Lesson 32: Installing Node and Running Scripts

32.1 JavaScript on Your Own Computer

So far, your JavaScript has always run inside something else: Google's servers, a web page, Excel, Airtable, Obsidian. Each gave your code a particular set of tools, and kept it away from everything else. Node.js runs JavaScript directly on your own computer, as an ordinary program. With it, your code can do what the others couldn't: read and write any file you can, process folders full of data, run commands, and, in the next part of the book, act as a web server.

The language is the same. let, if, loops, functions, objects, array methods, async and await all work exactly as you know them. What changes are the tools around it: there's no document, because there's no web page, and no SpreadsheetApp. Instead, Node has its own built-in tools for files, folders and the network, which the next few lessons cover.

The club has plenty of work for it: sorting a folder of garden photos, combining harvest logs exported from several places, analyzing a season of data, and building small tools Maya can run with one command.

32.2 Installing Node

You'll need a computer where you can install software: Windows, Mac or Linux. (A Chromebook or school-managed computer may not allow it. Check with your school, or use a free online environment such as GitHub Codespaces.) Here's the question, in a new chat with your learner profile, updated for Node.js as shown at the end of this lesson:

The steps are right, including the most important choice: the LTS version, which stands for Long Term Support and is the stable one. But notice the version numbers in the examples: v20. Node releases a new version every six months, and by the time you read this, the current LTS will be a larger number. The assistant is remembering what was current when it learned, not looking. When instructions include version numbers, take the number from the official site, nodejs.org, not from the assistant.

On a Mac, the installer on nodejs.org works the same way.

32.3 The Terminal

A terminal is a window where you type commands instead of clicking. It's how you run Node, and it's worth getting comfortable with, because many programming tools work this way. On Windows, the terminal is PowerShell or Command Prompt; on a Mac, it's the Terminal app.

The key idea is that a terminal is always in a folder, called the current working directory. The prompt usually shows it, something like C:\Users\maya> on Windows or maya@laptop ~ % on a Mac, where ~ means your home folder. Commands work relative to that folder, the same way relative paths worked for web pages in Lesson 24. Publishing a Site for Free.

A handful of commands covers most of what you'll need:

To do this	Windows (PowerShell)	Mac
See which folder you're in	pwd (or look at the prompt)	pwd
List what's in the folder	dir (or ls)	ls
Go into a folder	cd Documents	cd Documents
Go up one folder	cd ..	cd ..
Clear the screen	cls (or clear)	clear

Two shortcuts save a lot of typing: press Tab to finish a file or folder name you've started typing, and press the up arrow to bring back your last command.

32.4 Visual Studio Code

You can write JavaScript files in any text editor, but a code editor makes it much easier. Visual Studio Code, usually called VS Code, is free from code.visualstudio.com, and it's what most JavaScript programmers use. It colors your code, points out some mistakes as you type, and has a terminal built in.

The way to work in VS Code:

1. Make a folder for a project, such as garden-scripts in your Documents folder.
2. In VS Code, choose File, then Open Folder, and open that folder.
3. Create files with the New File button in the Explorer panel on the left.
4. Open a terminal with Terminal, then New Terminal. It opens at the bottom of the window, already in your project folder.

That last point matters more than it seems, as you're about to see.

32.5 Your First Script

Here's a first script, in the same chat:

Read. It's plain JavaScript, using the Date methods from Lesson 14. Email, Calendar, and Dates. The reasoning in the explanation is careful: getDay() is 0 to 6, so 6 - dayOfWeek can't be negative, and on a Saturday the answer is 0. You can run the same code right here, since it doesn't use anything specific to Node:

const today = new Date()
const dayOfWeek = today.getDay()
const daysUntilSaturday = 6 - dayOfWeek
console.log(`Days until the next garden club workday: ${daysUntilSaturday}`)

Now run it with Node. Suppose you save it as countdown.js in your Documents folder, open a terminal from the Start menu, and type node countdown.js:

That's exactly right, and the error message is more helpful than it looks: it shows the full path where Node looked, C:\Users\maya\countdown.js, which tells you the terminal was in C:\Users\maya, not in Documents. There are two fixes:

• Move the terminal to the file: type cd Documents, then node countdown.js again.
• Work in VS Code with the folder open, so the terminal starts in the right place.

This is the most common problem people have when they start with Node. When Node can't find a file, look at the path in the message, and compare it with where the file really is.

When it works, the terminal shows the script's console.log output:

Days until the next garden club workday: 3

32.6 Errors in Node

When a script has an error, Node stops and prints it in the terminal, with more detail than you've seen elsewhere. Try it: add a typo, such as today.getDya(), and run the script again. You'll see something like:

C:\Users\maya\Documents\garden-scripts\countdown.js:3
const dayOfWeek = today.getDya()
                        ^

TypeError: today.getDya is not a function
    at Object.<anonymous> (C:\Users\maya\Documents\garden-scripts\countdown.js:3:25)

The first line gives the file and the line number, 3. Then it shows the line, with a ^ pointing at the problem, and the error itself, a TypeError you'd recognize from any platform. The lines starting with at are the stack trace, which lists where the error happened and what called it. For now, the first line of it, with your file's name, is the useful one.

Two more things to know about running scripts:

• Stopping a script. If a script runs longer than you want, perhaps because of an infinite loop from Lesson 8. Loops and Repetition, press Ctrl+C in the terminal. It works on a Mac too, not Cmd+C.
• Browser code doesn't all work. Code that uses document or alert fails in Node with an error like ReferenceError: document is not defined, because there's no page. If you see that, the code was written for a browser.

32.7 Your Learner Profile

The environment line is the only change to your rules. Everything else you've learned comes along, which will matter in the next lessons: Node code uses modern JavaScript, including array methods and async and await, all of which you know.

32.8 Summary

Node.js runs JavaScript on your own computer, with the same language you know and different built-in tools: no document, but access to files, folders and the network. Install the LTS version from nodejs.org, taking the version number from the site rather than from an assistant's memory. A terminal is always in a folder, the current working directory, and commands like cd, ls or dir, and pwd let you move around and look. node filename.js runs a file, which Node looks for starting from the current working directory, so "Cannot find module" usually means the terminal is in the wrong folder. VS Code, with a project folder open, gives you an editor and a terminal that starts in the right place. Next, you'll split your code into modules, and use code other people have published.