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

useState and Type Inference

Rely on inference for primitives and use explicit unions for nullable or async state. Prevent type widening and invalid states in React local state management.
On this page

Type Inference Basics

  • TypeScript infers type from initial value.
  • Primitive initialization is usually safe without generics.
  • Nullable or async state requires explicit unions.

Example: Primitive State

const [count, setCount] = React.useState(0);
// inferred as number

Example: Nullable State

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

const [user, setUser] = React.useState<User | null>(null);

Avoid Type Widening

  • Do not initialize with empty object when a specific shape is expected.
  • Avoid casting as unknown as T to silence errors.
  • Model async states explicitly instead of using multiple booleans.

Discriminated Union Pattern

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

const [state, setState] = React.useState<ViewState>({ status: "loading" });

Failure Modes

  • Empty object initialization forcing unsafe casts later.
  • Multiple boolean flags creating impossible state combinations.
  • Null not included in union causing runtime access errors.
  • Over broad types hiding invalid UI states.

Operational Checklist

  • Async state uses union modeling.
  • No unsafe type assertions in state updates.
  • Derived values computed, not stored.
  • State models represent valid UI transitions only.
useReducer with Type Safety →