React useEffect Hook

An introduction on how to use useEffect Hook in your React application

Wednesday, November 10, 2021

What is a useEffect Hook?

TL;DR

React useEffect is an alternative to the "old" class lifecycle methods/hooks. It is used to manage side effects, such as network requests or run a piece of code when the component is mounted, updated, or unmounted.

Longer version

Before React v16.8, we can only enable a component to react to state changes using lifecycle methods.

How to define a useEffect

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123456useEffect(() => { //(1) declaration  // (2)effect  return () => {    // (3)cleanup   }}, /* (4)dependency array */)

Here's a quick explanation of the above code:

1. We can declare an effect by calling either React.useEffect or useEffect.
2. effect is the function that will be called when the component is mounted. OR when the dependency array changes.
3. cleanup is the function called when the effect is "unmounted".
4. dependency array is the array of values passed as a second argument to the effect function.
   • If there is no dependency array, the effect will be called every time the component is mounted.
   • If the array is empty, the effect will be called only once when the component is mounted.
   • If the array is not empty, the effect will be called every time the component is mounted and the dependency array changes.

Examples

Increment a counter every second until it reaches 10

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12345678910111213141516function App() {  const [count, setCount] = useState(0)
  useEffect(() => {    if (count < 10) {      const interval = setInterval(() => {        setCount(prev => prev + 1)      }, 1000)
      // cleanup function      return () => clearInterval(interval)    }  }, [count])
  // Render the component}
Link to Codepen

Basic fetch from an API

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1234567891011function App() {  const [data, setData] = useState([])
  useEffect(() => {    fetch('https://jsonplaceholder.typicode.com/users')      .then(res => res.json())      .then(data => setData(data))  }, [])
  // Render the component}
Link to Codepen

Fetching with loading indicator + error handling - then/catch/finally

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1234567891011121314151617181920212223function App() {  const [data, setData] = React.useState()  const [error, setError] = React.useState()  const [isLoading, setIsLoading] = React.useState(false)
  React.useEffect(() => {    setIsLoading(true)
    fetchData()      .then(data => {        setError(null)        setData(data)      })      .catch(data => {        // handle error case anyway you want        setError(data)        setData(null)      })      .finally(() => setIsLoading(false))  }, [])
  // Render the component}
Link to Codepen

Fetching with loading indicator + error handling - async/await

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123456789101112131415161718192021222324function App() {  const [data, setData] = React.useState()  const [error, setError] = React.useState()  const [isLoading, setIsLoading] = React.useState(false)
  React.useEffect(() => {    // yeah, this is weird    (async () => {      try {        setIsLoading(true)        const data = await fetchData()        setError(null)        setData(data)      } catch(e) {        // handle error case anyway you want        setError(e)        setData(null)      }      setIsLoading(false)    })()  }, [])
  // Render the component}
Link to Codepen

Store a value in localStorage everytime the key or value changes

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12345678910function App({ name }) {  const [value, setValue] = useState(() => localStorage.getItem(name))
  useEffect(() => {    localStorage.setItem(name, value)  }, [name, value])  // Ignore old keys for now
  // Render the component}
Link to Codepen

OR mimic a class lifecycle method

Check this blog on how to convert a class lifecycle methods to useEffect hooks

Additional Note

• useEffect can only be used in functional components
• The order of useEffect declarations is important.
• useEffect in a custom hook is a great way to promote side effect reusability. I will discuss this in another blog.

Conclusion

Compared to the old lifecycle methods, useEffect is much more powerful and flexible, making it an ideal choice when managing a side-effect.