When you have a lot of codes, you need to split them into separate files called modules.
Simply put, a JavaScript module is a file which contains some javascript codes that can be reused.
To make those code reusable, the module needs to declare what to be exported from the module, and on the other hand, the codes that depend the module need to declare what to be imported from the module.
It's a set of conventions to define:
- How to export functionalities from a module
- How to import functionalities from a module
- When loading the dependencies, how to load them (e.g. are they loaded in the same order as the
importorrequirestatements)
In ES5, there are two popular modular systems:
- AMD, which is adopted by the browsers
- CommonJS, which is adopted by Node.JS
In ES6, there is a module system defined to cover both browser environment and Node.JS environment, which is usually referred as ESM (ES6 Module).
For more details, read The mind-boggling universe of JavaScript Module strategies