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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.

Event Loop & Runtime

JavaScript is single-threaded with one call stack — the event loop enables async behavior by processing microtasks (promises) before macrotasks (setTimeout) between each cycle of the call stack.

DOM

The Document Object Model (DOM) is a tree-structured programming interface that lets JavaScript read, modify, and respond to a web page's content, structure, and styles in real time.

Memory Management & Garbage Collection

JavaScript automatically manages memory through garbage collection using a mark-and-sweep algorithm — objects are freed when they become unreachable from root references, but common patterns like forgotten timers and detached DOM nodes cause memory leaks.

Prototypes & Inheritance

Every JavaScript object has an internal [[Prototype]] link forming a chain — property lookups walk this chain until found or null. ES6 classes are syntactic sugar over this prototypal inheritance model.

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.

Performance Optimization

React.memo skips re-renders when props haven't changed, useMemo caches computed values, and useCallback caches function references — but these are targeted optimizations, not defaults for every component.

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.

Flexbox Layout

Flexbox is a one-dimensional layout model for distributing space and aligning items along a main axis and cross axis, with container properties controlling flow direction and item properties controlling grow, shrink, and basis behavior.

CSS Grid Layout

CSS Grid provides two-dimensional layout control over rows and columns simultaneously, using grid-template definitions, the fr unit for fractional space, auto-fit/auto-fill for responsive grids, and named grid areas for semantic layout declarations.

Core Web Vitals & Performance Metrics

Core Web Vitals are Google's three key metrics (LCP, INP, CLS) that measure real-world user experience for loading speed, interactivity, and visual stability, directly impacting search rankings.