redux-101.js

const initialState = { name: '' };

function reducer(state = initialState, action) {
  switch (action.type) {
    case 'CHANGE_NAME':
      return { ...state, name: action.value };
  }
}

const action = {
  type: 'CHANGE_NAME',
  value: 'Arnaud'
};

reducer(initialState, action);
// { name: 'Arnaud' }

function createStore(reducer) {
  let state = {};

  return {
    getState() {
      return state;
    },
    dispatch(action) {
      state = reducer(state, action);
    }
  };
}

const store = createStore(reducer);
store.dispatch(action);
store.getState();
// { name: 'Arnaud' }