Testing

Components using Trans or useLingui require access to the context of I18nProvider. How you can wrap your component with the I18nProvider depends on the test library you use.

Here is a working example with react-testing-library, using the wrapper-property:

index.js

 import React from 'react'
 import { getByText, render, act } from '@testing-library/react'
 import { i18n } from '@lingui/core'
 import { I18nProvider } from '@lingui/react'

 import { messages } from './locales/en/messages'
 import { messages as csMessages } from './locales/cs/messages'
 import App from './App'

 i18n.load({
   en: messages,
   cs: csMessages
 })

 const TestingProvider = ({ children }: any) => (
   <I18nProvider i18n={i18n}>
     {children}
   </I18nProvider>
 )

 test('Content should be translated correctly in English' , () => {
   act(() => {
     i18n.activate('en')
   })
   const { getByTestId, container } = render(<App />, { wrapper: TestingProvider });
   expect(getByTestId('h3-title')).toBeInTheDocument()
   expect(getByText(container, "Language switcher example:")).toBeDefined()
 });

 test('Content should be translated correctly in Czech', () => {
   act(() => {
     i18n.activate('cs')
   })
   const { getByTestId, container } = render(<App />, { wrapper: TestingProvider });
   expect(getByTestId('h3-title')).toBeInTheDocument()
   expect(getByText(container, "Příklad přepínače jazyků:")).toBeDefined()
 });

You could define a custom renderer to re-use this TestingProvider, see react testing library - Custom Render