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

Failure Modes in React + TypeScript

Most failures come from boundary typing and unsafe casts. Use this playbook to detect any spread, stale closures, union gaps, and invalid DTO mapping before they become production incidents.
On this page

High Impact Failure Sources

  • Boundary typing errors: API, storage, URL params, third party callbacks.
  • Unsafe casts and any spread across components.
  • State models that allow invalid combinations.
  • Stale closures from missing dependencies in hooks.

Symptom to Cause Mapping

  • Random runtime crashes after deploy → unsafe cast or invalid boundary data.
  • UI shows old values → stale closure or missing dependency array items.
  • Errors hidden or unreachable → missing union variants or boolean soup.
  • Data shape mismatch after backend change → UI reading DTO directly instead of mapping.
  • Performance regression → any causes broad props, unstable context values, over wide state updates.

Fast Audit Checklist

  • Search for explicit any, unknown as, and large type assertions.
  • Verify all API boundaries validate and map DTO to ViewModel.
  • Confirm async state uses discriminated unions with explicit status.
  • Check useEffect and useCallback dependencies for completeness.
  • Verify context providers memoize object values.

Boundary Validation Pattern

function isRecord(x: unknown): x is Record<string, unknown> {
  return typeof x === "object" && x !== null;
}

function getString(r: Record<string, unknown>, k: string): string {
  const v = r[k];
  if (typeof v !== "string") throw new Error("invalid");
  return v;
}

type UserVM = { id: string; name: string };

function parseUserVM(x: unknown): UserVM {
  if (!isRecord(x)) throw new Error("invalid");
  return { id: getString(x, "id"), name: getString(x, "name") };
}

State Model Upgrade Pattern

type LoadState =
  | { status: "loading" }
  | { status: "error"; message: string }
  | { status: "ready"; data: UserVM[] };

Common Failure Modes

  • Any spreading from one file into the entire component tree.
  • DTO fields leaking into UI components, causing breakage on backend rename.
  • Non null assertions hiding undefined paths and crashing later.
  • Union gaps forcing unsafe casts and unreachable UI states.
  • Effects that set state and re trigger themselves, creating loops.

Incident Triage Playbook

  • Step 1: Reproduce with minimal state and log boundary inputs.
  • Step 2: Identify first unsafe cast in the data path.
  • Step 3: Add validation and mapping at the boundary.
  • Step 4: Replace boolean soup with union status state.
  • Step 5: Add tests for loading, error, empty, and ready paths.

Release Gate Checklist

  • No any at boundaries.
  • DTO mapping exists and is tested for required fields.
  • Unions cover all states and rendering is switch based.
  • Hooks have correct dependency arrays.
  • Context values are stable and memoized where needed.