# 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.


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