Frontend System Design: Build an Autocomplete
A structured walkthrough of the autocomplete design round: requirements, architecture, race-free fetching, caching, rendering, the ARIA combobox and metrics.
~7 min readread →
psst… 1610+ questions inside
Your interview study notes, finally organized.
The tech interview questions that come up again and again, with clear study notes for JavaScript, TypeScript, React, Angular, Java, Python, SQL, AWS and more, organized by topic and with the reasoning written in the margins. Finally, practice them like it's the real interview.
Closures, Ch. 1 ✦
function outer() { let count = 0; return function inner() { return ++count; }; } const next = outer(); next(); // 1 next(); // 2 ← why not 1?!
Q: var vs let?var → function scope, hoisted as undefined
let → block scope + TDZ!
let → block scope + TDZ!
Top 10 Q ★Asked in almost every JS round.
Pick a chapter (start anywhere, really)
See all chapters →Frontend
The browser, the language and the frameworks.JSCh. 01JavaScript
Closures, hoisting, prototypes & the event loop.
TSCh. 02TypeScript
Generics, narrowing, utility & mapped types.
ReactCh. 03React
Hooks, rendering, memoization & state.
AngularCh. 04Angular
Change detection, DI, signals & RxJS.
HTML/CSSCh. 05HTML & CSS
Semantics, the box model, layout & a11y.
{ }BonusCoding round
Debounce, Promise.all, LRU cache… write it, run the tests.
Backend
Servers, services and the languages behind them.NodeCh. 06Node.js
Event loop phases, streams & modules.
JavaCh. 07Java
OOP, collections, JVM, concurrency & streams.
SpringCh. 08Spring Boot
IoC, auto-configuration, JPA, REST & security.
PythonCh. 09Python
Data model, generators, decorators & the GIL.
MicroCh. 10Microservices
Service design, messaging, resilience & sagas.
Databases
Relational and document stores, queries and design.Cloud & DevOps
Containers, orchestration, cloud and delivery.Platforms
Enterprise platforms with their own interview loops.Fundamentals
Algorithms and architecture: asked everywhere.The practice room no sign-up, saved in your browser
Open the practice room →Quiz483 multiple-choice & “what's the output?” questions, with explanations.Flashcards1127 cards with spaced repetition, so they stick.Mock interviewTimed questions, model answers and a score report.Coding round14 challenges with tests that run in your browser.
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Latest notes fresh off the pen ✎
All 19 notes →Closures, Scope & the Classic setTimeout Loop
What lexical scope and closures really are, why the var + setTimeout loop prints 3 3 3, three ways to fix it, and where closures earn their keep in real code.
~6 min readread →
TypeScript Generics, Explained Through Interview Questions
Ten generics questions, from constraints and keyof to infer, distributive conditional types and variance, each with a short answer and code you can reproduce.
~6 min readread →
The Event Loop: Microtasks vs Macrotasks
How the call stack, task queue and microtask queue fit together, where rendering happens, how async/await schedules work, and output puzzles with answers.
~7 min readread →
useEffect Pitfalls Interviewers Love to Ask
Stale closures, missing cleanup, fetch races, Strict Mode's double run, infinite loops and effects you don't need, plus how useLayoutEffect differs in timing.
~7 min readread →
The this Keyword: Four Binding Rules and One Exception
How JavaScript decides what this means: default, implicit, explicit and new binding, which one wins, why arrow functions ignore them, and the classic bugs.
~6 min readread →
Tear-off cheat sheets print & stick on your wall
All cheat sheets →JavaScript Essentials
48 must-know facts & snippets
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TypeScriptFrom unknown to infer, on one page
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TypeScript Utility TypesPartial → Awaited, all of them
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ReactRendering to Server Components, on one page
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React HooksEvery hook, one example each
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AngularSignals to guards, standalone-first
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RxJS OperatorsThe 30 operators you'll actually use
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HTML & CSSSemantics to grid, cascade to stacking
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Node.jsEvent loop to streams, checked on Node 22
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JavaOOP to virtual threads, on one page
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Spring BootBeans to JWT and @Transactional, one page
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PythonCore Python to asyncio, on one page
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MicroservicesDDD to sagas, retries and tracing
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SQL & PostgreSQLJoins to MVCC, every query run on Postgres 16
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MongoDBBSON to sharding, with the defaults that matter
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DockerDockerfile to Compose, layers to PID 1
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KubernetesPods to Helm, plus a troubleshooting table
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AWSEC2 to IAM to DR, the services that come up
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Git & CI/CDStaging area to canary deploys, on one page
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ServiceNowGlideRecord to update sets, on one page
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Data Structures & AlgorithmsBig-O to DP, with Python templates
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System DesignRendering to caching, plus five mini-designs
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A note in your inbox, every Sunday.
One tricky interview question, fully explained. Takes 5 minutes to read with your coffee.