demo.test.tsx

import * as React from 'react'
import { render } from '@testing-library/react'

const DemoContext = React.createContext<any>(null)

describe('Demo', () => {
  const setup = DummyComponent => {
    const update = jest.fn()
    const utils = render(
      <DemoContext.Provider value={{ update }}>
        <DummyComponent />
      </DemoContext.Provider>
    )

    return {
      update,
      ...utils
    }
  }

  it('do not call update', async () => {
    const DummyUpdate: React.FC = () => {
      return null
    }

    const { update } = setup(DummyUpdate)
    expect(update).not.toHaveBeenCalled()
  })

  it('do call update', async () => {
    const DummyUpdate: React.FC = () => {
      const demoContext = React.useContext(DemoContext)

      React.useEffect(() => {
        demoContext.update()
      })
      return null
    }

    const { update } = setup(DummyUpdate)
    expect(update).toHaveBeenCalled()
  })
})