JavaScript for C#/Java developers — Mission

Version studied

Node.js 24.21.0, the current long-term support release (“Krypton”, LTS since October 2025), with its V8 engine 13.6 and npm 11.19. The language itself is ECMAScript, the standard that browsers and Node.js implement. Every example of this course is in code/javascript-for-csharp-java, next to its expected output. .github/workflows/javascript-examples.yml installs Node.js 24.21.0, .NET 10 and Java 25 on Linux, Windows and macOS, runs every example, error snippet and solution, and compares the outputs with the ones pasted in the lessons.

Why I’m learning this

I write C# and read a lot of Java, and I write JavaScript when I have to: a build script, a component of Guitar Alchemist’s front end, the configuration of this site. Each time, the syntax looks familiar enough that I stop paying attention, and then something surprises me: a 0 that becomes 0.5, a method that loses its object, a copy that isn’t one. I want to learn the rules behind those surprises, from the specification and from Node.js itself, instead of collecting them one bug at a time.

Who this course is for

You are comfortable with C# or Java: classes, interfaces, generics, exceptions, collections, LINQ or streams. You have read or written some JavaScript, and you have been surprised by it. Each lesson starts from what you already know, shows where JavaScript agrees and where it differs, and prints what Node.js says, including its errors.

This course is the base of two others. Static types belong to the coming TypeScript for C#/Java developers course, so this one only mentions TypeScript where it changes nothing at run time. The DOM, JSX and components belong to the React (Vite) course that follows TypeScript, so this course runs JavaScript in Node.js and only looks at the browser as another host.

JavaScript in one table

	C#	Java	JavaScript
Specification	C# language specification	Java Language Specification	ECMAScript, a new edition every June
Runs on	the CLR, after compilation to IL	the JVM, after compilation to bytecode	an engine that parses the source: V8 in Chrome and Node.js, SpiderMonkey in Firefox, JavaScriptCore in Safari
Types	static	static	dynamic: values have types, variables don’t
Numbers	int, long, double, decimal…	int, long, double…	number (a double) and bigint
Absent value	null	null	undefined and null
Objects	instances of classes	instances of classes	property bags linked to a prototype; class builds on that
Package manager	NuGet	Maven, Gradle	npm (or pnpm, Yarn)
Project file	.csproj	pom.xml, build.gradle	package.json
Modules	assemblies and namespaces	packages and modules	one module per file, ES modules or CommonJS

The data

The lessons use real code as material. Where a lesson quotes GuitarAlchemist/ga, it is at commit a826864, in the React front ends ReactComponents/ga-react-components and Apps/ga-client. Those files are TypeScript; the lessons reduce the lines they quote to plain JavaScript, which is what runs once the types are removed. This site, built with Astro, is the other real JavaScript project the lessons look at.

By the end of this course, I will be able to

• install Node.js on Windows, Linux and macOS, run a file, and manage a project with npm;
• say whether a file is an ES module or a CommonJS module, and make the two work together;
• predict what ==, +, || and if do with any pair of values, and choose the operator that doesn’t surprise;
• say what this is in any call, and keep it in a callback;
• use prototypes and classes, and copy and compare objects on purpose;
• write asynchronous code with promises and async/await, and explain the event loop;
• test, debug and publish a Node.js package.

Outline

#	Lesson	You already know
1	Node.js, npm and modules	dotnet run, NuGet, java, Maven, namespaces
2	Values and types	var, readonly/final, double, null, Equals
3	Functions, scope, closures and this	methods, lambdas, delegates, captured variables
4	Objects, prototypes and classes	classes, inheritance, properties, records
5	Arrays, iteration and collections (coming next)	List<T>, LINQ, streams, Dictionary, HashMap, IEnumerable, iterators
6	Errors and exceptions	try/catch/finally, exception types, using and try-with-resources
7	Asynchronous JavaScript: the event loop, promises, async/await	Task, async/await, CompletableFuture
8	The Node.js standard library: files, paths, streams and processes	System.IO, java.nio.file, Process
9	Testing and debugging: node:test, the inspector, ESLint	xUnit, JUnit, the Visual Studio and IntelliJ debuggers, analyzers
10	npm in depth: versions, lock files, workspaces, exports and publishing	NuGet and Maven Central
11	Text, regular expressions, dates and Intl	Regex, DateTime, java.time, CultureInfo
12	From Node.js to the browser: script modules, fetch and bundlers	ASP.NET static files, a front-end build
—	Journal

Resources

• ECMAScript® Language Specification, the living draft; the editions published by Ecma
• MDN — JavaScript reference and JavaScript guide
• Node.js 24 documentation
• npm documentation
• Node.js release schedule