React Basics

Basic idea

UI = f(state). Components return a virtual DOM tree; React reconciles it with the live DOM. State changes trigger re-renders of the affected subtree, not the whole page.

Key facts

• Components: function returning JSX.
• Hooks: useState, useEffect, useMemo, useRef, useContext, useReducer.
• Pure rendering: a component with the same props + state should return the same output.
• Keys identify list items for stable reconciliation.