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Math for Algorithms
The whole arc, from counting and indexing through to NP-completeness: 26 parts that build on each other. Every idea arrives because a real program needed it — you work it by hand on concrete numbers, then write it in JavaScript/TypeScript, then meet the identical task again in Kotlin, compiled and graded on real stdin/stdout.
It is one continuous track: inside the IDE, Previous / Next carry you lesson to lesson, all 165 in a row, with nothing to go re-find.
Coding Arena
Your interview IDE in the browser
Solve 99+ challenges with multi-file editing, live preview, terminal, and automated tests. React, TypeScript, Kotlin, DSA, and architecture — zero setup.
- Full IDE in the browser
- Automated tests (Vitest + Piston)
- React, TypeScript, and Kotlin
Recently updated & new
The courses we've worked on most lately — sort by what you're after.
React Patterns, Good Practices & Internals
Learn to reason about React: render vs commit, reconciliation, identity and Fiber, keys, state snapshots, stale closures, effects, refs, performance, async React and internals. 167 lessons built on prediction, experiments, production bugs and senior interview answers — all runnable in the IDE.
Updated Aug 9, 2026Azure: Zero to Hero
70 lessons covering Azure from cloud concepts to real-world production patterns. Portal, VMs, App Service, AKS, Cosmos DB, Key Vault — plus Entra ID/MSAL, Service Bus, Azure Artifacts, Helm, reliability/operations, and Infrastructure as Code deep dives.
Updated Jul 20, 2026Terraform: Zero to Production
44 lessons of Infrastructure as Code: HCL, providers, resources, variables, remote state, modules, multi-environment, and real-world patterns — with concrete, sanitized examples.
Updated Jul 20, 2026Coding Challenges
HackerRank-style problems. Solve in TypeScript or Kotlin, run in-browser, and get a confetti celebration when you pass all tests.
Updated Jul 20, 2026Kotlin — Coroutine drills (extra practice)
Intensive coroutine practice: launch/async, structured concurrency, cancellation & timeouts, Flow, Channels, Mutex and exception handling — short challenges with tests.
Updated Jul 15, 2026Kotlin — Reactor operator drills (extra practice)
Master Reactor operators with focused drills: map/flatMap, merge/zip, buffer/window, error handling, backpressure and schedulers — each challenge validated.
Updated Jul 15, 2026Browse by area
Find everything related to a topic in one place
Courses10
React Patterns, Good Practices & Internals
Learn to reason about React: render vs commit, reconciliation, identity and Fiber, keys, state snapshots, stale closures, effects, refs, performance, async React and internals. 167 lessons built on prediction, experiments, production bugs and senior interview answers — all runnable in the IDE.
React Course
Learn React from scratch. Components, props, hooks. Monaco editor with TypeScript/React autocomplete and live preview.
Next.js Course
Zero to hero in Next.js: App Router, Server & Client Components, data fetching & caching, Server Actions, middleware, metadata, and deployment. Monaco editor with live preview.
Vue.js Course
Zero to hero in Vue 3: reactivity, Composition API, components & slots, Vue Router, Pinia, and more. Monaco editor with live preview — real Vue running in the browser.
Svelte Course
Zero to hero in Svelte 5: runes ($state, $derived, $effect), components, transitions, stores, and more. Monaco editor with live preview — the real Svelte compiler running in the browser.
Angular Course
Zero to hero in Angular v22: standalone components, signals, control flow (@if/@for), dependency injection, services, routing, reactive forms, and more. Monaco editor with static code verification.
TypeScript Course
Learn TypeScript. Types, interfaces, unions. Monaco editor with autocomplete, run in-browser, and verify.
CSS Course
Learn CSS Flexbox. Monaco editor with live HTML+CSS preview and verify—learn by doing.
MobX: Interview Prep
24 lessons on MobX's reactivity model: observable, computed, action, autorun, React integration, store patterns, testing, and comparison with Redux/Zustand.
Apollo GraphQL
Master Apollo Client 4. Queries, mutations, cache, Suspense hooks, testing, and performance. Monaco editor with TypeScript.
