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Example

Component

<script>
  export let name

  let buttonText = 'Button'

  function handleClick() {
    buttonText = 'Button Clicked'
  }
</script>

<h1>Hello {name}!</h1>

<button on:click="{handleClick}">{buttonText}</button>

Test

// NOTE: jest-dom adds handy assertions to Jest and it is recommended, but not required.
import '@testing-library/jest-dom/extend-expect'

import { render, fireEvent } from '@testing-library/svelte'

import Comp from '../Comp'

test('shows proper heading when rendered', () => {
  const { getByText } = render(Comp, { name: 'World' })

  expect(getByText('Hello World!')).toBeInTheDocument()
})

// Note: This is as an async test as we are using `fireEvent`
test('changes button text on click', async () => {
  const { getByText } = render(Comp, { name: 'World' })
  const button = getByText('Button')

  // Using await when firing events is unique to the svelte testing library because
  // we have to wait for the next `tick` so that Svelte flushes all pending state changes.
  await fireEvent.click(button)

  expect(button).toHaveTextContent('Button Clicked')
})

For additional resources, patterns and best practices about testing svelte components and other svelte features take a look at svelte-society/recipes-mvp/testing.