Scenario

Say we need to make a React component that has following requirements:

Fetch data from network when it's first mounted and show the fetched data if succeeds
Show loading indicator while fetching
Show error message if fetch fails
Show a button when it's not fetching to refetch the data when user clicks on it.

Following those requirements, we make use of the new and shiny React hooks and come up with the following component

import React, { useState, useEffect } from 'react'

const App = () => {
  const [isLoading, setIsLoading] = useState(false)
  const [lastChecked, setLastChecked] = useState(Date.now())
  const [data, setData] = useState(null)
  const [error, setError] = useState(null)

  useEffect(() => {
    setIsLoading(true)

    fetch('SOME URL')
      .then((response) => response.json())
      .then((result) => {
        setIsLoading(false)
        setData(result.results)
      })
      .catch((error) => {
        setIsLoading(false)
        setError(error)
      })
  }, [lastChecked])

  return (
    <div>
      {isLoading && <p data-testid="loading">Loading ...</p>}
      {error && <p data-testid="error">Error</p>}
      {data && <p data-testid="data">{JSON.stringify(data, null, 2)}</p>}
      {!isLoading && (
        <button data-testid="check" onClick={() => setLastChecked(Date.now())}>
          Check
        </button>
      )}
    </div>
  )
}

export default App

Testing

Now we need to make automated tests to ensure this component follows the requirements. We can use React testing library to help us write the test code.

Requirement #1: Fetch and show data on mounted

Our App component uses fetch to fetch data from network. To test this first requirement, we need to mock fetch to return successfully. Then we check if an element with "data" test ID is rendered.

test('should render data', async () => {
  // mock fetch
  global.fetch = jest.fn().mockResolvedValue({
    json: jest.fn().mockResolvedValue({ results: 'something' }),
  })

  const { getByTestId } = render(<App />)

  await wait(() => expect(getByTestId('data')).toBeTruthy())
})

Requirement #2: Show error message when fetch fails

To test this requirement, we need to mock fetch to return unsuccessfully by rejecting the promise. Then we check if an element with "error" test ID is rendered.

test('should render error', async () => {
  // mock fetch
  global.fetch = jest.fn().mockRejectedValue({
    message: 'Something',
  })

  const { getByTestId } = render(<App />)

  await wait(() => expect(getByTestId('error')).toBeTruthy())
})

Requirement #3: Show loading when fetch is running

To test this requirement, we need to mock fetch in a way that it will resolve when we tell it to. So before we tell it to resolve, we check if an element with "loading" test ID is rendered. After we tell it to resolve, we check again if an element with "loading" test ID is not rendered.

test('should render loading', async () => {
  var shouldResolve = false

  // mock fetch
  global.fetch = jest.fn().mockImplementation(() => {
    return new Promise((resolve) => {
      const waitUntilShouldResolve = () => {
        if (shouldResolve) {
          resolve({
            json: jest.fn().mockResolvedValue({ results: 'something' }),
          })
        } else {
          setImmediate(waitUntilShouldResolve)
        }
      }

      waitUntilShouldResolve()
    })
  })

  const { getByTestId, queryByTestId } = render(<App />)

  await wait(() => expect(getByTestId('loading')).toBeTruthy())

  shouldResolve = true

  await wait(() => expect(queryByTestId('loading')).toBeNull())
})

Requirement #4: Refetch when button is clicked

To test this requirement, we need to mock fetch to first reject the promise. Then we simulate the button click to trigger the re-fetch and tell the mocked fetch to resolve successfully. Then we check if an element with "data" test ID is rendered.

test('should reload on button click', async () => {
  var shouldResolve = false
  var shouldReject = true

  // mock fetch
  global.fetch = jest.fn().mockImplementation(() => {
    return new Promise((resolve, reject) => {
      const waitUntilShouldResolve = () => {
        if (shouldReject) {
          reject({ message: 'something' })
        } else if (shouldResolve) {
          resolve({
            json: jest.fn().mockResolvedValue({ results: 'something' }),
          })
        } else {
          setImmediate(waitUntilShouldResolve)
        }
      }

      waitUntilShouldResolve()
    })
  })

  const { getByTestId } = render(<App />)

  await wait(() => expect(getByTestId('error')).toBeTruthy())

  shouldResolve = true
  shouldReject = false
  const button = getByTestId('check')
  fireEvent.click(button)

  await wait(() => expect(getByTestId('data')).toBeTruthy())
})

Repo

With those tests, we can make sure that App component will always follow the requirements even if we make some changes in the future. You can checkout the complete code in this repo. Let me know what you think of this way of testing on Twitter.