useIsClient vs useIsMounted

tip

useIsClient is primarily for handling SSR scenarios, while useIsMounted is for managing component lifecycle and preventing memory leaks or updates after unmounting.

Timing and Purpose

• useIsClient specifically tells you if the code is running in a browser vs server environment
• useIsMounted tells you if a component is currently mounted in the DOM

Implementation

• useIsClient uses useState directly and only sets to true once
• useIsMounted uses useRef and returns a callback function that checks the ref

Return Value

• useIsClient returns a boolean directly
• useIsMounted returns a function that returns a boolean

Here are scenarios where they would differ:

function Example() {
  const isClient = useIsClient()
  const isMounted = useIsMounted()

  // Scenario 1: Initial Server-Side Rendering
  console.log(isClient) // false during SSR, true after hydration
  console.log(isMounted()) // true in both cases

  // Scenario 2: Component Unmounting
  useEffect(() => {
    const timer = setTimeout(() => {
      console.log(isClient) // true (doesn't change)
      console.log(isMounted()) // false (component unmounted)
    }, 1000)

    return () => clearTimeout(timer)
  }, [])

  return null
}

Server-Side Rendering

• useIsClient will be false during SSR and initial render, then true after hydration
• useIsMounted will be true as soon as the component mounts, regardless of SSR

Component Lifecycle

• useIsClient stays true once set, even if the component unmounts
• useIsMounted becomes false when the component unmounts

Race Conditions

• useIsClient is better for conditional rendering based on client/server environment
• useIsMounted is better for preventing state updates after unmounting

// Example where `useIsMounted` is more appropriate
function DataFetcher() {
  const isMounted = useIsMounted()

  useEffect(() => {
    fetchData().then((data) => {
      // Prevents memory leak and updates after unmount
      if (isMounted()) {
        setData(data)
      }
    })
  }, [])
}

// Example where `useIsClient` is more appropriate
function BrowserFeature() {
  const isClient = useIsClient()

  // Only renders browser-specific features after hydration
  return isClient ? <WindowSizeDisplay /> : null
}