[React] Ensure all React useEffect Effects Run Synchronously in Tests with react-testing-library

Thanks to react-testing-library our tests are free of implementation details, so when we refactor components to hooks we generally don't need to make any changes to our tests. However, useEffectis slightly different from componentDidMount in that it's actually executed asynchronously after the render has taken place. So all of our query tests which relied on the HTTP requests being sent immediately after render are failing. Let's use the flushEffects utility from react-testing-library to ensure that the pending effect callbacks are run before we make assertions.

Component code:

import {useContext, useReducer, useEffect} from 'react'
import * as GitHub from '../../../github-client'

function Query ({query, variables, children, normalize = data => data}) {
  const client = useContext(GitHub.Context)
  const defaultState = {loaded: false, fetching: false, data: null, error: null}
  const [state, setState] = useReducer(
    (state, newState) => ({...state, ...newState}),
    defaultState)
  useEffect(() => {
    setState({fetching: true})
    client
      .request(query, variables)
      .then(res =>
        setState({
          data: normalize(res),
          error: null,
          loaded: true,
          fetching: false,
        }),
      )
      .catch(error =>
        setState({
          error,
          data: null,
          loaded: false,
          fetching: false,
        }),
      )
  }, [query, variables]) // trigger the effects when 'query' or 'variables' changes
  return children(state)
}

export default Query

Test Code:

import {render as rtlRender, wait, flushEffects} from 'react-testing-library'

import React from 'react'
import {render as rtlRender, wait, flushEffects} from 'react-testing-library'
import * as GitHubClient from '../../../../github-client'
import Query from '../query'

const fakeResponse = {fakeData: {}}
const fakeClient = {request: jest.fn(() => Promise.resolve(fakeResponse))}

beforeEach(() => {
  fakeClient.request.mockClear()
})

function renderQuery({
  client = fakeClient,
  children = jest.fn(() => null),
  query = '',
  variables = {},
  normalize,
  ...options
} = {}) {
  const props = {query, variables, children, normalize}
  const utils = rtlRender(
    <GitHubClient.Provider client={client}>
      <Query {...props} />
    </GitHubClient.Provider>,
    options,
  )
  return {
    ...utils,
    rerender: options =>
      renderQuery({
        container: utils.container,
        children,
        query,
        variables,
        normalize,
        ...options,
      }),
    client,
    query,
    variables,
    children,
  }
}

test('query makes requests to the client on mount', async () => {
  const {children, client, variables, query} = renderQuery()
  flushEffects();
  expect(children).toHaveBeenCalledTimes()
  expect(children).toHaveBeenCalledWith({
    data: null,
    error: null,
    fetching: true,
    loaded: false,
  })
  expect(client.request).toHaveBeenCalledTimes()
  expect(client.request).toHaveBeenCalledWith(query, variables)

  children.mockClear()
  await wait()

  expect(children).toHaveBeenCalledTimes()
  expect(children).toHaveBeenCalledWith({
    data: fakeResponse,
    error: null,
    fetching: false,
    loaded: true,
  })
})

test('does not request if rerendered and nothing changed', async () => {
  const {children, client, rerender} = renderQuery()
  flushEffects();
  await wait()
  children.mockClear()
  client.request.mockClear()
  rerender()
  flushEffects();
  await wait()
  expect(client.request).toHaveBeenCalledTimes()
  expect(children).toHaveBeenCalledTimes() // does still re-render children.
})

test('makes request if rerendered with new variables', async () => {
  const {client, query, rerender} = renderQuery({
    variables: {username: 'fred'},
  })
  flushEffects();
  await wait()
  client.request.mockClear()
  const newVariables = {username: 'george'}
  rerender({variables: newVariables})
  flushEffects();
  await wait()
  expect(client.request).toHaveBeenCalledTimes()
  expect(client.request).toHaveBeenCalledWith(query, newVariables)
})

test('makes request if rerendered with new query', async () => {
  const {client, variables, rerender} = renderQuery({
    query: `query neat() {}`,
  })
  flushEffects();
  await wait()
  client.request.mockClear()
  const newQuery = `query nice() {}`
  rerender({query: newQuery})
  flushEffects();
  await wait()
  expect(client.request).toHaveBeenCalledTimes()
  expect(client.request).toHaveBeenCalledWith(newQuery, variables)
})

test('normalize allows modifying data', async () => {
  const normalize = data => ({normalizedData: data})
  const {children} = renderQuery({normalize})
  flushEffects();
  await wait()
  expect(children).toHaveBeenCalledWith({
    data: {normalizedData: fakeResponse},
    error: null,
    fetching: false,
    loaded: true,
  })
})