Separation of concerns is one of the key principles in software design, and since each entity in the code has to be isolated from others, it makes sense to put them into separate files to simplify the navigation and ensure isolation.
Modern JS applications consist of modules that can have exports and imports. JS modules export some entities and may consume exported entities from other modules.
In this book, we will use the latest JS syntax with classes, arrow functions, spread operators, and so on. If you are not familiar with this syntax, you can always refer to it here: http://exploringjs.com.
The simplest JS module looks like this:
// A.js:export const noop = () => {};
