You start with one useState . Then another. Then a third. Before you know it, a single event handler calls four setters in sequence and you've lost track of which combination of state values is valid. Sound familiar? 👉 Try it in practice: Calculator with reducer React gives us two built-in tools for component state: useState for independent values, and useReducer for grouped state transitions. The problem is that nobody tells you where the line is. You only notice you crossed it when the code starts hurting. Here's how to recognize that moment — and what to do about it. The smell: state that moves together The clearest signal is when a single user action needs to update multiple state values at once.…