So far we’ve worked with a single file, index.js.

You can add a second file to the program.

Call it lib.js for example.

In this file we can write a sum function:

function sum(a, b) {
  return a + b
}

and we can add a little line to export it:

function sum(a, b) {
  return a + b
}
exports.sum = sum

Now in index.js we can require this function:

const { sum } = require('./lib.js')

and then we can invoke it like this:

const { sum } = require('./lib.js')

console.log('test')
console.log(sum(23, 2))

It works:

This is the Node.js require syntax, also called CommonJS.

A lot of programs use this, so you have to know about it, and it’s been the way to import files and modules in Node.js for years.

Lately, there’s been a new way and I want to introduce it because in most projects we’ll do, we’ll use it. It’s ES Modules, which I’ve introduced in Module 4.

Using this new way, instead of using require we’ll use import:

function sum(a, b) {
  return a + b
}

export { sum }

import { sum } from './lib.js'

console.log('test')
console.log(sum(23, 2))

It’s kind of similar to what we did before, but the syntax is different.

Now if you run the program again… it doesn’t work! ‼️

Why? Well, we need to read what the error is telling us.

If you’re new to programming, this is not the last time you’ll run into problems.

We run into problems like this all the time.

The key thing is knowing how to solve them.

See, there’s an error message: “SyntaxError: Cannot use import statement outside a module”.

Search that on Google.

You may find my article in the first few positions 😄

Click that, I’ll point you in the right direction:

So there is the solution.

We need to tell Node.js that this is a module.

How?

Create a package.json file, and add this code to it:

{
  "type": "module"
}

Things are now working because we told Node.js that this is a module and should use ES modules syntax.

Lessons in this unit:

0:	Introduction
1:	Installing Node.js on your computer
2:	How to write your first Node.js program
3:	▶︎ Importing other files
4:	Using npm to install packages
5:	Using built-in modules
6:	How to use the Node.js REPL
7:	Reading files with Node
8:	Writing files with Node
9:	Build an HTTP Server
10:	The Node Event emitter
11:	How to read environment variables from Node.js
12:	Node Buffers
13:	Node.js Streams
14:	How to use promises and await with Node.js callback-based functions
15:	Modern package managers (pnpm and Bun)