useIsClient vs useIsMounted

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:

tsx

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

tsx

// Example where `useIsMounted` is more appropriatefunction DataFetcher() {  const isMounted = useIsMounted()
  useEffect(() => {    fetchData().then((data) => {      // Prevents memory leak and updates after unmount      if (isMounted()) {        setData(data)      }    })  }, [])}

tsx

// Example where `useIsClient` is more appropriatefunction BrowserFeature() {  const isClient = useIsClient()
  // Only renders browser-specific features after hydration  return isClient ? <WindowSizeDisplay /> : null}