Useeffect Umnount
useEffect(() => {
return () => {
console.log("cleaned up");
};
}, []);
Dark Dotterel
useEffect(() => {
return () => {
console.log("cleaned up");
};
}, []); useEffect(() => {
//your code goes here
return () => {
//your cleanup code codes here
};
},[]);import React, { useEffect } from 'react';
export const App: React.FC = () => {
useEffect(() => {
}, [/*Here can enter some value to call again the content inside useEffect*/])
return (
<div>Use Effect!</div>
);
}import React, { useState, useEffect } from 'react';
function Example() {
const [count, setCount] = useState(0);
useEffect(() => { document.title = `You clicked ${count} times`; });
return (
<div>
<p>You clicked {count} times</p>
<button onClick={() => setCount(count + 1)}>
Click me
</button>
</div>
);
}import React, { useEffect, useState } from "react";
import ReactDOM from "react-dom";
function Reddit() {
// Initialize state to hold the posts
const [posts, setPosts] = useState([]);
// effect functions can't be async, so declare the
// async function inside the effect, then call it
useEffect(() => {
async function fetchData() {
// Call fetch as usual
const res = await fetch(
"https://www.reddit.com/r/reactjs.json"
);
// Pull out the data as usual
const json = await res.json();
// Save the posts into state
// (look at the Network tab to see why the path is like this)
setPosts(json.data.children.map(c => c.data));
}
fetchData();
}); // <-- we didn't pass a value. what do you think will happen?
// Render as usual
return (
<ul>
{posts.map(post => (
<li key={post.id}>{post.title}</li>
))}
</ul>
);
}
ReactDOM.render(
<Reddit />,
document.querySelector("#root")
);import {useEffect, useState} from 'react';
export default function App() {
const [counter, setCounter] = useState(0);
// ✅ hook is called at top level (not conditionally)
useEffect(() => {
if (counter > 0) {
console.log('hello world');
}
});
return (
<div>
<button onClick={() => setCounter(counter + 1)}>toggle loading</button>
<h1>Hello world</h1>
</div>
);
}