counter.spec.js

import React from 'react'
import Counter from '../Counter'
import {shallow} from 'enzyme'
import { wrap } from 'module';

describe('<Counter />', () => {

  it('should start with zero', () => {
    const wrapper = shallow(<Counter />)
    expect(wrapper.state().counter).toBe(0)
  })

  it('should start counter with initial props value when defined', () => {
    const wrapper = shallow(<Counter initial={3}/>)
    expect(wrapper.state().counter).toBe(3)
  })

  it('should call componentWillReceiveProps when start with initial props value', ()=> {
    const spy = jest.spyOn(Counter.prototype, 'componentWillReceiveProps')    
    const wrapper = shallow(<Counter initial={2}/>)
    wrapper.setProps({
      initial: 1
    })
    wrapper.setProps({
      initial: 2
    })
    expect(spy).toHaveBeenCalled()
    expect(Counter.prototype.componentWillReceiveProps.mock.calls.length).toBe(2)
  })
  
  it('should get correct state after increment', () => {
    const wrapper = shallow(<Counter initial={3}/>)
    wrapper.instance().increment()
    wrapper.update()
    expect(wrapper.state().counter).toBe(4)
  })

  it('should render inside counter class', () => {
    const wrapper = shallow(<Counter/>)
    expect(wrapper.find('.counter').text()).toBe("0")
  })

  it('should call correctly on click', () => {
    const wrapper = shallow(<Counter initial={12}/>)
    expect(wrapper.find('button[name="increment"]').length).toBe(1)    
    wrapper.find('button[name="increment"]').simulate('click')
    wrapper.update()
    expect(wrapper.state().counter).toBe(13)
  })

  it('should call expected arrow function method on button click', () => {
    const wrapper = shallow(<Counter initial={2}/>)    
    wrapper.instance().reset = jest.fn()
    wrapper.instance().forceUpdate()
    wrapper.find('button[name="reset"]').simulate('click')
    expect(wrapper.instance().reset).toHaveBeenCalled()
  })
})