What is React?

React is not a full framework like Angular. It is a UI library: you bring your own router, data layer, and build tooling—or adopt meta-frameworks like Next.js after this track.

Core ideas

• Components — reusable functions (or classes) that return UI descriptions
• Declarative rendering — you return JSX; React reconciles it with the DOM
• One-way data flow — props flow down; events and callbacks flow up
• Hooks — functions like useState and useEffect add state and lifecycle to function components

Compared to jQuery or vanilla JS

Vanilla code often reads: “find this button, attach listener, find that div, set innerHTML.” React reads: “when count is 3, render this markup.” The state is the source of truth; the UI follows.

Important interview questions and answers

1. Q: Is React a framework or a library?
   A: A library focused on the view layer. Routing, global state, and data fetching are typically separate choices.
2. Q: What is the Virtual DOM?
   A: An in-memory representation React compares between renders to compute minimal DOM updates—not a second DOM you manipulate directly.
3. Q: Why is React popular?
   A: Component model, large ecosystem, strong hiring demand, React Native for mobile, and mature patterns for complex UIs.

Self-check

1. Name two things React gives you that raw document.querySelector does not.
2. What does “one-way data flow” mean in one sentence?