Ultimately every UI is a collection of various (often contradictory) states. Example: Even typing into a text box on Hacker News is a new UI state: you have a

Ultimately every UI is a collection of various (often contradictory) states. Example: Even typing into a text box on Hacker News is a new UI state: you have a different set of shortcuts, the keyboard works slightly differently, the browser has to keep track of the input if you suddenly press back and then forward again (some browsers maintain input in these cases), undo has to work within the scope of the textbox etc. The more complex your UI becomes, the more contradictory states become, and need to be tracked. We're posting something over XHR/WebSockets? X amount of elements on the page have to be disabled, some state has to updated with save progress etc. We are logged in/logged out? We need to show a different state. There's an error in some part of the app? We need to block user from doing something and show errors etc. If you can split those into actual states and allow interface and actions based on the current state, it becomes easier to reason about what is going on ini your app