基本減速機

基本的減速器看起來像這樣：

// Import the action types to recognise them
import { ACTION_ERROR, ACTION_ENTITIES_LOADED, ACTION_ENTITY_CREATED } from './actions';

// Set up a default state
const initialState = {
    error: undefined,
    entities: []
};

// If no state is provided, we take the default state
export default (state = initilState, action) => {

    // Based on the type of action received, we calculate the new state
    switch(action.type) {

        // Set the error
        case ACTION_ERROR:
            // Note that we create a new object, copy the current state into it,
            // and then we make the relevant changes, so we don't mutate the state
            return Object.assign({}, state, { error: action.error });

        // Unset any error, and load the entitites
        case ACTION_ENTITIES_LOADED:
            return Object.assign({}, state, { 
                entities: action.entities,
                error: undefined
            };
        
        // Add only one entity. Again, note how we make a new entities array
        // combining the previous one with the new entity
        // instead of directly modifying it, so we don't mutate the state.
        case ACTION_ENTITY_CREATED:
            return Object.assign({}, state, {
                entities: [action.entity].concat(state.entities)
            }):

        // If the action is not relevant to this reducer, just return the state
        // as it was before.
        default:
            return state;
    }
};