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10 JavaScript questions that came up in almost every frontend interview I sat this year

I've been interviewing for frontend roles (SDE-2, around 4.5 years experience, React and Next.js) over the last few months and sat through a fair number of loops at product companies and mid-size startups. I kept a running doc of everything that got asked so I could spot patterns. Ten questions came up often enough that I'd now treat them as near guaranteed.

Sharing in case it saves someone else the trial and error.

Implement debounce from scratch, then explain when you'd use debounce vs throttle. This one showed up in almost every single loop, usually as the warm up before harder coding.
Closures. Not just the definition, but where you use them in React and the classic closure-inside-a-loop output question with var vs let.
Hoisting, and how it differs from closures. Usually followed by var vs let vs const and when you actually hit a ReferenceError.
The event loop. Call stack, microtask queue, macrotask queue, followed by predicting output from a snippet mixing setTimeout and Promise.then.
The this keyword. Normal vs arrow functions, and what happens when you detach a method from its object. const a = obj.getName; a()
Prototypal inheritance and the prototype chain, and how it differs from class inheritance.
Deep copy vs shallow copy. Explain the difference, then implement a deep clone. Interviewers usually push on why JSON.parse(JSON.stringify()) is not good enough.
Flatten a nested array, first without Array.flat() and then with it.
Promise vs async/await. When you prefer each, and how error handling actually differs between them.
Event delegation. Why it exists and how you'd apply it to a list with thousands of rows.

Two things that surprised me. Polyfills for call/apply/bind and the Promise combinators came up far less than the prep content online suggests. And output prediction questions were much more common than I expected, often used as a filter before anyone let me write real code.

Curious whether this matches what others have seen recently, or if it's specific to the kind of companies I was talking to.

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