Core Code
Core Code Modern Dev Handbook
REACT Contents
React Fundamentals
React Architecture Overview JSX and Rendering Model Component Design Principles Props and Data Flow State Management with useState Side Effects with useEffect Event Handling Patterns Conditional Rendering Strategies Lists, Keys, and Reconciliation Forms and Controlled Components Component Composition Lifting State and Shared State Basic Routing with React Router Failure Modes in Beginner React Apps
React + TypeScript
Typing Functional Components Typing Props and Children useState and Type Inference useReducer with Type Safety Typing Event Handlers Generic Components Utility Types in React Discriminated Unions for UI State Strict Mode and Null Safety API Response Typing Strategies Form Typing Patterns Avoiding any and Unsafe Casts Type-Safe Context API Failure Modes in React + TypeScript
React Advanced Patterns
Custom Hooks Architecture Context vs Prop Drilling Tradeoffs Compound Components Pattern Controlled vs Uncontrolled Abstractions Render Props and Pattern Alternatives State Machines in React Performance Optimization Playbook Memoization Strategies Code Splitting and Lazy Loading Error Boundaries and Recovery Portal Architecture Large Scale Folder Structure Testing Strategy: Unit and Integration Common React Anti Patterns
React Production
CSR vs SSR vs SSG Hydration and Mismatch Failures Next.js Architecture Overview App Router Deep Dive Authentication Patterns Token Storage Security Role Based UI Control API Integration Patterns Rate Limiting and UI Resilience Frontend Security Checklist XSS in Modern React Observability in Frontend Bundle Optimization Production Incident Playbook
Distributed Systems
Distributed System Fundamentals CAP Theorem in Practice Consistency Models Replication Strategies Leader Election Concepts Consensus Algorithms Overview Event Driven Architectures Message Queues and Delivery Guarantees Idempotency Patterns Saga Pattern Circuit Breakers Backpressure and Flow Control Observability in Distributed Systems Failure Scenarios and Postmortems

Observability in Frontend

Frontends need metrics, logs, and traces to debug user impact. Learn RUM signals, error aggregation, session correlation, performance budgets, and incident workflows for production React apps.
On this page

Observability Goals

  • Detect user impact quickly.
  • Explain failures with context: route, release, device, network.
  • Reduce mean time to resolution with actionable signals.

Core Signals

  • Errors: uncaught exceptions, unhandled promise rejections, error boundary events.
  • Performance: LCP, CLS, INP, TTFB, hydration time, long tasks.
  • Availability: API error rates by endpoint and route.
  • UX: rage clicks, broken navigation, form abandon rate where relevant.

Event Correlation

  • Attach correlation ids: session id, request id, trace id if available.
  • Attach release metadata: build version, commit hash, deployment region.
  • Ensure privacy controls: avoid PII and secrets in telemetry payloads.

Instrument Error Boundaries

  • Report boundary caught errors with route and feature identifiers.
  • Record recovery actions: retry clicks and navigation away.

Performance Budgets

  • Set budgets for bundle size, LCP, and hydration time.
  • Fail builds or warn on regressions.
  • Track real user metrics, not only lab metrics.

Operational Dashboards

  • Error rate by release and route.
  • Core web vitals by device class and network type.
  • API latency and 429 rate limiting events.
  • Auth failures and redirect loop detection.

Failure Modes

  • No release tagging, cannot correlate spikes to a deploy.
  • PII leaks into logs, creating compliance incidents.
  • Over sampling hides low frequency but high impact bugs.
  • Client logs are too noisy and lack actionable context.

Incident Playbook

  • Step 1: Identify impacted routes and release versions.
  • Step 2: Check if failure is client crash, API error, or auth loop.
  • Step 3: Correlate with backend traces using request ids.
  • Step 4: Roll back or gate the feature, then ship a fix.
  • Step 5: Add regression tests and telemetry for the root cause.

Production Checklist

  • Errors, performance, and API failures are instrumented.
  • Telemetry includes route and release metadata.
  • No PII or secrets in client logs.
  • Budgets exist and regressions are visible in CI.
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