Notes · React 19.2.5
Everything a React developer should know, from JSX to the fiber tree.
Each topic is explained in plain English first, then shown: diagrams, step-through animations and real output recorded from one small example app. Every note ends with interview questions you can quiz yourself on.
Seven modules, from the basics to how React works inside.
From the raw DOM to JSX, components, props, forms, error boundaries and keys: nothing about React’s core API feels like magic.
State, effects and their cleanup, the component lifecycle, lifting state, refs, unique IDs, useState vs useReducer, and what makes components re-render.
The escape hatches: reducers, context with use, portals, layout effects, imperative handles, flushSync and external stores.
Component API design: composition, latest ref, compound components, slots, prop getters, state initializers, state reducers and control props.
Render less and respond faster: element and context optimization, concurrent rendering, code splitting, expensive calculations, memo for lists and windowing.
Data fetching with use, Suspense and error boundaries, transitions, optimistic UI, suspending images and avoiding waterfalls.
How React works under the hood: reconciliation, Fiber, the work loop, commit, scheduling and hooks, checked against the React 19.2.5 source.
Practice
76 questions across React internals, JSX and fundamentals, optimization, hooks and patterns, answered the way a senior engineer would: a precise definition, then the key points. Quiz yourself and mark what you know.
Start practising →