/ajax/libs/react/16.13.1/umd/react.production.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/react-dom/16.13.1/umd/react-dom.production.min.js"></script>
<div id="app"></div>
Better solution: Uncontrolled inputs
Another technique in your case would be to use an uncontrolled <input> by using a ref and only updating the search string on click of the button instead of on change of the input value.
function Header() {
const [searchString, setString] = useState('');
const inputRef = useRef();
const activeSearch = () => {
setString(inputRef.current.value);
}
return (
<div>
<input ref={inputRef} />
<button onClick={activeSearch}>Search</button>
{searchString.length > 0 && <Home query={searchString} />}
</div>
);
}
const { useState, useEffect, useRef } = React;
const FAKE_DELAY = 5; // seconds
function Home({ query }) {
const [results, setResults] = useState(null);
useEffect(() => {
let queryChanged = false;
console.log('Fetch search results for', query);
setTimeout(() => {
if (queryChanged) {
console.log('Query changed since last fetch, results discarded for', query);
return;
}
setResults(['example', 'result', 'for', query])
}, FAKE_DELAY * 1000);
return () => {
// Prevent race-condition
queryChanged = true;
setResults(null);
};
}, [query]);
return (
<div>
{results ? (
<ul>
{results.map((result) => (
<li>{result}</li>
))}
</ul>
) : `Loading... (${FAKE_DELAY} seconds)`}
</div>
);
}
function Header() {
const [searchString, setString] = useState('');
const inputRef = useRef();
const activeSearch = () => {
setString(inputRef.current.value);
}
return (
<div>
<input
placeholder='Search here'
ref={inputRef}
/>
<button onClick={activeSearch}>Search</button>
{searchString.length > 0 && <Home query={searchString} />}
</div>
);
}
ReactDOM.render(<Header />, document.querySelector("#app"))
<script src="https://cdnjs.cloudflare.com/ajax/libs/react/16.13.1/umd/react.production.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/react-dom/16.13.1/umd/react-dom.production.min.js"></script>
<div id="app"></div>
Passing the state around
[The following line] brings the Home component inside the Header component, which makes duplicate
{searchString.length > 0 && <Home query={searchString} />}
In order to make the Header component reusable, the quickest way would be to lift the state up.
// No state needed in this component, we now receive
// a callback function instead.
function Header({ onSubmit }) {
const inputRef = useRef();
const activeSearch = () => {
// Uses the callback function instead of a state setter.
onSubmit(inputRef.current.value);
}
return (
<div>
<input ref={inputRef} />
<button onClick={activeSearch}>Search</button>
</div>
);
}
function App() {
// State lifted up to the parent (App) component.
const [searchString, setString] = useState('');
return (
<div className='App'>
<Header onSubmit={setString} />
{searchString.length > 0 && <Home query={searchString} />}
<Footer />
</div>
);
}
If that solution is still too limited, there are other ways to pass data around which would be off-topic to bring them all up in this answer, so I'll link some more information instead:
