Polyfills and JavaScript Utility Implementation Questions

Rebuilding standard JavaScript behavior and common utility libraries from scratch, the machine-coding interview genre where a candidate re-creates a familiar API to prove they understand its mechanics. Covers a minimal Promise built from first principles (chaining, asynchronous handlers, single settlement, thenable adoption, error flow) and Promise.allSettled-style combinators; polyfills for browser APIs such as IntersectionObserver, and a keyed virtual DOM diff; utilities such as debounce (leading and trailing, cancel, flush), throttle, and React hooks that debounce a value or a callback with stable identity and no stale closures; deep clone (cycles, shared references, Dates, RegExps), shallow and deep equality (typed arrays, Maps and Sets, NaN and -0), nested object helpers (safe path get, flatten and unflatten, deep merge), curry with placeholders and partial application, and memoize with cache keys and WeakMap-based caches; and small infrastructure such as event emitters (on, off, once, emit, context binding, listener leaks). Emphasizes edge cases, correct this binding, argument handling, spec fidelity, and complexity. Using these built-ins as a language feature, the event loop and async/await semantics, DOM traversal and event propagation, functional programming as a paradigm, and generic algorithm puzzles are covered elsewhere.