javascript-notes.md

CodeNFacts · Programming Language

JavaScript

The language that runs in every browser on Earth - and, thanks to Node.js, on the server too. This page is a complete, project-ready reference: full syllabus, cheat sheets, diagrams, and the theory behind why JS works the way it does.

Chapters

Topics Covered

Diagrams

About JavaScript

One language, every corner of software

Where it's used

Frontend UI, backend services with Node.js, mobile apps (React Native), desktop apps (Electron), game engines, browser extensions, automation scripts, and increasingly AI-assisted tooling. Almost no software category is untouched by JS.

Why it endures

It's the only language that ships natively in every browser, so the frontend is never optional. Add a fast JIT-compiled runtime, an enormous npm ecosystem, and a syntax that's approachable for beginners yet powerful enough for large systems.

Core features

Dynamic typing, prototype-based objects, first-class functions, closures, and an event-driven, non-blocking model built around the event loop — this combination is what makes async code (network calls, timers, UI events) feel natural.

Where it's headed

WebAssembly for performance-critical code running alongside JS, server components blurring frontend/backend lines, edge runtimes shrinking cold-start times, and AI-assisted coding tools making JS's already-low barrier to entry even lower.

The theory worth understanding early

JavaScript is single-threaded — only one thing runs at a time on the Call Stack. What makes it feel concurrent is the event loop: long-running work (timers, network requests) is handed off to the browser or Node's C++ APIs, and only the follow-up callback returns to the Call Stack later. Understanding this one idea explains hoisting quirks, why async/await reads top-to-bottom while still being non-blocking, and why a Promise callback always runs before a setTimeout callback, even at 0ms.

Full Syllabus

Every topic, in order

What JavaScript is, where it runs, and how it went from a 10-day prototype to the language of the web.

▹High-level, interpreted, object-oriented, event-driven language
▹Powers Frontend, Backend, Mobile, Desktop, Games, APIs, Automation, AI/ML
▹Runs in the browser (Chrome, Firefox, Edge) and on the server via Node.js
▹Created by Brendan Eich in 1995, built in just 10 days
▹Renamed twice — Mocha → LiveScript → JavaScript

console.log("Hello World");
// Output: Hello World

Quick Reference

Cheat sheets

Array Methods

push()add to end

pop()remove from end

map()transform every item

filter()keep matching items

reduce()fold into one value

find()first match

includes()true / false check

sort()reorder in place

String Methods

lengthcharacter count

toUpperCase()ALL CAPS

slice(a,b)extract substring

trim()remove edge spaces

includes()contains check

split()string → array

replace()swap text

startsWith()prefix check

Comparison & Logic

==loose equality (coerces)

===strict equality

&&AND

||OR

!NOT

?? nullish coalescing

?:ternary shorthand

!!xforce to boolean

Data Types

Number10, 3.14

String"text"

Booleantrue / false

Undefineddeclared, no value

Nullexplicitly empty

BigInt123n

Symbolunique id

Object/Arraynon-primitive

Visual Notes

Diagrams & sketches

The Event Loop

Closures

Scope Chain

Keep this reference offline

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