Challenges3
Coding Arena
99+ interview-grade challenges in a full IDE inside your browser. React, TypeScript, Kotlin, DSA, and architecture. Multi-file editor, live preview, terminal, and automated tests.
React Challenges
Master React hooks, rendering behavior, TypeScript patterns, and performance. Solutions unlock after 10 attempts.
Apollo GraphQL Challenges
Test your Apollo Client mastery. Cache, optimistic UI, Suspense, error handling, and more. Solutions unlock after 10 attempts.
Courses27
Kotlin — The Interview Round
16 runnable challenges on the questions everyone thinks they know: == vs === and the Int cache, copy() dropping body properties, extensions that cannot override, smart casts that are refused, inline and non-local return. Predict, run, explain.
Kotlin Spring Boot — The Interview Round
7 challenges on what Spring does to your objects: self-invocation disabling @Transactional, constructor vs field injection, a stateful singleton leaking between requests, the Filter/Interceptor/Advice order, N+1, rollback rules and the lost update.
Kotlin & Coroutines Course
The most complete and up-to-date Kotlin course — 68 bilingual lessons split into Beginner, Intermediate and Advanced, each with a Visualize-tab animation and runnable challenges that execute in-browser on JetBrains' free compiler. Everything from the basics through coroutines, Flow, sequences, DSLs, generics, sealed classes and algorithms.
Kotlin — Design Patterns Masterclass
A chapter-by-chapter authored course: OOP and the five SOLID principles, the full Gang-of-Four catalogue retold in idiomatic Kotlin, and a Ride-Sharing capstone. 36 lessons · 43 challenges · animated walkthroughs.
Kotlin — Refactoring Masterclass
Refactor real code step by step: code smells, extract function, replace conditionals with polymorphism, immutability and idiomatic Kotlin patterns — with tests validating every improvement.
Design Principles & SOLID
SOLID (SRP, OCP, LSP, ISP, DIP) plus DRY, KISS, YAGNI and composition over inheritance — each principle with a bad-vs-good example, a runnable Kotlin demo, a graded exercise, and an animated class diagram in Visualize.
Kotlin Coroutines & Reactive
The big reactive-stack course: suspend + WebFlux, coroutine↔Reactor bridging (awaitBody/awaitSingle/mono{}), Flow↔Reactor, structured concurrency, dispatchers, timeouts & retries — with runnable examples in the Arena IDE.
Kotlin Persistence
Talk to SQL and Mongo from Kotlin: JDBC, jOOQ, R2DBC, Spring Data + coroutines, reactive transactions, and MongoDB change streams as a Flow — 8 lessons with annotated examples.
Jackson with Kotlin
Production JSON in Kotlin with Jackson: the Kotlin module, annotations + @field:/@get: use-site targets, polymorphism over sealed classes, custom (de)serializers, trees/streaming, and WebFlux.
Kotlin Testing
Test Kotlin/Spring services: JUnit 5, MockK (incl. coEvery for suspend), kotlinx-coroutines-test (runTest), Reactor StepVerifier, Testcontainers, and the fakes-vs-mocks strategy.
C# Course — Zero to Hero
Master C# on .NET 8, live runnable: types & control flow → loops & arrays → methods → classes, inheritance & collections → exceptions → generics → delegates & events → regex → async/await → structs, enums & records. Every chapter ends with a real capstone, and every lesson ships SVG visualizations and stdin/stdout-graded practice.
Java Masterclass — Zero to fluent
Learn Java from scratch by writing real Java in a full Monaco editor: how the JVM runs your code, types, operators, control flow, loops, and the object model. 37 lessons with visualizations and practices graded instantly in the browser (no server, zero cost).
Data Structures & Algorithms
55 lessons and 220 graded exercises: Big-O, sorting (heap sort included), binary search (and binary search on the answer), linked lists, stacks and queues, hash tables, BSTs and AVL rotations, heaps, tries, graphs with BFS, DFS, topological sort, union-find, Dijkstra, Bellman–Ford, Floyd–Warshall, strongly connected components and minimum spanning trees, dynamic programming (including on strings, and Kadane), greedy and Huffman coding, sliding windows, monotonic stacks, intervals, palindromes by centre expansion, Rabin–Karp, Floyd cycle detection, Fenwick and segment trees, Fisher–Yates shuffling, number theory and designing an LRU cache — in real Java that compiles and runs in the Arena IDE, with a Visualize tab that animates every algorithm step by step.
