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

Typing Event Handlers

Type DOM events at boundaries, then extract primitives immediately. Prevent handler bugs from mixed element types, stale closures, and event object leakage across async boundaries.
On this page

Boundary Rule

  • Type the event at the UI boundary, then convert it to domain primitives.
  • Do not pass event objects to business logic or async code.
  • Prefer onChange signatures like (value) instead of (event).

Common Event Types

  • React.ChangeEvent for inputs and selects.
  • React.MouseEvent for clicks.
  • React.FormEvent for form submit.

Example: Extract Primitive Value

function SearchBox({ value, onValue }) {
  function onChange(e: React.ChangeEvent<HTMLInputElement>) {
    onValue(e.target.value);
  }

  return <input value={value} onChange={onChange} />;
}

Example: Submit Handler

function Form({ onSave }) {
  async function onSubmit(e: React.FormEvent<HTMLFormElement>) {
    e.preventDefault();
    await onSave();
  }

  return (
    <form onSubmit={onSubmit}>
      <button>Save</button>
    </form>
  );
}

Mixed Element Pitfall

  • A generic handler typed for one element can break when reused for another element.
  • Use separate handlers or type a union only when logic is truly shared.

Failure Modes

  • Passing event objects into async functions and using them later.
  • Incorrect event type causing unsafe casts.
  • Stale closures reading old state inside handlers.
  • Handlers doing heavy work on the main thread.

Production Checklist

  • Handlers translate DOM events into domain intent quickly.
  • Only primitives cross component boundaries.
  • Submit handlers have in flight guards.
  • High frequency handlers are debounced or throttled.
Generic Components →