2learn

React

React.js

What it is

React is a JavaScript library for building user interfaces out of small, reusable components. You describe what the screen should look like for a given state, and React works out the smallest set of changes needed to make the browser match that description. It is a library rather than a framework: routing, data fetching and styling stay separate choices that you make yourself.

Why it matters

Updating the DOM by hand is where most early front-end bugs live. You forget to remove a listener, you update one of the three places the same value lives, and the screen slowly stops matching your data. React removes that class of mistake by making the rendered output a pure function of state: change the state, and the UI is rebuilt for you. That is why it dominates the job market — the mental model is small enough to learn in a day and strong enough to carry a large application. It is also the ecosystem everything else is written for, from component libraries to testing tools.

In this track

  1. Rendering and JSX (02-frontend/29-react/01-rendering-and-jsx) — how a component becomes DOM, and the JSX syntax you use to describe it.
  2. Components and props (02-frontend/29-react/02-components-and-props) — splitting a screen into components and passing data down without mutating it.
  3. State and hooks (02-frontend/29-react/03-state-and-hooks) — making a component remember values between renders with useState.
  4. Effects and data fetching (02-frontend/29-react/04-effects-and-data-fetching) — running side effects after render and loading data without racing.

Prerequisites

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