Hand-picked tutorials, guides and examples on Interview Questions — explained simply.
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Short answer: a closure is a function together with the variables from the scope where it was created. The function keeps access to those variables even after the outer function has returned. function makeCounter() { let count = 0; // captured by the inner fun...
Short answer: when state changes, React calls your components to produce a new tree of plain JavaScript objects describing the UI (often called the virtual DOM). It compares this tree with the previous one ( reconciliation ) and applies only the necessary chan...
Short answer: WHERE filters individual rows before grouping. HAVING filters groups after GROUP BY , so it can use aggregate functions like COUNT and SUM . -- Customers who placed more than 5 paid orders in 2026 SELECT customer_id, COUNT(*) AS paid_orders, SUM(...
System design interviews are open-ended on purpose. Interviewers want to see structured thinking and trade-off reasoning more than a "correct" diagram. This framework works for almost any prompt. 1. Clarify requirements (5 minutes) Functional : what must it do...
Short answer: JavaScript runs on a single thread with a call stack. Async work (timers, network, I/O) is handled by the environment, which queues callbacks. The event loop takes the next callback when the stack is empty, always draining all microtasks before t...
Short answer: keys tell React which list item is which between renders. With stable keys, React can move, insert and remove items correctly, keeping each item's DOM node and state attached to the right data. {todos.map((todo) => ( <TodoItem key={todo.id} todo=...
Short answer: joins combine rows from two tables based on a condition. They differ in what happens to rows that have no match. -- customers: (1, Asha), (2, Ravi), (3, Meera) -- orders: (101, customer 1), (102, customer 1), (103, customer 9) INNER JOIN: only ma...
A classic warm-up question that touches ID generation, read-heavy scaling and caching. Requirements Create a short URL for a long URL; optionally a custom alias and expiry. Redirect quickly (under ~50 ms), very high availability. Scale: ~100M new links per mon...
Short answer: === (strict equality) compares value and type with no conversion. == (loose equality) converts the operands to a common type first, which leads to surprising results. 1 === '1' // false - different types 1 == '1' // true - '1' converted to 1 0 ==...
Short answer: useEffect runs after React has committed the render to the screen. With no dependency array it runs after every render; with [] only after mount; with [a, b] after mount and whenever a or b changed. The cleanup runs before the next effect and on...
A favourite interview question: given employees(id, name, department, salary) , find the Nth highest salary. Mention how you handle ties and missing values. 1. DISTINCT with LIMIT/OFFSET -- 3rd highest distinct salary SELECT DISTINCT salary FROM employees ORDE...
Rate limiters protect services from abuse and overload. This question tests algorithm choice and distributed-state thinking. Requirements Limit requests per user/API key (for example 100 per minute), with different tiers. Low added latency (a few milliseconds)...
Short answer: var is function-scoped and hoisted with the value undefined . let and const are block-scoped and can't be used before their declaration (the temporal dead zone). const can't be reassigned. Scope if (true) { var a = 1; let b = 2; } console.log(a);...
Short answer: a controlled input's value comes from React state and changes through onChange . An uncontrolled input keeps its own value in the DOM; you read it when needed (via a ref or FormData ). Controlled function Search() { const [query, setQuery] = useS...
Short answer: an index is a separate, sorted data structure (usually a B-tree) that maps column values to row locations, so the database can find matching rows without scanning the whole table, much like a book's index. CREATE INDEX idx_orders_customer ON orde...
Caching appears in almost every design. Strong answers name the cache layer, the pattern, how data stays fresh and what happens when the cache fails. Layers of caching Browser - Cache-Control headers for static assets and some API responses. CDN - images, scri...
Short answer: for regular functions, this is decided by how the function is called , not where it's defined. Arrow functions don't have their own this ; they use the this of the surrounding scope. The rules, in order new : new User() - this is the new object....
Short answer: useMemo caches a computed value , useCallback caches a function , and React.memo skips re-rendering a component when its props haven't changed. const visible = useMemo(() => products.filter((p) => p.inStock), [products]); // value const onSelect...
Short answer: normalisation organises tables to reduce duplicated data and prevent update anomalies. Each normal form removes a specific kind of redundancy. Starting point orders_raw order_id | customer | customer_city | products | product_prices 101 | Asha |...
Short answer: a load balancer spreads incoming requests across multiple servers so no single server is overloaded, and routes around servers that fail health checks. Layer 4 vs Layer 7 L4 (transport) : routes TCP/UDP connections by IP and port. Very fast, prot...
