01 · React Fundamentals

Q1

What is React?

In one line

React is a JavaScript library for building user interfaces out of reusable components, created by Meta.

The four ideas that actually define it:

Idea	What it means
Component-based	The UI is split into small independent pieces you compose together, like Lego.
Declarative	You describe what the screen should look like for a given state. React works out how to update the DOM.
Virtual DOM	React keeps a copy of the UI in memory, compares it after a change, and touches only the parts of the real DOM that differ.
One-way data flow	Data flows down from parent to child. That makes bugs easy to trace.

Declarative vs imperative — the difference in code

// Imperative (plain JS): you give step-by-step instructions
const btn = document.getElementById("btn");
btn.addEventListener("click", () => {
  document.getElementById("label").innerText = "Clicked!";   // you touch the DOM yourself
});

// Declarative (React): you describe the result
{clicked ? "Clicked!" : "Not clicked"}                       // React updates the DOM for you

30-second interview answer

React is a JavaScript library from Meta for building user interfaces. You build the UI from small reusable components and describe what each should look like for a given state; React then uses a Virtual DOM to work out the minimum real DOM updates needed. Data flows one way, from parent to child, which makes the app predictable. It handles the view layer only — routing, state management and HTTP come from separate libraries.

Q2

Is React a framework or a library?

In one line

A library. React only handles the UI — everything else is your choice.

React deliberately leaves out most of what an application needs:

Need	React gives you	You add
UI components	✅ Yes	—
Routing	❌ No	React Router
Global state	Context only	Redux Toolkit, Zustand
HTTP calls	❌ No	fetch, Axios
Forms	❌ No	React Hook Form
Build tooling	❌ No	Vite

Nuance worth adding

People sometimes call React a framework because of the ecosystem around it, and Next.js genuinely is a framework built on React. But React itself is a library. Saying that distinction out loud shows you understand it rather than reciting it.

30-second interview answer

React is a library — it only covers the view layer. Routing, state management, HTTP and forms all come from separate packages that you choose. That's the trade-off: more flexibility, but the architecture decisions are yours. Next.js, which is built on React, is the framework version of it.

Q3

What is the difference between a library and a framework?

In one line

You call a library. A framework calls you. That reversal is called inversion of control.

A useful comparison from your own .NET world: Dapper is a library — you write the SQL and call it when you want. ASP.NET MVC is a framework — it owns the request pipeline and calls your controller action at the moment it decides.

	Library	Framework
Who's in control	You	The framework
Structure	You decide	Prescribed
Freedom	High	Lower, but consistent
Examples	React, Dapper, Axios	Angular, ASP.NET Core, Spring

30-second interview answer

The difference is inversion of control. With a library, your code is in charge and calls the library when it needs it. With a framework, the framework is in charge and calls your code at the points it defines. Dapper is a library, ASP.NET Core MVC is a framework — and in the frontend, React is a library while Angular is a framework.

Q4

React vs Angular?

In one line

React is a flexible UI library where you assemble your own stack; Angular is a complete opinionated framework with everything already decided.

	React	Angular
Type	Library (UI only)	Full framework
Language	JS or TypeScript	TypeScript (enforced)
Templates	JSX — markup inside JS	HTML templates + directives (`ngIf`, `ngFor`)
Data binding	One-way	Two-way available (`[(ngModel)]`)
DI	Not built in	Built-in dependency injection
Routing / HTTP	Separate packages	Built in
Async style	Promises / async-await	RxJS observables
Learning curve	Gentler start	Steeper — more concepts up front
Best for	Flexible apps, fast-moving teams	Large enterprise apps wanting one standard way

How to answer without sounding biased

Never say "React is better". Say what each is good at, then name a situation for each. A senior engineer picks tools for a context; a junior defends a favourite.

30-second interview answer

Angular is a complete framework — TypeScript, dependency injection, routing, HTTP and RxJS all come in the box, with one prescribed way of doing things, which suits large enterprise teams that want consistency. React is a UI library: it gives you components and leaves routing, state and HTTP to you, so you get flexibility and a lighter learning curve but you own the architecture decisions. I'd pick Angular where a large team needs one enforced standard, and React where I want to move fast and shape the stack around the product.