useEffect Hook Not Firing After State Change

Question

I have two sibling components that share state via context in react. The shared state between the components is an array.

If I update arr state in one component, I want the other component to listen for that update and do something accordingly. When I use useEffect in the second component, I listen for changes in the arr state variable.

For example:

// App Component -------
const App = props => {
 const { arr, setArr } = useContext(GlobalContext)
  
 const handleChange = () => {
   const newArr = arr
   [10, 20, 30, 40].map(v => {
     newArr.push(v)
     setArr(newArr)
   })

  return (...)
}

// App2 (Sibling) Component 
const App2 = props => {
  const { arr, setArr } = useContext(GlobalContext)
  const [localArr, setLocalArr] = useState(0)

  useEffect(
    () => {
      updateLocalState()
    },
    // fire if "arr" gets updated
    [arr]
  )

  const updateLocalState = () => {
    setLocalArr(localArr + 1)
  }

  return (...)
}

The useEffect hook is only fired on the initial render, though the state of arr updates.

I know that declaring a new variable const newArr = arr to my state variable is a reference, so newArr.push(v) is technically a state mutation. However, the state still updates, no warning is thrown, and useEffect does nothing.

Why does useEffect not get called though the state gets updated? Is it because of the state mutation?

Second Question: Why is there no warning or error thrown regarding a state mutation? State mutations are dangerous - If it happens, I'd expect some sort of warning.

Live demo here:

Answer 1

The array you pass as second argument to useEffect only checks if the elements in the array are === to the elements in it in the previous render. const newArr = arr; will lead to newArr === arr since it doesn't create a new array, which is not what you want.

Create a new array with all the elements in arr and it will work as expected.

const App = props => {
 const { arr, setArr } = useContext(GlobalContext)

 const handleChange = () => {
   const newArr = [...arr]
   [10, 20, 30, 40].forEach(v => {
     newArr.push(v)
   })
   setArr(newArr)
 }

  return <>{/* ... */}</>
}

Answer 2

When you want to update array using useState hook. Make sure to spread the array into new array and update the new array so that your useEffect listening for this state will be called.

UseEffect will not call in the below code snippet as you are directly updating array.

const [skills, selectedSkills] = useState([])

     const onSelect = (selectedList) => {
            selectedSkills(selectedList)
        }
     useEffect(() => {
            MyLogger('useEffect called')
        }, [skills])

UseEffect will call in the below code snippet as we are keeping new reference to the array.

const [skills, selectedSkills] = useState([])

     const onSelect = (selectedList) => {
            const tempSelectedList = [...selectedList]
            selectedSkills(tempSelectedList)
        }
     useEffect(() => {
            MyLogger('useEffect called')
        }, [skills])