Frontend Architecture Foundations

Use these patterns across JavaScript, TypeScript, Angular, and React projects to keep codebases maintainable.

Layering and boundaries

• UI vs domain vs data: Separate rendering (components/UI) from domain logic and data access. Keep side effects at the edges.
• State ownership: Define single owners for pieces of state; pass data down and events up. Prefer explicit state machines for complex flows.
• API boundaries: Centralize HTTP clients, auth, and serialization. Represent server contracts with types and validation.

Data flow patterns

• Unidirectional data: Prefer predictable flows (props/context in React, inputs/outputs in Angular) over implicit globals.
• Streams and async: Use RxJS in Angular or lightweight observables/promises in React/vanilla. Keep subscription lifecycles explicit.
• Caching strategies: Stale-while-revalidate, background refresh, and optimistic updates with rollback on failure.

UI composition

• Component design: Small, focused components; elevate shared concerns (loading, errors) into reusable wrappers.
• Styling: Co-locate styles (CSS Modules, Tailwind, or scoped styles). Keep tokens (colors, spacing) centralized.
• Accessibility: Semantic HTML first; ensure keyboard support and focus management for interactive widgets.

Scalable and responsive design

• Responsive layout system: Use flexible grids, fluid typography, and container queries where possible; keep breakpoints consistent across the app.
• Design tokens: Centralize spacing, color, and typography to keep large teams aligned and reduce style drift.
• Progressive enhancement: Start from mobile-first constraints, then add complexity for larger viewports and higher bandwidth.
• Component resilience: Design UI to handle long text, empty states, slow data, and variable content density without breaking layout.

Performance discipline

• Rendering costs: Avoid unnecessary renders by memoizing values and handlers; virtualize large lists; split bundles by route or feature.
• Network: Defer non-critical fetches, compress payloads, and paginate/stream large results.
• Instrumentation: Measure with Web Vitals and browser Performance APIs; set budgets and alerts.

Testing strategy

• Pyramid balance: Lots of unit tests for pure logic, integration tests around stateful flows, few e2e smoke tests.
• Contract testing: Validate API clients against fixtures/schemas; keep mocks consistent across tests.
• Accessibility checks: Include a11y assertions (roles, names) in component tests.

Deployment and environments

• Configs per environment: Separate build-time and runtime config; avoid leaking secrets to the client.
• Feature flags: Gate risky changes; default to safe behavior and clean up flags regularly.
• Observability: Log with context, capture errors with stack traces, and monitor user-impacting metrics.

Micro frontends

• When to use: Large orgs with many teams, independent release cycles, or distinct product areas.
• Integration options: Module federation, iframe isolation, or server-side composition with shared design systems.
• Shared dependencies: Align versions for React/Angular and UI libraries; avoid duplication in bundles.
• Cross-app communication: Use event contracts, shared state adapters, or message buses with strict schemas.
• Ownership boundaries: Define clear domain ownership, routing contracts, and compatibility guarantees.

Interview-ready checklists

• Explain trade-offs between client-side routing, SSR/SSG, and progressive hydration.
• Walk through how you avoid prop drilling (context, event buses, state managers) and when to choose each.
• Describe how you’d harden a frontend against API errors, slow networks, and flaky dependencies.