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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.

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.

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.

Debounce & Throttle

Debounce delays execution until a pause in events, while throttle limits execution to a fixed interval — both are essential for controlling high-frequency event handlers.

React Internals

React's reconciliation algorithm diffs virtual DOM trees in O(n) using two heuristics — different element types tear down the subtree, same types compare attributes — while the Fiber architecture makes this work interruptible so the browser stays responsive during large updates.

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.

Generics

Generics let you write functions, classes, and types that work with any type while preserving type safety — the type parameter <T> acts as a variable at the type level, inferred from usage or constrained with extends to require specific shapes.

Runtime Performance & Profiling

Runtime performance focuses on what happens after the page loads: identifying long tasks that block the main thread, avoiding layout thrashing, using requestAnimationFrame for smooth animations, and offloading heavy computation to Web Workers.

Rendering Strategies & Critical Rendering Path

Choosing between CSR, SSR, SSG, and ISR determines when and where HTML is generated, while understanding the Critical Rendering Path (DOM, CSSOM, render tree, layout, paint) reveals how browsers turn that HTML into visible pixels.

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.