The answer
async: download in parallel, execute as soon as ready (pauses parsing). defer: download in parallel, execute after HTML parsing completes (in order). Use defer for most scripts; async for independent scripts like analytics.
Top JavaScript Interview Topics for Frontend Developers
What is the difference between async and defer in script loading?
More FAQs in Top JavaScript Interview Topics for Frontend Developers
Closures, event loop, this keyword, call/apply/bind, promises/async-await, prototypes, debounce/throttle, currying, event bubbling/delegation, polyfills, CORS, and async/defer. Study each deeply and implement from scratch.
Closures are the foundation of many interview questions: data privacy, currying, debounce, throttle, memoize, and the loop closure bug. Understanding closures is essential for almost every JS interview.
Implementing debounce or throttle from scratch (Flipkart, Walmart), polyfills for map/filter/reduce/bind, the event loop output question (sync, microtask, macrotask order), and currying (sum(1)(2)(3) - Amazon).
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