TypeScript, React and Next.js Guide for Developers

What TypeScript is and is not, why types are erased at runtime, setting up a project, a tsconfig explained line by line, and reading your first compiler errors.

Primitives, unions, literal types, any versus unknown versus never, inference, narrowing and discriminated unions.

Object types, interface versus type, structural typing, excess-property checks, classes, enums versus unions, as const and satisfies.

Function types, overloads, this parameters, generics and constraints, a typed fetch wrapper and a typed event emitter.

Built-in helper types, mapped and conditional types, infer, template literal types, and when not to be clever.

Typed React and Express, plus Zod validation and branded types, because types vanish at runtime.

Reading long errors bottom-up, the common error codes, and a measured step-by-step migration to strict mode.

What “render” really means, how React finds what changed between renders, why keys matter and how batching works.

Effect dependencies, stale closures, useRef vs state, useLayoutEffect and the idea behind “You Might Not Need an Effect”.

Transitions, Suspense boundaries, streaming SSR, hydration errors and the line between server and client components.

How to sort state into kinds (UI, server, URL, form) and pick a tool for each kind, instead of arguing about libraries.

Parse, style, layout, paint and composite. Also layout thrashing, and where tasks, microtasks and requestAnimationFrame fit.

What each metric measures, its “good” limit at the 75th percentile, why INP replaced FID, and the difference between field data and lab data.

The React Profiler, context updates that re-render every consumer, props that change each time, using memo on purpose and not everywhere, and what the React Compiler changes.

Bundle analysis, code splitting by route, image optimisation, font loading, preload and preconnect, and Server Components with streaming.

Test what users see, not how the code is built. Mock the network, not your modules. Keep end-to-end tests for the key flows only.

Queries by role, user-event, async UI, mocking the network with MSW, forms and error states.

Playwright locators, auto-waiting, traces and isolation, with four flaky tests reproduced and fixed.

Semantic HTML first, focus in modals, ARIA only when needed, and testing with the keyboard.

Generics, narrowing, utility types, tsconfig flags, typing React and APIs, and the errors that confuse people.

Component design, hooks, rendering cost, forms, data fetching, React 19, and code-review style questions.

App Router, Server and Client Components, caching, Server Actions, proxy, rendering modes and deployment.

Redux Toolkit, TanStack Query v5, Zustand and Jotai, server state vs client state, optimistic updates, hydration with the App Router, forms with Zod.

Generic constraints, discriminated unions, infer, mapped types, satisfies, and keeping API response types correct from the server to the UI.