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 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 { useState, useEffect } from 'react';
function App() {
const [pieceOfState, setPieceOfState] = useState(0);
useEffect(() => {
console.log('I'm in useEffect!');
console.log('This will be called whenever an instance of this component mounts');
console.log('or whenever pieceOfState is updated');
}, [pieceOfState]);
return (
<div>
<button onClick={() => setPieceOfState(pieceOfState + 1)}>Click Me to Update pieceOfState!</button>
</div>
);
}
useEffect(()=>{
// It will run everytime
})
// onMount
useEffect(()=>{
// It will run only one time
},[])
// component didUpdate
useEffect(()=>{
// it will run when dependency array change
},[data])
// component willUnmount
useEffect(()=>{
// for cleaning up the code
return(()=>{
})
})
If you’re familiar with React class lifecycle methods,
you can think of useEffect Hook as componentDidMount,
componentDidUpdate, and componentWillUnmount combined.
useEffect is a hook for encapsulating code that has 'side effects,' and is like a combination of componentDidMount , componentDidUpdate , and componentWillUnmount . Previously, functional components didn't have access to the component life cycle, but with useEffect you can tap into it.23.1.2019
//This is a basic example, carefully read the docs as
//useEffect it's like componentDidMount/WillUnmount
function Example() {
const [count, setCount] = useState(0);
// Similar to componentDidMount and componentDidUpdate:
useEffect(() => {
// Update the document title using the browser API
document.title = `You clicked ${count} times`;
});
return (
<div>
<p>You clicked {count} times</p>
<button onClick={() => setCount(count + 1)}>
Click me
</button>
</div>
);
}
most common cases how useEffect implementation:
- fetch data from API,
- directly updating the DOM,
- use timers and real-time data displays (such as BTC current price)
- validating input field
- trigger animation on new array value
-
// 1- EVERY RERENDER
useEffect( () => {
console.log("It runs everytime this component rerenders")
});
// 2- ON MOUNT
useEffect( () => {
console.log("It Only runs once (When the component gets mounted)")
}, []);
// 3- DEPENDING ON CONDITION
useEffect( () => {
console.log("It runs everytime the condition is changed")
}, [condition]); //basically the dependency array determines when the rerender happens
// 4- DEPENDING ON CONDITION, BUT WITH "clean up"
useEffect( () => {
console.log("It runs everytime my condition is changed")
return () => {
console.log("Use this return as a 'clean up tool' (it runs before the actual code)")
}
}, [condition]);
useEffect is a hook for encapsulating code that has 'side effects,' and is like a combination of componentDidMount , componentDidUpdate , and componentWillUnmount . Previously, functional components didn't have access to the component life cycle, but with useEffect you can tap into it.23.1.2019
The useEffect Hook allows you to perform side effects in your components.
Some examples of side effects are: fetching data, directly updating the DOM, and timers.
useEffect accepts two arguments. The second argument is optional.
useEffect(<function>, <dependency>)
What does useEffect do? By using this Hook, you tell React that
your component needs to do something after render.
React will remember the function you passed (we’ll refer to it
as our “effect”), and call it later after performing the DOM updates.
//1.
function App() {
const [isOn, setIsOn] = useState(false);
useEffect(() => {
const interval = setInterval(() => console.log('tick'), 1000);
return () => clearInterval(interval);
});
}
//2.
function App() {
const [isOn, setIsOn] = useState(false);
useEffect(() => {
const interval = setInterval(() => console.log('tick'), 1000);
return () => clearInterval(interval);
}, []);
}
//3.
function App() {
const [isOn, setIsOn] = useState(false);
useEffect(() => {
const interval = setInterval(() => console.log('tick'), 1000);
return () => clearInterval(interval);
}, [isOn]);
}
The first will run the effect on mount and whenever the state changes. The clean up will be called on state change and on unmount.
The second will only run the effect once on mount and the clean up will only get called on unmount.
The last will run the effect on mount and whenever the isOn state changes. The clean up will be called when isOn changes and on unmount.
In your examples, the first and last examples will behave the same because the only state that will change is isOn. If the first example had more state, that effect would also refire if the other state were to change.
I guess I should also add is that the order of things would be like: mount: -> run effect, state change: run clean up -> run effect, unmount -> run clean up.
jsximport { useEffect, useState } from 'react';function MyComponent({ prop }) { const [state, setState] = useState(''); useEffect(() => { // Runs ONCE after initial rendering // and after every rendering ONLY IF `prop` or `state` changes }, [prop, state]);}
//useEffect
//the whole purpose of useEffect is to:
//observe state
//observe initial state when the array is empty
//Target specific State variable and do something every time changes