Data Structures & Algorithms · Kotlin
The same essential algorithms — sorting, binary search, lists, stacks, queues, hashing, BSTs and heaps — in idiomatic Kotlin that compiles and runs in the Arena IDE, with step-by-step animations in the Visualize tab.
Gradle for Kotlin
The JVM build system with the Kotlin DSL: build.gradle.kts, the Kotlin plugin, the kotlin{} extension, dependencies, repositories, tasks, version catalogs and multi-module builds — with annotated examples and a quiz per lesson.
Kotlin Deep Dive — Masterclass
A from-the-ground-up Kotlin masterclass: essentials → OOP → coroutines → flows → an advanced type system. Live runnable Kotlin 1.8.20, inline visualizations, pitfall/deeper/tip/FAQ callouts, and stdin/stdout-graded practice.
Kotlin Java InterOp
Call Java from Kotlin. Platform types, nullability, SAM conversions, getters/setters, and mapped types.
Kotlin + Spring Boot
Build real backends with Spring Boot and Kotlin: IoC/DI, REST, JPA, security, JWT, microservices and deployment. 20 chapters, 126 lessons on an embedded mini-Spring runtime.
Kotlin + Spring Boot 4
DI, coroutines, and controller/service/repo layering in Spring Boot 4, plus what no other course covers: OkHttp + Failsafe clients (retries/circuit breakers) for flaky external APIs, polyglot persistence (Postgres + Liquibase, Redis cache-aside, Mongo), fire-and-forget coroutine writes, a config-driven rules engine, and Testcontainers + MockK + Datadog metrics — 26 lessons built around a real vehicle-valuation service.
Reactive Kotlin — Reactor, Coroutines & WebFlux
The reactive Kotlin stack the way it actually ships in production, modelled on a real vehicle-inventory import service: Kotlin idioms → Project Reactor (Mono/Flux) → coroutines & interop → Spring WebFlux. Everything compiles and runs on Kotlin 1.8.20 with faithful teaching primitives — the semantics are real, the code runs.
Kotlin Path — guided roadmap
A beginner-to-advanced route through all 5 Kotlin courses, with a recommended order and your unified progress.
Java 17: Interview Prep
40 deep lessons covering Java 17 from primitives and OOP to streams, concurrency, JVM internals, and modern features (records, sealed, pattern matching, virtual threads).
Go Course — Zero to Hero
11 chapters of modern Go (1.23+), runnable inline: values & variables, control flow, structs & methods, interfaces, idiomatic errors, and the concurrency model with goroutines and channels. Every lesson ships prose, runnable examples, and instantly graded practice.
Rust Course — Zero to Hero
Ownership, borrowing, structs and enums, error handling with Option/Result, collections, traits and generics, lifetimes, closures and iterators, fearless concurrency, and testing — 16 lessons with annotated examples and a CLI capstone.
Learn Python — Zero to Hero
A full walk through modern Python — variables, types, conditionals, loops, strings, lists, dicts, functions, comprehensions and classes — all practiced in the Arena with your own in-browser IDE, live console.log, and stdin/stdout tests running server-side on CPython 3.10.
Spring Reactive Programming
Master reactive programming with Spring: Reactive Manifesto, Project Reactor, Mono & Flux, backpressure, Spring WebFlux, and reactive patterns with Kotlin.
Reactor Flux — Complete Guide
Interactive reference for all Flux operators: creation, transformation, filtering, combining, backpressure, scheduling, and more with runnable Kotlin examples.
Challenges4
Coding Arena
99+ interview-grade challenges in a full IDE inside your browser. React, TypeScript, Kotlin, DSA, and architecture. Multi-file editor, live preview, terminal, and automated tests.
