UseEffect

useEffect() is an escape hatch for synchronizing with external systems (APIs, DOM manipulation, third-party libraries, browser APIs). It should not be used for orchestrating the state. React linters will warn against that.

useEffect() documentation | You Might Not Need an Effect

Dependency Patterns

With dependencies - runs after the initial render and when dependencies change:

useEffect(() => {
  fetchData(userId);
}, [userId]);

No dependencies - runs after every render:

useEffect(() => {
  console.log('Runs on every render');
});

Empty array - runs once after initial mount:

useEffect(() => {
  const connection = connectToServer();
}, []);

Cleanup Function

Return a function from useEffect() to clean up side effects. It runs before the effect re-executes and when the component unmounts.

useEffect(() => {
  const subscription = api.subscribe(id);
  
  return () => {
    subscription.unsubscribe();
  };
}, [id]);

Common cleanup use cases: clearing timers, canceling requests, unsubscribing from events.

Dev Strict Mode

In development, React runs effects twice to help find bugs. This only happens in Strict Mode.

useEffect(() => {
  console.log('You should see me twice in dev mode');
  return () => console.log('And me in between!');
}, []);

Strict Mode documentation

useEffectEvent()

Allows defining event handlers that always have access to the latest state and props without needing to include them in the dependency array.

const onVisit = useEffectEvent((url) => {
  logAnalytics(url, shoppingCart);
});

useEffect(() => {
  onVisit(url);
}, [url]);

useEffectEvent documentation

Timeline

There could be more nuances to this, but generally the order of things is as follows:

1. Component render logic
2. React commits to DOM
3. useLayoutEffect() cleanup
4. useLayoutEffect() effect
5. Browser paints
6. useEffect() cleanup
7. useEffect() effect

Important notes:

• useLayoutEffect() runs before the browser paints, blocking visual updates until it's done.
• Cleanup functions capture state/props from the previous render (old scope).