Modules

Modules are the primary construct which we organize our JavaScript code.

At first glance, a JavaScript module is just a JavaScript file; it arises naturally when splitting large JavaScript programs into several sections each stored in a separate file.

Additionally, modules create scope. The variables declared in a module have "file scope"; they are not accessible outside of the module. A module system provides mechanism to export any value declared in a module and import them in another module when needed. Therefore, modules are another way (in addition to Class and Closure) to create encapsulation.

For a long time, JavaScript modules were implemented via third-party libraries (not built into the language). ECMAScript 2015 (ES6) was the first time that JavaScript provided built-in modules. We will focus on ES6 modules.¹