Cannot Use Import Statement Outside a Module: ESM vs CommonJS
Fix SyntaxError: Cannot use import statement outside a module in Node and browsers: type module, .mjs, require of ESM and __dirname in ESM.
~8 min readread →
psst… explained, not just answered
Developer interview questions & practice.
Prepare with explained interview questions and study notes for JavaScript, TypeScript, React, Angular, Node.js, Java, Spring Boot and SQL, 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.Databases
Relational and document stores, queries and design.Cloud & DevOps
Containers, orchestration, cloud and delivery.Fundamentals
Algorithms and architecture: asked everywhere.The practice room no sign-up, saved in your browser
Open the practice room →QuizMultiple-choice & “what's the output?” questions, each with an explanation.FlashcardsInterview questions as cards, with spaced repetition so they stick.Mock interviewTimed questions, model answers and a score report.Coding roundSmall coding challenges with tests that run in your browser.
Question of the day
One quiz question a day, with the reasoning behind the answer. It appears here once the page has loaded.
Good places to start picked by hand ✎
All notes →React useEffect Cleanup: When It Runs and How to Avoid Leaks
When React runs useEffect cleanup, why Strict Mode mounts twice, and how to clean up listeners, timers, sockets and fetches without leaks.
~7 min readread →
Can't bind to 'ngModel' since it isn't a known property of 'input' in Angular
Fix Angular NG8002 Can't bind to 'ngModel' since it isn't a known property of 'input': import FormsModule, then fix formControl and inputs.
~7 min readread →
TypeScript TS2345: Not Assignable to Parameter of Type 'never'
Fix TS2345 'Argument of type string is not assignable to parameter of type never' from empty arrays, function unions and exhaustive checks.
~7 min readread →
ClassNotFoundException vs NoClassDefFoundError in Java
ClassNotFoundException vs NoClassDefFoundError in Java: what each error means, why classes go missing at runtime and how to fix the classpath.
~8 min readread →
Whitelabel Error Page 404 in Spring Boot: How to Fix It
Seeing the Whitelabel Error Page with status 404 in Spring Boot 3? Check the mapping, component scan, @RestController and context path.
~7 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 →
SQL & PostgreSQLJoins to MVCC, every query run on Postgres 16
open & print →
DockerDockerfile to Compose, layers to PID 1
open & print →
Git & CI/CDStaging area to canary deploys, 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 →