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Developer interview questions & practice.

Prepare with explained interview questions and 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.

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chapters:JavaScript TypeScript React Angular HTML & CSS

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!

Try it yourself ★Explain why the closure remembers count.

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Frontend

The browser, the language and the frameworks.

JSCh. 01JavaScript

Closures, hoisting, prototypes & the event loop.

99 Qs · 54 notes0%

TSCh. 02TypeScript

Generics, narrowing, utility & mapped types.

84 Qs · 43 notes0%

ReactCh. 03React

Hooks, rendering, memoization & state.

353 Qs · 42 notes0%

AngularCh. 04Angular

Change detection, DI, signals & RxJS.

344 Qs · 40 notes0%

HTML/CSSCh. 05HTML & CSS

Semantics, the box model, layout & a11y.

85 Qs · 22 notes0%

NextCh. 24Next.js

App Router, Server Components, caching & rendering.

50 Qs · 8 notes0%

{ }BonusCoding round

Debounce, Promise.all, LRU cache… write it, run the tests.

14 challenges0 solved

Backend

Servers, services and the languages behind them.

NodeCh. 06Node.js

Event loop phases, streams & modules.

87 Qs · 37 notes0%

OOP, collections, JVM, concurrency & streams.

78 Qs · 31 notes0%

SpringCh. 08Spring Boot

IoC, auto-configuration, JPA, REST & security.

84 Qs · 40 notes0%

PythonCh. 09Python

Data model, generators, decorators & the GIL.

82 Qs · 28 notes0%

MicroCh. 10Microservices

Service design, messaging, resilience & sagas.

79 Qs · 21 notes0%

Goroutines, channels, interfaces & errors.

50 Qs · 8 notes0%

KafkaCh. 26Apache Kafka

Topics, partitions, consumer groups & delivery.

50 Qs · 8 notes0%

Databases

Relational and document stores, queries and design.

SQLCh. 11SQL & PostgreSQL

Joins, indexes, transactions & query tuning.

80 Qs · 38 notes0%

MongoCh. 12MongoDB

Documents, schema design, indexes & aggregation.

81 Qs · 20 notes0%

RedisCh. 27Redis

Data structures, caching, persistence & scaling.

50 Qs · 8 notes0%

Cloud & DevOps

Containers, orchestration, cloud and delivery.

DockerCh. 13Docker

Images, layers, Dockerfiles, volumes & networks.

82 Qs · 26 notes0%

K8sCh. 14Kubernetes

Pods, deployments, services & scaling.

78 Qs · 25 notes0%

EC2, S3, IAM, VPC, Lambda & architecture.

80 Qs · 20 notes0%

GitCh. 16Git & CI/CD

Branching, rebase vs merge, pipelines & releases.

81 Qs · 25 notes0%

Platforms

Enterprise platforms with their own interview loops.

SNOWCh. 17ServiceNow

ITSM, scripting, flows, ACLs & integrations.

79 Qs · 20 notes0%

Fundamentals

Algorithms and architecture: asked everywhere.

DSACh. 18Data Structures & Algorithms

Big-O, arrays, trees, graphs & DP patterns.

84 Qs · 21 notes0%

DesignCh. 19System Design

Feeds, autocomplete, caching & performance.

90 Qs · 32 notes0%

NetCh. 20Networking

HTTP, DNS, TCP and request troubleshooting.

19 Qs · 20 notes0%

OSCh. 21Operating Systems

Processes, threads, memory and concurrency.

19 Qs · 20 notes0%

TestingCh. 22Software Testing

Test strategy, isolation, edge cases and debugging.

19 Qs · 29 notes0%

PeopleCh. 23Behavioral & HR

STAR stories, collaboration and project discussions.

19 Qs · 20 notes0%

AI & Machine Learning

From classic ML to LLM apps in production.

MLCh. 28Machine Learning

Bias-variance, metrics, trees & ensembles.

50 Qs · 8 notes0%

DLCh. 29Deep Learning

Backprop, optimizers, CNNs & transformers.

50 Qs · 8 notes0%

AICh. 30AI & LLM Engineering

RAG, embeddings, agents, evals & guardrails.

50 Qs · 8 notes0%

MLOpsCh. 31MLOps

Pipelines, serving, monitoring & drift.

50 Qs · 8 notes0%

StatsCh. 32Statistics & Data Science

Probability, A/B tests & hypothesis testing.

50 Qs · 8 notes0%

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Quiz1107 multiple-choice & “what's the output?” questions, with explanations.Flashcards1529 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 ✎

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read ✓Docker · easy

Containers vs Virtual Machines: Key Differences Explained

Containers share the host kernel through namespaces and cgroups; VMs boot their own OS on a hypervisor. Compare isolation, startup, size and fit.

~6 min readread →

read ✓HTML & CSS · mid

Script async vs defer vs type=module: Loading Order Explained

When a plain script blocks parsing, why async runs in arrival order and defer in document order, and how module scripts fit, tested in Chrome.

~6 min readread →

read ✓Java · easy

Abstract Class vs Interface in Java: When to Use Each

Abstract classes share state and constructors; interfaces define capabilities a class mixes in. See what default methods and sealed types change.

~7 min readread →

read ✓Java · easy

Checked vs Unchecked Exceptions in Java

Checked exceptions must be caught or declared; unchecked ones needn't be. See the compiler rules, the lambda problem and when to use each.

~7 min readread →

read ✓Java · mid

HashMap vs ConcurrentHashMap in Java: Key Differences

HashMap isn't thread-safe, synchronizedMap locks every call, and ConcurrentHashMap offers lock-free reads and atomic updates. When to use each.

~7 min readread →

read ✓Java · easy

String vs StringBuilder vs StringBuffer in Java

String is immutable, StringBuilder is a fast mutable buffer and StringBuffer adds locks. Why += in a loop is quadratic and which one to use.

~6 min readread →

Tear-off cheat sheets print & stick on your wall

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JavaScript Essentials

48 must-know facts & snippets

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From unknown to infer, on one page

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TypeScript Utility Types

Partial → Awaited, all of them

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Rendering to Server Components, on one page

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Every hook, one example each

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Signals to guards, standalone-first

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The 30 operators you'll actually use

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Semantics to grid, cascade to stacking

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App Router, RSC, caching and actions

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Event loop to streams, checked on Node 22

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OOP to virtual threads, on one page

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Beans to JWT and @Transactional, one page

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Core Python to asyncio, on one page

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DDD to sagas, retries and tracing

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SQL & PostgreSQL

Joins to MVCC, every query run on Postgres 16

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BSON to sharding, with the defaults that matter

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Goroutines to generics, slices to context

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Dockerfile to Compose, layers to PID 1

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Pods to Helm, plus a troubleshooting table

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Data types to Cluster slots, Redis 7.4/8

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EC2 to IAM to DR, the services that come up

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Staging area to canary deploys, on one page

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Partitions to transactions, KRaft era

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GlideRecord to update sets, on one page

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Data Structures & Algorithms

Big-O to DP, with Python templates

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Rendering to caching, plus five mini-designs

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Statistics & Data Science

Bayes to CUPED, p-values to regression

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Machine Learning

Bias-variance to boosting, metrics to leakage

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Backprop to KV cache, Adam to LoRA

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AI & LLM Engineering

Tokens to agents, RAG to guardrails

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Pipelines to PSI, registry to rollback

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