Modules in JavaScript

Today, in Node.js, there are two ways to create and use JavaScript modules. One is historical, non-standard, and the other is standard (ESM for ECMAScript Module) and has been implemented in Node since version 12 experimentally, and since Node 16 the API is stable.

We will only look at the standard way.

In JavaScript, each file is a module if we use the ESM syntax.

Syntax

It is the import / export syntax:

index.js

import process from 'node:process'

console.log(process.env.PATH)

process is a native Node module that we import into the index.js module with the import keyword.

Default Export

In the previous example, process is the default export: there are no curly braces around it.

Here is another example of a default export:

index.js

import add from './utils.js'

console.log(add(1, 2))

utils.js

export default function add(a, b) { // `default` keyword
  return a + b
}

Named Export

Here, add is a named export: curly braces are required around it (note the absence of the default keyword in utils.js)

index.js

import { add } from './utils.js'

console.log(add(1, 2))

utils.js

export function add(a, b) { // No `default` keyword
  return a + b
}