old_contents: import { expect } from 'chai'
  import types from '../../src/constants/actionTypes'
  import { onClickMenuButton } from '../../src/actions/nav'
  
  describe('actions/nav.js', () => {
    it('should toggle sidebar menu when click on menu button', () => {
      const isExpanded = false 
      
      expect(onClickMenuButton(isExpanded)).to.deep.equal(Object.assign({}, {
        type: types.ON_CLICK_MENU_BUTTON,
        isExpanded: ! isExpanded
      }))
    })
  })
new_contents: import { expect } from 'chai'
  import types from '../../src/constants/actionTypes'
  import { onClickMenuButton } from '../../src/actions/nav'
  
  describe('actions/nav.js', () => {
    it('should toggle sidebar menu when click on menu button', () => {
      const isExpanded = false 
      
      expect(onClickMenuButton(isExpanded)).to.deep.equal({
        type: types.ON_CLICK_MENU_BUTTON,
        isExpanded: ! isExpanded
      })
    })
  })
subject: Change from immutable > mutable object