Short answer: a promise represents a value that will be available later; it's pending, then fulfilled or rejected. async / await is syntax on top of promises: an async function always returns a promise, and await pauses the function until a promise settles. Sa...
Short answer: a component re-renders when its own state changes, when its parent re-renders, when a context it reads changes, or when a hook it uses triggers an update. 1. Its state changes const [count, setCount] = useState(0); setCount(1); // re-render setCo...
Short answer: ACID describes the guarantees a database transaction gives: A tomicity, C onsistency, I solation and D urability. BEGIN; UPDATE accounts SET balance = balance - 1000 WHERE id = 1; UPDATE accounts SET balance = balance + 1000 WHERE id = 2; COMMIT;...
"The database is the bottleneck. How do you scale it?" Walk through the options from cheapest to most complex. 1. Optimise and scale up Indexes, query fixes, connection pooling and caching first. Then a bigger machine (vertical scaling). Modern servers are pow...
Short answer: every JavaScript object has an internal link to another object, its prototype . When you read a property that the object doesn't have, JavaScript looks it up along this prototype chain until it finds it or reaches null . const animal = { speak()...
Short answer: props are inputs passed from a parent; a component can't change them. State is data a component owns and updates over time, which triggers a re-render. function Counter({ step }) { // step is a prop const [count, setCount] = useState(0); // count...
Short answer: both stack the results of two queries. UNION removes duplicate rows; UNION ALL keeps every row. Because removing duplicates needs a sort or hash step, UNION ALL is faster. -- Emails from two sources SELECT email FROM newsletter_subscribers UNION...
Short answer: in a distributed system, when a network partition happens (nodes can't talk to each other), you must choose between consistency (every read sees the latest write, or gets an error) and availability (every request gets a non-error response, possib...
Short answer: a shallow copy duplicates only the top level; nested objects are still shared. A deep copy duplicates every level, so changing the copy never affects the original. const original = { name: 'Asha', address: { city: 'Pune' } }; const shallow = { .....
Short answer: Server Components render only on the server. They can be async , read databases and files directly, and their code is never sent to the browser. Client Components (marked 'use client' ) handle interactivity with state, effects and event handlers....
Short answer: DELETE removes selected rows, TRUNCATE quickly removes all rows but keeps the table, and DROP removes the table itself, including its structure. DELETE FROM sessions WHERE expires_at < NOW(); -- some rows TRUNCATE TABLE import_staging; -- all row...
Short answer: use a message queue when work doesn't need to finish within the user's request, when producers and consumers should scale and fail independently, or when traffic bursts need buffering. Classic uses Sending emails, generating PDFs, resizing images...
Task: write debounce(fn, wait) that returns a function which only calls fn after it stops being called for wait milliseconds. Typical use: search-as-you-type. Solution function debounce(fn, wait) { let timer; return function (...args) { clearTimeout(timer); //...
Short answer: prop drilling is passing props through several intermediate components that don't use them, just to reach a deeply nested child. Fix it with composition first, then Context , then a state library for complex shared state. The problem <App user={u...
Short answer: a primary key uniquely identifies each row and can't be NULL (one per table). A unique key also enforces uniqueness but a table can have many, and they may allow NULLs. A foreign key links a column to a key in another table and enforces that the...
A popular question that combines APIs, queues, third-party integrations and reliability. Requirements Channels: mobile push, email, SMS, in-app. Triggered by other services ("order shipped") and by scheduled campaigns. Respect user preferences and quiet hours;...
Short answer: event delegation means attaching one listener to a parent element instead of one per child. It works because most DOM events bubble up from the element where they happened through its ancestors. <ul id="todo-list"> <li data-id="1">Buy milk <butto...
Short answer: useRef returns an object { current } that persists for the component's lifetime. Changing ref.current does not cause a re-render. It's used for DOM access and for mutable values that aren't part of what's displayed. 1. Accessing DOM elements func...
Short answer: all three number rows in a given order. They differ only in how they handle ties. SELECT name, score, ROW_NUMBER() OVER (ORDER BY score DESC) AS row_num, RANK() OVER (ORDER BY score DESC) AS rnk, DENSE_RANK() OVER (ORDER BY score DESC) AS dense F...
Short answer: consistent hashing distributes keys across servers so that adding or removing a server moves only a small fraction of keys, instead of almost all of them. The problem with modulo server = hash(key) % N N = 4 -> 5 servers: about 80% of keys now ma...
