Angular HTTP Error Handling: catchError and Safe Retries
Handle Angular HttpClient failures without stopping a search stream. Learn where catchError belongs, when retries are safe and how to test recovery.
~4 min readread →
psst… 2180+ questions inside
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.
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!
Try it yourself ★Explain why the closure remembers count.
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.DSACh. 18Data Structures & Algorithms
Big-O, arrays, trees, graphs & DP patterns.
DesignCh. 19System Design
Feeds, autocomplete, caching & performance.
NetCh. 20Networking
HTTP, DNS, TCP and request troubleshooting.
OSCh. 21Operating Systems
Processes, threads, memory and concurrency.
TestingCh. 22Software Testing
Test strategy, isolation, edge cases and debugging.
PeopleCh. 23Behavioral & HR
STAR stories, collaboration and project discussions.
The practice room no sign-up, saved in your browser
Open the practice room →Quiz927 multiple-choice & “what's the output?” questions, with explanations.Flashcards1259 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.
Question of the day
Loading today's question…
Latest notes fresh off the pen ✎
All 23 notes →Angular Cross-Field Validation with Reactive Forms
Build an Angular password-confirmation validator, show accessible errors and test matching, mismatching, empty and reset states in reactive forms.
~3 min readread →
Controlled vs Uncontrolled Inputs in React
Compare controlled and uncontrolled React inputs with examples, reset behavior, checkbox handling and fixes for the uncontrolled-to-controlled warning.
~3 min readread →
React Fetch Race Conditions: Cleanup and AbortController
Fix out-of-order React fetch results with cleanup and AbortController. Includes loading states, error handling and a practical interview explanation.
~3 min readread →
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 →
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 →
Tear-off cheat sheets print & stick on your wall
All cheat sheets →JavaScript Essentials
48 must-know facts & snippets
open & print →
TypeScriptFrom unknown to infer, on one page
open & print →
TypeScript Utility TypesPartial → Awaited, all of them
open & print →
ReactRendering to Server Components, on one page
open & print →
React HooksEvery hook, one example each
open & print →
AngularSignals to guards, standalone-first
open & print →
RxJS OperatorsThe 30 operators you'll actually use
open & print →
HTML & CSSSemantics to grid, cascade to stacking
open & print →
Node.jsEvent loop to streams, checked on Node 22
open & print →
JavaOOP to virtual threads, on one page
open & print →
Spring BootBeans to JWT and @Transactional, one page
open & print →
PythonCore Python to asyncio, on one page
open & print →
MicroservicesDDD to sagas, retries and tracing
open & print →
SQL & PostgreSQLJoins to MVCC, every query run on Postgres 16
open & print →
MongoDBBSON to sharding, with the defaults that matter
open & print →
DockerDockerfile to Compose, layers to PID 1
open & print →
KubernetesPods to Helm, plus a troubleshooting table
open & print →
AWSEC2 to IAM to DR, the services that come up
open & print →
Git & CI/CDStaging area to canary deploys, on one page
open & print →
ServiceNowGlideRecord to update sets, on one page
open & print →
Data Structures & AlgorithmsBig-O to DP, with Python templates
open & print →
System DesignRendering to caching, plus five mini-designs
open & print →
A note in your inbox, every Sunday.
One tricky interview question, fully explained. Takes 5 minutes to read with your coffee.