测试 Reducers

import counter from './counter';                                         
describe('counter reducers', () => {                                     
  it('should handle initial state', () => {                              
    expect(                                                         
      counter(undefined, {})                                             
    )                                                                    
    .toEqual(0)                                                         
  it('should handle INCREMENT_COUNTER', () => {                          
    expect(                                                         
      counter(0, {                                                       
        type: INCREMENT_COUNTER                                          
      })                                                                 
    )                                                                    
    .toEqual(1)                                                         
  });                                                                    
    expect(                                                         
      counter(1, {                                                       
        type: DECREMENT_COUNTER                                          
      })                                                                 
    )                                                                    
    .toEqual(0)                                                         
  });                                                                    
});