30 technical interview questions, explained the way you'd actually say them

A developer compiled 30 technical interview questions across JavaScript, React, and Node.js, with answers written in conversational language to help candidates explain concepts clearly under pressure. The guide covers topics like closures, event loop, hoisting, and React hooks, emphasizing practical verbal explanations over textbook definitions.

Most interview prep content gives you a definition. Real interviews test something different: can you explain your reasoning clearly, out loud, under a little pressure — not just recite the right words. I put together 30 questions across JavaScript, React, and Node.js. Every answer here is written the way you'd actually say it in an interview, not the way a textbook would write it. How to actually use this: cover the answer, try explaining it out loud in under 30 seconds, then read the answer. If you froze or rambled, that's the real signal — more than whether you technically knew the concept. A closure is a function that remembers the variables from where it was created, even after that outer function has finished running. It powers private variables, debouncing, memoization, and module patterns. setTimeout fn, 0 vs Promise.then — which runs first? The Promise wins. .then callbacks go into the microtask queue, which fully drains before the next macrotask like setTimeout runs — even with a 0ms delay. var break inside loops with closures, but let doesn't? var is function-scoped — every iteration shares the same variable. let is block-scoped, so each iteration gets its own fresh binding. == actually give you a different and wrong answer than === ? == does type coercion first — 0 == false and '' == 0 are both true. === compares type and value directly, no surprises. this break in callbacks with regular functions, but not arrow functions? Regular functions get this based on how they're called. Arrow functions inherit this lexically from where they were defined, so it stays consistent no matter how they're invoked. JS walks the prototype chain — the object, then its prototype, then that prototype's prototype — until it's found or it hits null . Search wants debounce — fire once after typing stops. Scroll wants throttle — fire at a steady max rate continuously. Promise.all vs Promise.race ? Use all when you need every result, and it should reject if any fail. Use race when you only care about whichever resolves first. Function declarations fully hoist. const fn = = {} only hoists the declaration, not the assignment, so calling it early throws. Currying transforms f a,b,c into f a b c . It's useful for creating reusable, partially-configured functions. React batches changes, diffs the old and new virtual trees, and only touches the specific real DOM nodes that actually changed. useEffect completely? The effect runs after every single render — a common source of infinite loops if the effect itself updates state. setState 3 times in one event handler — does React re-render 3 times? No. React batches updates within the same handler into a single re-render, and React 18 extends this batching further. key break things when a list reorders? React matches elements between renders by key . Using the index means React thinks the item at that position changed, not that it moved. A controlled input's value lives in React state and updates via onChange . An uncontrolled input's value is managed by the DOM itself, read via a ref when needed. useMemo vs. useCallback — what's the actual difference in what gets memoized? useMemo memoizes a computed value. useCallback memoizes the function itself — useful for stable references passed to memoized children. Context re-renders every consumer on any value change — bad for frequently-changing state. It's best for rarely-changing global data like theme or auth. React.memo , but it's still re-rendering every time. Why? React.memo does a shallow prop comparison. A new object, array, or function reference each render looks different even if the underlying data is the same. A function whose name starts with use and that calls other hooks inside it. The naming convention is how React's linter enforces the rules of hooks. try/catch work for catching rendering errors in React? try/catch only catches synchronous errors in code you directly run. Error Boundaries hook into React's own lifecycle to catch errors during rendering. JavaScript execution is single-threaded, but I/O operations get delegated to a thread pool. The event loop picks up completed I/O and runs the callbacks, never blocking on the wait. Reading the whole file into memory holds it all in RAM per request. Streams process and send data in small chunks, keeping memory usage flat regardless of file size. next actually do in Express middleware, and what breaks if you forget it? It passes control to the next handler in the chain. Forget it, and the request just hangs forever with no response ever sent. Not easily. JWTs are stateless, so you'd need extra infrastructure like a blocklist or short expiry with refresh tokens to revoke one early. It lets asynchronous code read top-to-bottom like synchronous code, with normal try/catch for errors — much easier to reason about than chained callbacks. The cluster module forks multiple copies of the process across cores, sharing the same port, so incoming requests get distributed across the workers. .env file to Git a serious mistake, not just messy? It usually holds real secrets. Once committed, it's in the Git history permanently — deleting the file later doesn't remove the old commits. GET to delete a resource, even if it technically works? GET is supposed to be safe and idempotent. Browsers and crawlers can pre-fetch GET URLs, which could accidentally trigger a deletion. Express checks a function's parameter count to identify it as an error handler. A 4-argument signature is specifically what signals that role. package.json , same npm install — can two people end up with different dependency versions? Yes, without a package-lock.json . Version ranges can resolve differently over time, and the lockfile pins exact versions for reproducible installs. Reading an answer and being able to say it clearly under pressure are two different skills. I've been building Sparlog https://sparlog.com — an AI that actually interviews you: you explain your reasoning out loud while writing code, and it scores clarity and correctness together, the way a real interviewer would. It's free to try, currently in open beta. Would genuinely love feedback if you give it a shot. Also made a free downloadable PDF version of this list if you'd rather have it offline: sparlog.com/resources https://sparlog.com/resources