modules.md

Modules

You may have noticed that there are two different ways of importing code between files being used in this project. The backend uses require and the frontend uses import. It is important to understand how to use both module systems.

This page provides an overview of the different systems, along with an explanation of why both are used.

Contents:

• History
• Backend - CommonJS
• Frontend - ES Modules

History

JavaScript was originally a purely frontend, browser-based language, without any module system whatsoever. As frontend scripts began to grow in size, and browser applications became more complex, there was a need to develop a way to load files (modules) from other files.

A number of different module systems were developed, called AMD, UMD and CommonJS. When Node.js was released in 2009, CommonJS was chosen as the native module system. You will recognise it as the module system you have used so far, which uses require and module.exports.

In 2015, a major update to JavaScript was released, called ES6. This included a new, definitive, standardised module system named ES Modules or (ESM for short). The syntax for this is import and export which can be found in this project's frontend.

Adoption of ES Modules has so far been slow, mainly due to the existing prevalance of CommonJS packages and the popularity of NodeJS, but it is getting better all the time. At some point soon NodeJS will switch over properly to ESM, but until then, CommonJS will remain the standard for backend code.

Backend - CommonJS

We can set the export of a file using module.exports. Each file can have one export only , which means that if we want to export multiple files, we need to put them together into an object.

Exporting one value:

module.exports = MyClass;

Which can then be imported from another file using require:

const MyClass = require("./path/to/other/file.js");

Exporting multiple values:

We can export multiple values by collecting them together into an object, and exporting that.

module.exports = {
  MyFirstClass: MyFirstClass,
  MySecondClass: MySecondClass,
  myFunction: myFunction,
  // etc...
};

Which can then be imported through destructuring:

const { MyFirstClass, myFunction } = require("./path/to/other/file.js");

Frontend - ES Modules

Instead of require and module.exports, ES Modules use import and export.

Exporting a single value

To export a single value, we use the default keyword:

export default MyClass;

and then import it with import, using from to specify the file path:

import MyClass from "./path/to/other/file.js";

Exporting multiple values

To export multiple values, we can use the export keyword more than once:

export function add (a, b){
  return a + b;
};

export function multiply (a, b){
  return a * b;
};

which can then be imported using destructuring:

import { add, multiply } from "./path/to/other/file.js";