Frontend Development Last-Minute Revision

30 quick revision notes for Frontend Development

1. ES module imports are static, which lets bundlers tree-shake unused exports; CommonJS require cannot be shaken reliably.
2. Nullish coalescing ?? falls back only for null or undefined, while || also replaces 0, false and empty strings.
3. Promise.allSettled waits for every promise and reports each outcome, while Promise.all rejects on the first failure.
4. await inside forEach does not wait; use for...of for sequential work or Promise.all with map for parallel work.
5. Microtasks like promise callbacks always run before the next macrotask such as a setTimeout callback.
6. AbortController cancels in-flight fetch requests and prevents stale responses from overwriting newer results.
7. unknown forces you to narrow a value before use, making it the safe alternative to any for external data.
8. Discriminated unions with a shared status field make impossible UI states unrepresentable in TypeScript.
9. satisfies validates a value against a type while keeping the value's more specific inferred type.
10. TypeScript types are erased at runtime, so API responses still need runtime validation with tools like Zod.
11. Web Workers offload heavy computation from the main thread; service workers intercept network requests for offline support.
12. Stale-while-revalidate serves cached content instantly and updates the cache in the background.
13. IntersectionObserver replaces scroll listeners for lazy loading, infinite scroll and impression tracking.
14. Always check event.origin in message listeners and pass an exact target origin to postMessage.
15. Cascade layers decide priority between groups of styles regardless of selector specificity.
16. :where() always has zero specificity, while :is() takes the specificity of its most specific argument.
17. WCAG AA requires 4.5 to 1 contrast for normal text and 3 to 1 for large text and UI components.
18. Use 100dvh or 100svh instead of 100vh for full-height mobile layouts affected by browser toolbars.
19. Good Core Web Vitals are LCP up to 2.5 seconds, INP up to 200 milliseconds and CLS up to 0.1.
20. Any main-thread task longer than 50 milliseconds is a long task and can delay user interactions.
21. Hashed asset filenames can be cached for a year with immutable, while HTML should use no-cache.
22. Fonts must be preloaded with the crossorigin attribute, otherwise the browser fetches them twice.
23. Field data from real users decides Core Web Vitals ranking, not a single Lighthouse lab score.
24. Vite serves native ES modules in development and uses Rollup or Rolldown for optimised production builds.
25. Frontend environment variables are embedded into the bundle, so they must never contain secrets.
26. Commit the lockfile and use npm ci in CI so every environment installs identical dependency versions.
27. Testing Library recommends getByRole queries because they mirror how users and assistive technology find elements.
28. Replace fixed sleeps with auto-waiting assertions to remove most causes of flaky end-to-end tests.
29. Separate server state, global client state and local UI state; tools like TanStack Query manage server state.
30. Store auth tokens in HttpOnly Secure SameSite cookies rather than localStorage to limit XSS damage.

Night-before checklist

• Revise your saved questions on this page once, aloud.
• Read the quick notes and mark any you cannot explain in one line.
• Rehearse a two-minute self-introduction and two project stories.
• Keep CV, ID, laptop charger and interview link ready; sleep on time.