Redux is a predictable state container built on three core principles: a single store, read-only state changed only via actions, and pure reducers that compute the next state. Data flows one way, making changes traceable.
1. Single source of truth — the whole app's state lives in ONE store object
2. State is read-only — you never mutate it; you DISPATCH actions to change it
3. Changes via pure reducers — (state, action) => newState, no side effects
// STORE — holds the single state tree
const store = configureStore({ reducer: rootReducer });
// ACTION — a plain object describing what happened
const action = { type: "counter/incremented", payload: 1 };
// REDUCER — pure function: given state + action, returns NEW state
function counterReducer(state = { value: 0 }, action) {
switch (action.type) {
case "counter/incremented":
return { ...state, value: state.value + action.payload }; // new object
default:
return state;
}
}
// DISPATCH — the only way to trigger a change
store.dispatch(action);
// SELECT — read state
store.getState().counter.value;
UI dispatches an ACTION → REDUCER computes new STATE → STORE updates →
subscribed components re-render → user interacts → dispatches the next action
This strict one-way flow means every state change is the result of a dispatched action processed by a pure function — fully traceable.
import { useSelector, useDispatch } from "react-redux";
function Counter() {
const count = useSelector(state => state.counter.value); // read
const dispatch = useDispatch();
return <button onClick={() => dispatch(increment())}>{count}</button>; // write
}
Predictability — every change is an explicit, logged action
Debuggability — time-travel debugging, action replay (Redux DevTools)
Testability — pure reducers are trivial to unit test
(Note: modern Redux uses Redux Toolkit, which cuts the boilerplate dramatically while keeping these concepts.)
Redux's core concepts — single store, actions, pure reducers, one-way flow — make application state predictable and traceable, which is its main value for large apps.
Understanding store/action/reducer/dispatch/select is essential for the many codebases that use Redux, and the principles (immutable updates, pure reducers, unidirectional flow) recur across nearly every state library.
The trade-off is boilerplate, which Redux Toolkit addresses while preserving these fundamentals.