To understand Redux “under the hood,” let’s break down its core principles and how they work together:

1. The Store:

• Single Source of Truth:
  • The Redux store is a single JavaScript object that holds the entire application’s state.
  • This centralized store makes it easy to manage and access state from any part of the application.
• createStore(reducer, [preloadedState], [enhancer]):
  • This function creates the Redux store.
  • It takes a reducer function as its primary argument.
  • Optionally, it can take an initial state (preloadedState) and store enhancers.
• Store Methods:
  • getState(): Returns the current state of the application.
  • dispatch(action): Dispatches an action to update the state.
  • subscribe(listener): Registers a listener function that will be called¹ whenever the state changes.
  • replaceReducer(nextReducer): Replaces the current reducer with a new reducer.

2. Actions:

• Plain JavaScript Objects:
  • Actions are plain JavaScript objects that describe what happened in the application.
  • They must have a type property that indicates the type of action.²
• Action Creators:
  • Action creators are functions that create and return action objects.
  • They simplify the process of creating actions.
• dispatch(action):
  • The dispatch function sends an action to the Redux store.

3. Reducers:

• Pure Functions:
  • Reducers are pure functions that take the current state and an action as arguments and return a new state.
  • They³ must be pure, meaning they should not have side effects and should always return the same output for the same input.
• State Immutability:
  • Reducers must treat the state as immutable. Instead of modifying the existing state, they should create a new state object.
• (state, action) => newState:
  • The reducer function’s signature.
• combineReducers(reducers):
  • This function combines multiple reducers into a single root reducer.

4. The Data Flow:

1. Action Dispatch:
   • A component or other part of the application dispatches an action using store.dispatch(action).
2. Reducer Execution:
   • The Redux store calls the reducer function, passing in the current state and the dispatched action.
3. State Update:
   • The reducer creates a new state object based on the action and returns it.
4. Store Update:
   • The Redux store updates its internal state with the new state returned by the reducer.
5. Listener Notification:
   • The Redux store notifies all registered listeners (components that are subscribed to the store) that the state has changed.
6. Component Re-rendering:
   • Components that are subscribed to the store re-render to reflect the new state.

Key Concepts:

• Immutability: Redux relies heavily on immutability to ensure predictable state updates.
• Pure Functions: Reducers are pure functions, which makes them easy to test and reason about.
• Single Store: The single store simplifies state management and makes it easier to debug.
• Middleware: Middleware enhances Redux’s functionality by intercepting actions before they reach the reducer. This is used for things like asynchronous actions (e.g., Redux Thunk, Redux Saga).

In essence:

Redux provides a predictable and manageable way to handle application state by enforcing a strict data flow and relying on pure functions and immutability.