JavaScript

The complete language: types, functions, objects, async, modules, DOM.

1. 1Variables and typesconst/let, primitive types, === and handy operators.
2. 2Conditions and loopsEarly return, an object map instead of switch, for...of vs for...in.
3. 3Functions and closuresArrow functions, parameters, functions as values, closures.
4. 4Scope, hoisting and thisWhere variables live and why a method "loses" its this.
5. 5Strings, numbers and IntlEveryday methods, floating-point traps and proper formatting.
6. 6Regular expressionsPieces, quantifiers, groups, flags and when not to use regex.
7. 7Dates and timeTimestamps, UTC vs local, Date's traps, calculations and formatting with Intl.
8. 8Array methodsmap, filter, reduce, find and chaining.
9. 9Objects and destructuringShorthand, spread, destructuring, Object.entries and groupBy.
10. 10References and immutabilityValue vs reference, immutable updates, shallow vs deep copy.
11. 11Map and SetCollections of unique values and dictionaries with any type of key.
12. 12Classes and prototypesclass, private fields, getters, extends and what's under the hood.
13. 13Error handlingtry/catch/finally, throwing properly, custom errors and when not to catch anything.
14. 14Async: Promise and awaitThe event loop, async/await, running in parallel with Promise.all.
15. 15fetch, JSON and AbortControllerCorrect requests, checking res.ok, query params and cancellation.
16. 16Modules: import / exportNamed vs default, dynamic import, tree shaking and barrel files.
17. 17DOM and eventsSelecting, safe updates, events and event delegation.