Lesson 1 of 5 · 15–25 minutes

Running JavaScript with Node.js, and arrays

🏠The everyday picture

Until now your JavaScript lived inside a web page, like a cook who only works in one restaurant's kitchen. Node.js gives the same cook a kitchen of their own. And an array is the cook's numbered row of jars.

📘What it really is

Node.js runs JavaScript outside the browser, straight from the terminal. In Level 3 you'll write programs that don't need a page at all. (You met Node.js in Choosing Your Tools, Lesson 1.)
Install the LTS (long-term support) version from nodejs.org. Check it worked: node --version.
Run a file with node catalogue.js. console.log now prints in the terminal.
An array is an ordered list. Each item has a position, its index, counted from 0:

const products = ["Notebook", "Pen", "Bag"];
console.log(products[0]);       // Notebook
console.log(products.length);   // 3
console.log(products[products.length - 1]);  // Bag, the last item

An array can hold anything: numbers, text, other arrays, objects. Usually every item is the same kind of thing.
const stops the variable being pointed at a different array, but the array's contents can still change.

🖱️Try it

Install Node.js (LTS) and check node --version in a new terminal.
Make a folder level-3, open it in VS Code, run git init, and create catalogue.js.
Put the code above in it and run node catalogue.js.
Add a fourth product and print the new last item.

✅Quick check

An array has 5 items. What's the index of the last one?

Show the answer

Indexes start at 0, so the last is length - 1.

Can you add an item to an array declared with const?

Show the answer

Yes. const means the name always points to the same array; its contents can still change.