JavaScript

The language every frontend and Node job assumes you already know — values, functions, this, and the async model that trips almost everyone up once.

Start the course Practise this with real questions

In this course

Values, types, and the equality traptypeof, truthy and falsy, and the one comparison operator to stop reaching for.7 min
Scope, hoisting, and closuresvar against let and const, what hoisting actually moves, and the loop variable trap.8 min
Arrays, objects, and the methods people actually usemap, filter and reduce over a hand-written loop, and the two ways to copy an object.8 min
this, and the four ways it gets setCall-site rules, arrow functions, and the callback that loses its this.8 min
From callbacks to async/awaitWhat a promise actually is, why await only pauses the function it is in, and the try/catch that never catches.8 min
Coercion: what +, Number() and sort do to your valuesThe conversions JavaScript performs without asking — string concatenation, Number versus parseInt, the default sort, and floating-point sums.9 min
Prototypes, classes, and what new actually doesProperty lookup walks a chain of objects. Classes are syntax over that chain — and new is four steps you can write yourself.10 min
call, apply and bindSetting this by hand — call and apply invoke now, bind returns a new function with this and leading arguments fixed for good.8 min
Closures at work: private state, once, memoize, debounce, curryThe closure patterns interviews ask you to write on the spot — a private counter, once, memoize with a sound cache key, debounce and curry.10 min
Array methods: searching, sorting, and the async loop trapfind, some and every; comparators that actually sort; non-mutating copies; and why forEach does not wait for async callbacks.10 min
Defaults, rest, and copying without surprisesWhen a destructuring default applies, how spread order decides who wins, and what freeze, JSON and structuredClone really copy.10 min
The event loop: tasks, microtasks, and why a then beats setTimeout(0)One thread, a queue of tasks, and a microtask queue that drains completely after each one. Predict the order of any mix of timers, promises and awaits.11 min
Errors: custom types, cause, finally, and failing async workThrow Error objects, subclass them so callers can tell failures apart, chain them with cause, and catch only what you can handle — sync or async.10 min
Modules: ESM and CommonJS, and what an import really gives yourequire returns a cached exports object you copy values out of; import binds to the live variable. Caching, circular dependencies, and each system's rules.11 min
Map, Set and WeakMap: when a plain object is the wrong toolWhy an object makes a poor dictionary, what Map and Set guarantee about order and keys, and what WeakMap is for. Then an LRU cache built on Map's order.10 min
Strings and regular expressions: lengths, flags and the stateful gStrings are immutable UTF-16 sequences, so length is not a character count. Regex flags, groups, greedy versus lazy, and the lastIndex trap.11 min