04 / LANGUAGES · JAVASCRIPT
JavaScript
Every file runs: `node <file>` prints one `ok:` line, or throws at the exact assertion that is false on your Node version. Edit, break, re-run.
01Arraysbuild / copy / slice vs splice / 2D-grid trap / workhorse methods / unique-count-group-zip-chunk / O(1) queue / holes02Sorting (comparators)comparator contract, lexicographic trap, multi-key tie-breakers, localeCompare, stability, top-N, derived keys03Stringsslicing, split/join, char codes, replace/replaceAll/regex, compare, templates, palindromes/anagrams/compression, unicode04Map & Setcounting idiom, grouping, Set algebra, compound-key trap, LRU via Map order, object-key pitfalls, WeakMap05Numbers & Mathdivision/modulo, parsing, toFixed/round, floating point, BigInt, Math grab-bag, gcd/lcm/primes/digits, bitwise06Objects & destructuringdestructuring forms, `?.` / `??` / `??=`, spread & immutable updates, pick/mapValues, JSON edge cases, freeze, getters07Classes & thisfull class anatomy, extends/super, the four `this` rules, bind/call/apply, prototypes without `class`, mixins, abstract-ish08Closures, scope & hoistingvar/let/const/TDZ, the for-loop closure, hoisting, counters/once/memoize, curry/partial/pipe, debounce/throttle, IIFE09Async & Promisesevent-loop order, all/allSettled/race/any, sequential vs concurrent, concurrency pool, retry+backoff, timeout/AbortController10Regextest/match/matchAll, replace with fns, greedy vs lazy, lookaround, backrefs, escaping, `/g` lastIndex trap, validators, tokenizer11Equality & coercion gotchas`==` table, Object.is, typeof, truthy/falsy, `+` coercion, comparison coercion, the classic trivia, safe patterns12Errors, generators & misccustom errors with cause, try/finally semantics, iterator protocol, generators, Symbols, Dates, CJS vs ESM13Industry-coding project skeletons: in-memory DB + banking systemCodeSignal "Industry Coding" shapes: in-memory DB (TTL, backup/restore) + banking system (ranking, scheduled payments, merge) in Practice