Why is the array.length method always one step slower than the input step in the handleChange event handler?

Question

I'm trying to create a list that filters based on input and only displays results if there are 10 or less objects, but array.length always lags behind the input.

const [countriesToShow, setCountriesToShow] = useState([])
    const [newSearch, setSearch] = useState('')
    const [showSearched, setShowShearched] = useState(true)
    const [notificationMessage, setNotificationMessage] = useState(null)

    useEffect(() => {
      console.log(countriesToShow.length)
    },[countriesToShow])
  
    const handleSearchChange = (event) => {
      setCountriesToShow(countries.filter(country =>
        country.name.common.toLowerCase().includes(event.target.value.toLowerCase())))
      setSearch(event.target.value)
      if (event.target.value.trim().length === 0) {
        setNotificationMessage(null)
        setShowShearched(false)
      }
      else if (countriesToShow.length <= 10) {
        setNotificationMessage(null)
        setShowShearched(true)
        console.log(countriesToShow.length)
      }
      else {
        setNotificationMessage('list too long')
        setShowShearched(false)
        console.log(countriesToShow.length)
      }
    }

I managed to print out the length correctly with the help of Effect Hook, but I'm confused on how to implement this into 'else if (countriesToShow.length <= 10)' as it still lags behind the input.

Answer 1

When you call handleSearchChange and then call setCountriesToShow to update the state:

setCountriesToShow(
  countries.filter(country =>
    country
      .name
      .common
      .toLowerCase()
      .includes(event.target.value.toLowerCase())
  )
)

You triggered a rerender. The newly updated state value will only become available on the upcoming re-render, which is why it lags behind.

If you want to use the value below, you need to store it in a variable first:

const handleSearchChange = (event) => {
  const newCountriesToShow = countries.filter(country =>
    country
      .name
      .common
      .toLowerCase()
      .includes(event.target.value.toLowerCase())
  )

  setCountriesToShow(newCountriesToShow)

  setSearch(event.target.value)

  if (event.target.value.trim().length === 0) {
    setNotificationMessage(null)
    setShowShearched(false)
  } else if (newCountriesToShow.length <= 10) {
    setNotificationMessage(null)
    setShowShearched(true)
  } else {
    setNotificationMessage('list too long')
    setShowShearched(false)
  }
}