SharePoint Framework (SPFx) is a fantastic development model on top of (modern) SharePoint, for user interface extensibility, and it have evolved tremendously over the last year since it became general available. The framework is based on JavaScript extensibility in a controlled manner, compared to the older JavaScript injection mechanisms we used to extend (classic) SharePoint, that comes with a lot of power.
Using SharePoint Framework our JavaScript has access to the whole DOM in the browser, meaning that we can do essentially what we want with the user interface – however, of course, we shouldn’t, only certain parts of the DOM are allowed/supported for modification.