[React Testing] Test componentDidCatch handler Error Boundaries

Error boundary:

import React from 'react'
import { reportError } from './components/extra/api'

export default class ErrorBoundary extends React.Component {
  constructor(props) {
    super(props)
    this.state = { hasError: false }
  }

  static defaultProps = {
    fallback: <h1>Something went wrong.</h1>,
  }

  static getDerivedStateFromError(error) {
    return { hasError: true }
  }

  componentDidCatch(error, errorInfo) {
    console.log(error, errorInfo)
    reportError(error, errorInfo)
  }

  render() {
    if (this.state.hasError) {
      return this.props.fallback
    }

    return this.props.children
  }
}

What we want to test is 'reportError' was called when error happens

Test:

import React from 'react'
import { render, fireEvent } from '@testing-library/react'
import { ErrorBoundary } from './error-boundary'
import { reportError as mockReportError } from './components/extra/api'

function Bomb(shouldThrow) {
  if (shouldThrow) {
    throw new Error('Bomb')
  } else {
    return null
  }
}

jest.mock('./components/extra/api')

test('calls reportError and renders that there was a problem', () => {
  mockReportError.mockResolvedValueOnce({ success: true })
  const { rerender } = render(
    <ErrorBoundary>
      <Bomb />
    </ErrorBoundary>,
  )

  rerender(
    <ErrorBoundary>
      <Bomb shouldThrow={true} />
    </ErrorBoundary>,
  )

  const error = expect.any(Error)
  const errorInfo = { componentStack: expect.stringContaining('Bomb') }
  expect(mockReportError).toHaveBeenCalledWith(error, errorInfo)
  expect(mockReportError).toHaveBeenCalledTimes(1)
})

// Clearn the mock impl
afterEach(() => {
  jest.clearAllMocks()
})

Notice:

  const error = expect.any(Error)
  const errorInfo = { componentStack: expect.stringContaining('Bomb') }

Both uses 'expect' static methods.

expect.any(): https://jestjs.io/docs/en/expect#expectanyconstructor

expect.stirngContiaining(): https://jestjs.io/docs/en/expect#expectstringcontainingstring

In the testin, we mock the whole 'api' module with jest.fn(), just provide the mock implementation for 'reportError':

mockReportError.mockResolvedValueOnce({ success: true })

Remember to claer the mock Implmentation after each test:

afterEach(() => {
  jest.clearAllMocks()
})