reducers_todos.js

import { ADD_TODO } from 'constants/ActionTypes'

export const initialState = [
  {
    text: 'Use Redux',
    completed: false,
    id: 0
  }
]

export default function todos(state = initialState, action) {
  switch (action.type) {
    case ADD_TODO:
      return [
        {
          id: state.reduce((maxId, todo) => Math.max(todo.id, maxId), -1) + 1,
          completed: false,
          text: action.text
        },
        ...state
      ]
    default:
      return state
  }
}

test.js

import expect from 'expect'
import reducer from 'reducers_todos'
import { initialState as stub } from 'reducers_todos'
import * as types from 'constants/ActionTypes'

var expectedState = { ...stub }
expectedState.push({
  text: 'Run the tests',
  completed: false,
  id: 0
})

it('should handle ADD_TODO', () => {
    expect(
      reducer(initialState, {
        type: types.ADD_TODO,
        text: 'Run the tests'
      })
    ).toEqual(expectedState)
})