reducer-example.js

function todoReducer(state, action) {
  switch (action.type) {
    case 'ADD_TODO':
      return [action.payload, ...state];
    default:
      return state;
  }
}

const actions = [
  {
    type: 'ADD_TODO',
    payload: 'Learn Redux'
  },
  {
    type: 'ADD_TODO',
    payload: 'Learn React'
  }
];

const initialState = [];

const currentState = actions.reduce(todoReducer, initialState);

assert.equals([
  'Learn Redux',
  'Learn React'
], currentState);


const nums = [1, 2, 3];
const sum = nums.reduce((res, value) => {
  return res + value;
}, 0);