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>
);
}useEffect(() => {
const data = // axios call
return () =>{
// unmount
};
}, [third]); // if Empty -> didMountuseEffect(() => {
// Update the document title using the browser API
}, []);import React, { useState, useEffect } from 'react';
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>
);import React, { useState, useEffect } from 'react';
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>
);
}useEffect(() => {
document.title = `You clicked ${count} times`;
}, [count]); useEffect(() => {
/*Inside*/
});//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. useEffect(() => {
alert('Requested data from server...');
get(forecastType).then((response) => {
setData(response.data);
});
}, [forecastType]);