Kotlin · HackerRank-style algorithms
Backend interview classics in Kotlin: Two Sum, LRU cache, token-bucket rate limiter, sliding window, KMP, circuit breaker, topological sort, Trie autocomplete, running median. Run server-side.
Kotlin — Coroutine drills (extra practice)
Intensive coroutine practice: launch/async, structured concurrency, cancellation & timeouts, Flow, Channels, Mutex and exception handling — short challenges with tests.
Kotlin — Reactor operator drills (extra practice)
Master Reactor operators with focused drills: map/flatMap, merge/zip, buffer/window, error handling, backpressure and schedulers — each challenge validated.
Games5
Kotlin Type Alchemist
Combine Kotlin ingredients in the cauldron. Learn val, null safety, collections, data class, scope functions, and DSLs.
Spring Boot Defender
Tower defense where Spring Boot annotations defend your API from exceptions. Learn @RestController, @Service, @Transactional and more.
Coroutine Conductor
Orchestrate coroutines on a visual timeline. Learn launch, async/await, withContext, Flow and more without blocking the main thread.
Reactor Pipes
Reactive pipeline puzzle. Learn Reactor operators (map, filter, take, sort, reduce) by building pipelines.
Reactor Flow Simulator
Visual marble diagram simulator. Learn multi-stream operators (merge, zip, flatMap, error handling) by building reactive pipelines.
Courses2
Machine Learning Bootcamp
9 tracks. Zero to hero — Python, statistics, classical ML, NLP, and deep learning, all running in your browser with direct links to the math course.
Mathematics for Machine Learning
Interactive visualizations covering linear algebra, calculus, probability, and ML applications
Courses2
Azure: Zero to Hero
70 lessons covering Azure from cloud concepts to real-world production patterns. Portal, VMs, App Service, AKS, Cosmos DB, Key Vault — plus Entra ID/MSAL, Service Bus, Azure Artifacts, Helm, reliability/operations, and Infrastructure as Code deep dives.
Terraform: Zero to Production
44 lessons of Infrastructure as Code: HCL, providers, resources, variables, remote state, modules, multi-environment, and real-world patterns — with concrete, sanitized examples.
Courses4
Math for Algorithms
165 lessons across 26 parts — the largest course on the site. Mathematics through code: indexes, modulo, ranges, search, recursion, graphs and complexity, all the way to NP-completeness. By hand first, then in JS/TS, then the same task again in compiled Kotlin (162 graded drills).
React Patterns, Good Practices & Internals
Learn to reason about React: render vs commit, reconciliation, identity and Fiber, keys, state snapshots, stale closures, effects, refs, performance, async React and internals. 167 lessons built on prediction, experiments, production bugs and senior interview answers — all runnable in the IDE.
Kotlin — The Interview Round
16 runnable challenges on the questions everyone thinks they know: == vs === and the Int cache, copy() dropping body properties, extensions that cannot override, smart casts that are refused, inline and non-local return. Predict, run, explain.
Kotlin Spring Boot — The Interview Round
7 challenges on what Spring does to your objects: self-invocation disabling @Transactional, constructor vs field injection, a stateful singleton leaking between requests, the Filter/Interceptor/Advice order, N+1, rollback rules and the lost update.
Challenges3
Coding Arena
99+ interview-grade challenges in a full IDE inside your browser. React, TypeScript, Kotlin, DSA, and architecture. Multi-file editor, live preview, terminal, and automated tests.
Coding Challenges
HackerRank-style problems. Solve in TypeScript or Kotlin, run in-browser, and get a confetti celebration when you pass all tests.
Multiplayer Challenges
Compete in real-time. Same challenge, split screen, first to solve wins.
Interview3
React Interviews
Prepare for React technical interviews. Questions on hooks, patterns, optimization, and React 19.
Soft Skills
40 behavioral interview questions with STAR frameworks. Communication, leadership, and teamwork.
Backend Kotlin & Spring
Staff/Principal Engineer guide. Kotlin 2.2, Spring Boot 4, JPA, Coroutines, Hexagonal Architecture, idempotency, and system design.