Set up your computer

What is JavaScript?

JavaScript is a programming language. It was made in 1995 to make web pages react to clicks, and today it runs almost everywhere: in every browser, on servers, in phone apps and in tools you use every day.

In How programs run you saw that a language needs an engine and a runtime around it. In the browser, the browser is the runtime. On your computer and on servers, the runtime is Node.js. Node.js is what turns JavaScript into a language for backends, and ZudoJS needs Node.js 24 or newer.

You can already run JavaScript on these pages. Press Run in browser:

console.log("Hello from Node.js!");

Hello from Node.js!

console.log prints whatever you give it. It is the first tool every JavaScript developer reaches for when they want to see what their code is doing. The browser terminal is for trying things quickly. The real backend will run on your computer, so the rest of this lesson installs Node.js there.

Install Node.js

Open a terminal first. If you are not sure how, Your developer environment shows you on Windows, macOS and Linux.

1. Go to nodejs.org/en/download.
2. Download the LTS version for your system. LTS means long-term support: the stable version most companies run. It must be version 24 or newer.
3. Run the installer and accept the defaults.
4. Close your terminal and open a new one, so it finds the new program.

Now check that it worked. --version asks a program which version it is:

$ node --version
v24.19.0
$ npm --version
11.19.0

Your numbers can be higher. What matters is that node --version starts with v24 or more. npm comes with Node.js. It installs packages, which are pieces of code other people published, such as ZudoJS.

Try JavaScript in the REPL

Type node on its own and press Enter. Node.js starts the REPL: Read, Evaluate, Print, Loop. It reads one line of JavaScript, runs it, prints the result, and waits for the next. The prompt changes to >. Here is a real session:

$ node
Welcome to Node.js v24.19.0.
Type ".help" for more information.
> 1 + 2
3
> 10 * 3 - 4
26
> "Buy " + "milk"
'Buy milk'
> const tasks = ["Buy milk", "Write report"]
undefined
> tasks.length
2
> console.log("Hello from the REPL")
Hello from the REPL
undefined
> .exit

• Each line after > is what was typed. The line under it is the result.
• 'Buy milk' is text, so the REPL shows it in quotes.
• const tasks = … stores a list under the name tasks. Storing a value produces no result, so the REPL prints undefined, which means "no value". The next line uses the stored list: it has 2 items.
• console.log printed its text, then the REPL printed undefined because console.log itself gives back no value.
• .exit leaves the REPL. Pressing Ctrl + C twice, or Ctrl + D, does the same.

The REPL is great for a quick question, like "what does this give?". Everything you type is lost when you leave it. For real programs, you write files.

Run your first program on your computer

Make a folder for the course and go into it:

$ mkdir hello-zudo
$ cd hello-zudo

Open the folder in your editor with code ., or with File → Open Folder. Create a file called hello.js with the same line as before:

console.log("Hello from Node.js!");

Save it, then give the file to Node.js:

$ node hello.js
Hello from Node.js!

node hello.js means "run the file hello.js in the current folder". If the file is in another folder, give its path, such as node src/app.js. When you change the file, save it and run the same command again. Press Up to get the command back.

You just ran JavaScript outside a browser. That is all a backend is at the start: a JavaScript file that Node.js runs.

Turn the folder into a project

Every Node.js project has a file called package.json. It records the project's name and the packages it uses. npm can write one for you:

$ npm init -y
Wrote to ~/hello-zudo/package.json:

{
  "name": "hello-zudo",
  "version": "1.0.0",
  "description": "",
  "main": "hello.js",
  "scripts": {
    "test": "echo \"Error: no test specified\" && exit 1"
  },
  "keywords": [],
  "author": "",
  "license": "ISC",
  "type": "commonjs"
}

On your computer the first line shows the full path of your folder. Look at the last line of the file: "type": "commonjs". JavaScript has two ways of splitting code into files. CommonJS is the old one. ES modules are the modern one, and every ZudoJS package uses them. Switch the project over now:

$ npm pkg set type=module

That command prints nothing when it works. Open package.json and the last line now says "type": "module". You will learn what modules are in the lesson on modules. Every project in this course starts with these two commands.

Statements and comments

A program is a list of statements, instructions that run one after another from top to bottom. Each statement usually sits on its own line and ends with a semicolon (;).

A comment is a note for people. Node.js skips it completely. // starts a comment that runs to the end of the line. /* and */ wrap a comment over several lines. Create tasks.js:

// This program prints a short task list.
// Node.js skips every line that starts with two slashes.

console.log("My tasks for today:");
console.log("- Buy milk"); // a comment can also end a line
console.log("- Write report");

/*
  A comment can span several lines.
  It is a good place to explain why the code is written this way.
*/
console.log("Tasks:", 2, "Done:", 0);

My tasks for today:
- Buy milk
- Write report
Tasks: 2 Done: 0

Four console.log statements ran, so four lines were printed. None of the comments appear in the output. The last line shows that console.log can take several values separated by commas: it prints them on one line with a space between them. Text goes in quotes; numbers do not.

Run it on your computer with node tasks.js and you see the same four lines.

Your first errors

Everybody types mistakes. The trick is to read what Node.js tells you. Here are the two you will meet first.

A missing quote

This typo.js forgets the closing ":

console.log("Hello from Node.js!);

$ node typo.js
file://~/hello-zudo/typo.js:1
console.log("Hello from Node.js!);
            ^^^^^^^^^^^^^^^^^^^^^^

SyntaxError: Invalid or unexpected token
    at compileSourceTextModule (node:internal/modules/esm/utils:318:16)
…

Node.js v24.19.0

This is a syntax error, as you saw in How programs run: the file breaks JavaScript's grammar, so nothing in it runs. The first line points at the file and the line number (typo.js:1). The ^^^ marks start at the unclosed text. "Unexpected token" means "I found something here that does not belong". On your computer the first line shows the full path.

A capital letter

JavaScript is case-sensitive: console and Console are different names. This one is not a grammar mistake, so the file starts, and then fails at that line:

Console.log("Hello from Node.js!");

$ node capital.js
file://~/hello-zudo/capital.js:1
Console.log("Hello from Node.js!");
^

ReferenceError: Console is not defined
    at file://~/hello-zudo/capital.js:1:1
…

Node.js v24.19.0

ReferenceError means "you used a name that does not exist". The fix is a small c. You can see the error's name and message without the crash by catching it:

try {
  Console.log("Hello from Node.js!");
} catch (error) {
  console.log(error.name + ":", error.message);
}

ReferenceError: Console is not defined

When you get an error, read three things: the file and line at the top, the error name (SyntaxError, ReferenceError, TypeError), and the message after it. Skip the at node:internal… lines: they are inside Node.js, not your code.

Practice

console.log("My name is Ada");
console.log("I want to build a task API");
console.log("Lessons in this course:", 84);

My name is Ada
I want to build a task API
Lessons in this course: 84

console.log("Minutes in a week:", 7 * 24 * 60);

Minutes in a week: 10080

console.log("Shopping list:);
Console.log("- Bread");
console.log("- Milk"

console.log("Shopping list:");
console.log("- Bread");
console.log("- Milk");

Shopping list:
- Bread
- Milk

Recap

• JavaScript is the language; Node.js is the runtime that runs it on your computer and on servers. You need Node.js 24 or newer: check with node --version.
• node alone opens the REPL for quick experiments. node file.js runs a file.
• npm init -y creates package.json, and npm pkg set type=module switches it to ES modules.
• A program is statements run top to bottom. Comments (// and /* */) are notes for people.
• Read errors in three parts: file and line, error name, message.

Next you will learn the building blocks of every program: values and variables.