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Closures

A closure is a function that retains access to variables from its outer (enclosing) scope even after the outer function has returned, enabled by JavaScript's lexical scoping.

Promises & Async/Await

Promises represent eventual completion or failure of async operations with three states (pending, fulfilled, rejected), and async/await provides syntactic sugar for writing promise-based code that reads like synchronous code.

Error Handling

JavaScript provides try/catch/finally for synchronous error handling and .catch() or try/catch with async/await for asynchronous errors, with built-in error types like TypeError, ReferenceError, and SyntaxError.

Hooks

Hooks let functional components manage state (useState), run side effects (useEffect), access refs (useRef), and extract reusable logic into custom hooks — all following strict rules about call order.

State

State is internal, mutable data owned by a component — updates must be immutable (creating new objects, not mutating existing ones) and are batched asynchronously in React 18+ for performance.

Error Boundaries

Error boundaries are class components that catch JavaScript errors during rendering and lifecycle methods, displaying a fallback UI instead of crashing the entire app — but they don't catch errors in event handlers, async code, or SSR.

TypeScript Fundamentals

TypeScript is a typed superset of JavaScript that compiles to plain JS — it catches type errors at compile time rather than runtime, enables intelligent IDE tooling (autocomplete, refactoring, go-to-definition), and has become the industry standard for production JavaScript applications.

Interfaces vs Type Aliases

Interfaces define object shapes and can be extended with extends or merged via declaration merging — type aliases can represent any type (unions, tuples, primitives) but can't be merged, and the practical rule is to use interface for objects and type for everything else.

Lazy Loading Techniques

Lazy loading defers the loading of non-critical resources until they are needed, reducing initial page weight and improving Time to Interactive through native attributes, Intersection Observer, and dynamic imports.

Caching Strategies

Web caching stores previously fetched resources at various layers (browser, CDN, service worker) to reduce network requests, lower latency, and enable offline access in production applications.

Unit Testing Fundamentals

Why testing matters, what unit tests are, what to test versus what to skip, and how automated tests improve code quality, enable safe refactoring, and serve as living documentation.

Responsive Design Fundamentals

Responsive design combines the viewport meta tag, media queries, fluid units (%, vw, rem, clamp()), and mobile-first methodology to create layouts that adapt gracefully from mobile screens to large desktops.