Event handling in React

React's event system has two interesting things going on: synthetic events and root-level delegation.

The basics.

<button onClick={handle}>Click</button>

• camelCase event name (onClick, not onclick).
• Function reference, not a string.
• Handler receives a SyntheticEvent, not the raw DOM event.

function handle(e: React.MouseEvent<HTMLButtonElement>) {
  e.preventDefault();
  e.stopPropagation();
  console.log(e.nativeEvent); // raw DOM event if you need it
}

SyntheticEvent. A thin cross-browser wrapper. Same API (target, currentTarget, preventDefault, stopPropagation) regardless of browser. Pre-React 17, events were pooled (reused across handlers); accessing them async required e.persist(). Since React 17, pooling is gone — e is yours to keep.

Delegation: events attach to the root, not the element.

When you write <button onClick={...}>, React doesn't actually add a click listener to that button. It maintains a single delegated listener on the React root (in React 17+; on document pre-17) and dispatches to the right handler based on the event target.

Implications:

• Adding/removing 1000 buttons with onClick does NOT add 1000 native listeners. Cheap.
• stopPropagation() on a native listener at document level can prevent React's delegated handler from ever firing. Order matters.
• For events that bubble (most) this works. For events that don't bubble (focus, blur, scroll, mouseenter/mouseleave) React uses workarounds — synthetic onFocus/onBlur go through capture phase. onMouseEnter / onMouseLeave are simulated via tracking.

Common handlers.

<input onChange={e => setVal(e.target.value)} />     // not onInput
<form onSubmit={e => { e.preventDefault(); ... }} />
<a onClick={e => { e.preventDefault(); navigate(...); }} />
<div onKeyDown={e => { if (e.key === "Enter") ...; }} />

onChange in React fires on every keystroke (unlike the native DOM change which fires on blur). React treats onChange and onInput as the same for inputs.

When to bypass React's event system.

• Native passive scroll listeners. React's synthetic onScroll doesn't support { passive: true } directly. For high-frequency scroll handlers on mobile, use useEffect + addEventListener("scroll", h, { passive: true }).
• Events outside the React tree. Document/window-level shortcuts: useEffect(() => { document.addEventListener("keydown", ...); return () => ... }, []).
• Wheel and touch events that need to call preventDefault synchronously — Chrome warns when synthetic handlers do this. Use a native non-passive listener.
• ResizeObserver, IntersectionObserver, MutationObserver — not events but the modern equivalents for layout/visibility/DOM changes; far cheaper than scroll / mutation listeners.

Stable handler identity (for memoization).

const onClick = useCallback(() => save(id), [id]);

Inline arrow functions create a new reference each render. For children wrapped in React.memo, that breaks the bailout. (React 19+ compiler handles this automatically.)

Common gotchas.

• e.target vs e.currentTarget. target is the deepest element under the cursor; currentTarget is the element the handler is attached to. Use currentTarget for "the button I clicked," target for delegation patterns.
• onClick on a <div> is a11y debt. Use <button> for actions, <a> for navigation. If you must use a div, add role="button", tabIndex={0}, and a keyboard handler.
• Passing args to handlers. onClick={() => save(id)} allocates a new closure each render. For memo children, lift the binding into useCallback or pass the id via the row component, not the handler.
• Don't return false from handlers — that's jQuery. Call e.preventDefault() and/or e.stopPropagation().

SyntheticEvent vs native — when does it matter?

• 99% of the time, treat them the same.
• Mobile gesture libraries (PointerEvent, custom touch sequences) typically prefer native listeners.
• For pixel-perfect coordinate math in fast pointer movements, e.nativeEvent gives you the unwrapped event.

Senior framing. Two ideas: synthetic events for cross-browser portability, root delegation for performance. Plus the practical lane: use native listeners when you need passive scroll, document-level listeners, or precise pointer math.