State Machines for UI: Modeling Complex Flows Without Spaghetti

Complex UI flows—multi-step forms, wizards, modals with multiple outcomes—often turn into a tangle of booleans and "if loading and not error and submitted." State machines model these as a finite set of states and transitions: only certain actions are valid in each state, and the next state is explicit. This post shows how to apply that on the frontend without a heavy library.

Why state machines help

• No invalid combinations: You can't be "loading" and "submitted" and "error" at once if the machine only allows one state at a time.
• Clear behavior: Every state has a defined set of possible transitions; the code reflects that.
• Testable: You test "in state X, event Y leads to state Z" without mocking the whole UI.
• Easier to extend: Adding a new step or outcome means adding states and transitions, not more conditionals.

A simple example: async button

Instead of loading, error, success as separate flags, model the button as a small state machine: