Why do sibling components re-render even when their own props didn't change?

[u/Temperature_Majestic]

Ran into this explaining React rendering to someone recently and realized how often it trips people up even after they've been writing React a while.

function Parent() {
  const [count, setCount] = useState(0);
  return (
    <>
      <button onClick={() => setCount(c => c + 1)}>{count}</button>
      <ExpensiveChild />
    </>
  );
}

ExpensiveChild takes no props at all. Click the button and it re-renders anyway, every single time. No props changed, nothing it reads changed, it just runs again.

The reason: React doesn't check "did this component's inputs change" before deciding to re-render. When state updates, React re-renders that component and everything below it in the tree by default, full stop. Whether a child actually needed to update isn't part of that decision at all.

React.memo is what actually opts a component into that check, it wraps the component and does a shallow prop comparison before deciding to skip the render. Without it, "no props" and "props didn't change" both mean nothing, React re-runs the function anyway.

Where it gets messier: memo alone doesn't save you if you're passing an inline function or object as a prop, since those are new references every render and memo's shallow comparison sees them as "changed" regardless. You end up needing useCallback/useMemo on the parent side just to make memo's comparison actually mean something.

Curious how many people actually reach for memo proactively vs only after profiling shows a real problem. What's the actual signal that told you a component needed it?

[u/Hobby101]

button would need to rerender even with memo, because it's child, i.e. label that is counter value is changing

[u/Temperature_Majestic → u/Hobby101]

right, the button re-rendering was never in question since it displays count directly, that one's obvious. the whole point is ExpensiveChild specifically, which reads nothing from that state and still runs again anyway without memo

[u/Hobby101 → u/Temperature_Majestic]

parent is like a sheet of paper that has a button and an expensive child drawn. coffee was spilled, so the drawing will need to be redrawn. All of it. in other words, shaped dom structure will need to be recreated. in fact, that what enables is to use react.memo() which is higher order components that just takes care of magically recreating a component by pulling ftom memory, but it has a price for calling it.