use-memo

Rule Details

useMemo is for computing and caching expensive values, not for side effects. Without a return value, useMemo returns undefined, which defeats its purpose and likely indicates you’re using the wrong hook.

Invalid

Examples of incorrect code for this rule:

// ❌ No return value
function Component({ data }) {
  const processed = useMemo(() => {
    data.forEach(item => console.log(item));
    // Missing return!
  }, [data]);

  return <div>{processed}</div>; // Always undefined
}

Valid

Examples of correct code for this rule:

// ✅ Returns computed value
function Component({ data }) {
  const processed = useMemo(() => {
    return data.map(item => item * 2);
  }, [data]);

  return <div>{processed}</div>;
}

Troubleshooting

I need to run side effects when dependencies change

You might try to use useMemo for side effects:

// ❌ Wrong: Side effects in useMemo
function Component({user}) {
  // No return value, just side effect
  useMemo(() => {
    analytics.track('UserViewed', {userId: user.id});
  }, [user.id]);

  // Not assigned to a variable
  useMemo(() => {
    return analytics.track('UserViewed', {userId: user.id});
  }, [user.id]);
}

If the side effect needs to happen in response to user interaction, it’s best to colocate the side effect with the event:

// ✅ Good: Side effects in event handlers
function Component({user}) {
  const handleClick = () => {
    analytics.track('ButtonClicked', {userId: user.id});
    // Other click logic...
  };

  return <button onClick={handleClick}>Click me</button>;
}

If the side effect sychronizes React state with some external state (or vice versa), use useEffect:

// ✅ Good: Synchronization in useEffect
function Component({theme}) {
  useEffect(() => {
    localStorage.setItem('preferredTheme', theme);
    document.body.className = theme;
  }, [theme]);

  return <div>Current theme: {theme}</div>;
}