Interview prep

Each question is phrased the way an interviewer asks it. The answer is collapsed so you can try first. Below it is why the answer holds and the follow-up you are likely to get. Language fundamentals are listed below the React topics.

719questions

231output puzzles

25topics

JavaScript - the language 3 topics

The half of an interview that is not about frameworks: what the engine does before your code runs, and what the operators actually compare.

Scope, hoisting and the TDZ

What the engine does to a scope before it runs a line of it - and why that makes var undefined, let a ReferenceError, and "moved to the top" the wrong story.

34 questions · 14 output

Closures, this, and currying

Scope is decided where a function is written; this is decided how it is called. The loop that prints 3,3,3, the method that loses its object, and the arrow that fixes one bug by causing another.

34 questions · 14 output

Types, coercion and equality

The == algorithm recited in six lines, the eight falsy values, why 0.1 + 0.2 misses, and what to write when JSON.stringify is not a real object comparison.

38 questions · 20 output

JavaScript - time and events 3 topics

One thread, and everything that makes it look like more than one. This is where output-order questions come from.

The event loop, microtasks and output order

One stack, two queues and a rule that answers almost every "what does this log" question - plus the misconception that async moves work to another thread, said out loud and killed.

32 questions · 14 output

Callbacks, promises and async await

Inversion of control is the real problem callbacks had, not nesting. Then the four bugs that actually reach code review: the sequential await, the forgotten await, the async forEach, and the constructor anti-pattern.

35 questions · 17 output

Event emitters, debounce and throttle

Publish/subscribe in twenty lines, and the difference between debounce and throttle stated so you cannot mix them up - with the cancel, the cache key and the leak everyone forgets.

28 questions · 11 output

React - how it works 3 topics

What React is doing between your setState and the pixels, said precisely enough to answer a follow-up.

Virtual DOM, reconciliation and diffing

What the virtual DOM actually is, why the diff is O(n) instead of O(n^3), and the honest answer to "doesn't keeping a second copy of the DOM make it slower".

38 questions · 10 output

Components, state, props and lifecycle

The vocabulary round, graded on judgement: why props are read-only, why state looks asynchronous, the whole class lifecycle, and the && that renders a bare 0 onto the page.

34 questions · 11 output

Re-renders, memo and optimisation

Why it re-rendered, whether that mattered, and the four structural fixes to try before reaching for memo - plus the answer to "your list re-renders on every keystroke".

32 questions · 10 output

React - the coding round 4 topics

Forms, fetching, custom hooks, rendering a tree. The things you are asked to write with someone watching.

Hooks: useState, useEffect and the rest

Why three setCount(count + 1) calls advance by one, why the interval logs zero forever, and why both are the same closure question - plus the five custom hooks worth typing from memory.

41 questions · 15 output

Forms: controlled, uncontrolled and validated

Who owns the value, and when to show the error. One state object with a generic onChange, a touched map so nothing shouts at an empty page, and every widget's own gotcha.

32 questions · 13 output

Fetching data, and the race condition

fetch does not reject on a 404, there are four states rather than two, and an older request can resolve last and win. Both fixes, then the honest comparison of Context, Redux, Zustand and React Query.

32 questions · 13 output

Passing data, context and routing

Props down, callbacks up, lift to a common parent - and composition before context, which is the fix nobody names. Then react-router end to end, portals, and the recursive file tree.

35 questions · 13 output

HTML, CSS and the rest 4 topics

The warm-up questions. Short answers, and the one follow-up each of them usually gets.

HTML: semantics, forms and meta

The tag is not chosen for its default styling - it is chosen for the accessibility tree, the keyboard behaviour and the form participation that CSS cannot give you back.

40 questions · 12 output

CSS: box model, layout and positioning

The values the browser resolves rather than the properties you type: box-model arithmetic, which ancestor an absolute element measures from, and why a z-index of 9999 loses to a 1.

42 questions · 16 output

The DOM, events and storage

Three phases, one main thread: which handler fires first, why an anonymous listener cannot be removed, what a token in localStorage costs, and the pipeline behind LCP, INP and CLS.

38 questions · 16 output

REST, HTTP and Git

Both halves are really one question - what happens when something is attempted twice. Safe versus idempotent, PUT versus PATCH, and what rewriting history does to a colleague's clone.

42 questions · 12 output

DSA - name the pattern 1 topic

Forty problems in plain words. Say which tool each one wants before you open it: the skill between reading the lessons and solving a problem cold.

Name the pattern

Sliding window or prefix sums, heap or sort, BFS or union-find. Each problem hides one detail that gives the tool away; the answer is the tool and the tell, and the follow-up is the change that breaks it.

40 questions

System Design - the whiteboard round 6 topics

Twenty-five design prompts in the words an interviewer uses. Each opens on the one decision the design hinges on, then the numbers that force it, then the twist they add next, and the lesson that teaches each piece.

Read-heavy systems

URL shortener, home timeline, typeahead, image hosting, pastebin. When reads dominate, the hit rate decides the design, not the read rate.

5 questions

Write-heavy and streaming systems

Metrics, ad-click counts, notifications, logging, a web crawler. When writes dominate, a log is the spine, and the hinge is the partition key and what happens when consumers fall behind.

5 questions

Storage and consistency

A key-value store, a cache, a rate limiter, unique IDs, payments that never charge twice. Name the consistency the product needs before naming the storage.

5 questions

Real-time and location

Chat with presence, ride-hailing dispatch, nearby places, a collaborative editor, video streaming. How the server reaches the client, and how many connections one box holds.

5 questions

Many hands on few rows

Concert tickets, a flash sale, hotel rooms, a leaderboard, an exchange. A lock, a version check or a single-writer queue, and what each costs under a spike.

5 questions

Data systems: which trade-off is this?

Twenty-five situations in plain words: a saved edit that vanished, a disk writing more than the app sends, a count that changed after it was shown. Name the idea and the detail that gave it away, then the change that turns it into a different question.

25 questions

Networking - the wire round 1 topic

One request followed down the stack: DNS, TCP, TLS, HTTP, and the routers underneath. For each step, what it promises, what it costs in round trips and how it fails.

Networking: from URL to packet

What happens when you type a URL, why DNS is on UDP, the handshake, loss, QUIC on a train, TLS, where 200 ms went, bufferbloat, BGP, NAT, SYN floods, and why a WebSocket dies at 60 seconds.

22 questions