All user actions are based on events.
The user do an action, mainly with the mouse on the screen and this action trigger an event. Events can be a right mouse click, a mouse move, a mouse over, a mouse wheel on an object on the map (notes, routes, waypoints) or on the user interface (UI), a drag and drop operation on the map or on the UI...
This event is captured by an event listener (one of the classes with the name finishing with ...EL) and the event listener call a method, mainly in one of the global objects in the core module / folder.
When the action is performed, the map and the UI have to be updated with the new/modifed/deleted data. So at the end of the procedure a new event is dispatched to the document with the data ( see the coreLib/EventDispatcher.js file ). This second event is captured by an event listener on the document (see AppLoader.#addEventsListeners ( ) method). This event listener call the appropriate methods of the classes MapEditor and UI to update the map and the UI.
Seems to be simple, but remember that there are quite 200 event listeners in the app.
Some user actions are displaying a context menu. In that case the action is stopped till the user select something in the context menu and then the action selected in the context menu executed.
All the context menus are derived from the BaseContextMenu class and are based on the Promise API.
In the same way, some user actions are displaying a dialog. The action is suspended till the user click on the ok button and then restarted.
All dialogs are derived from the BaseDialog class and are also based on the Promise API.
When the html page is loaded, some global objects are build (see in the technical documentation Global with name starting with the...).
Then the AppLoader.loadApp ( ) method is called from the file main.js. This method will first load all the json and csv files with the application configutation from the server.
When the configuration is completely launched, the method call:
Don't search. You will not find the function keyword in the application. Everything is based on classes and objects!
Arrows functions only are used when Javascript expect a function (and arrows functions are usefull to preserve the value of the this keyword).
And remember that event listeners can also be objects.