Explain the three fundamental principles of Redux and the concepts of store, actions, and reducers.

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Redux is a predictable state container based on three fundamental principles.

Principle one is single source of truth. All application state is stored in a single object called the store. This simplifies debugging because you can inspect the complete state at any time.

Principle two is that state is read-only. The only way to change state is to emit an action, an object describing what happened. You never mutate state directly.

Principle three is that changes are made with pure functions. Reducers are pure functions that take the previous state and an action, and return the new state. They never modify existing state.

The key concepts work together. The Store is the object containing application state. There is only one per application. You access state with getState, dispatch actions with dispatch, and subscribe to changes with subscribe.

Actions are objects describing what happened. They must have a type property and optionally a payload. Action creators are functions that return actions.

Reducers are pure functions that calculate the new state. They use a switch statement on action type and always return new state objects rather than mutating existing ones. Always include a default case that returns current state.

The data flow works as follows. User interacts with the interface. An action is dispatched. The reducer calculates new state. The store notifies subscribed components. The interface updates.