React

House conventions for React 18/19. Apply them to components you are writing or changing — don't refactor untouched files to match unless asked.

Conventions

• Server Component by default; 'use client' only where you need it — state, effects, event handlers, browser APIs. Push the boundary as far down the tree as it will go: every component above a 'use client' marker also ships to the browser. Everything crossing that boundary must be serializable — a function, a class instance, or a Date inside a prop object fails at runtime, not at build.
• Read state through the functional updater inside anything that outlives the render — intervals, subscriptions, debounced callbacks. setCount(count + 1) inside a useEffect(…, []) captures count from the first render forever and silently sets 1 every tick; setCount(c => c + 1) always sees current state.
• Effects are for synchronising with something outside React — a subscription, a timer, a non-React widget. Data transformation belongs in render, derived values in a plain variable, and event responses in the handler. Most useEffect bugs are an effect that should never have existed.
• Every effect that starts something returns a cleanup that stops it. Strict mode double-invokes effects in development precisely to make a missing cleanup visible — treat that as the signal it is, not noise.
• Keys must be stable identity, not position. An array index as key makes React reuse the wrong DOM node when the list reorders, and the symptom shows up as state attached to the wrong row.
• Memoise for a measured reason. memo, useMemo, and useCallback each cost a comparison and add code to read. They earn their place when a child is genuinely expensive or is itself memoised and a new reference would defeat it — not as a default wrapper.
• State lives at its natural level. useState locally, lifted only as far as the shared ancestor. Reach for Zustand when genuinely global client state exists; TanStack Query for server state, because cache invalidation and refetching are its job and hand-rolling them in effects goes wrong quietly.
• Error boundaries around anything that can fail independently — a route, a widget, a third-party embed. Without one, a render error unmounts the whole tree and the user gets a blank page.
• Suspense boundaries where you want a fallback, placed so the loading UI matches the layout that will replace it. A boundary at the root turns any pending fetch into a full-page spinner.
• React 19: useActionState and <form action> for mutations, useOptimistic for immediate feedback, use() to read a promise or context conditionally. ref is a normal prop — forwardRef is no longer needed. useFormStatus only reports on a form above it, so the submit button reading pending state has to be its own component nested inside the <form> — calling it in the component that renders the form always returns pending: false.
• useTransition when a keystroke triggers expensive work. Update the input urgently and mark the derived work as a transition, so React can interrupt it; without this, typing into a field that filters a large list feels broken even though nothing is.
• Semantic HTML first, ARIA only to fill the gap. A <button> brings focus handling, keyboard activation, and role for free; a <div role="button"> means reimplementing all three correctly.
• Query tests by role and label, not test IDs. getByRole('button', { name: /submit/i }) fails when the control stops being reachable the way a user reaches it — which is the bug you want the test to catch. getByTestId passes regardless and is the fallback for elements with no accessible name.