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This example demonstrates how to use jest-preview with Svelte and Svelte Testing Library

Install Jest

This example follows Component Testing in Svelte from This Dot Labs, credit to Ignacio Falk. Please follow this tutorial if your project is not setup to use Jest yet.

Install and use Jest Preview

Install Jest Preview

npm install --save-dev jest-preview

Configure code transforms as follow:

// jest.config.js
transform: {
// ...
"^.+\\.(css|scss|sass|less)$": "jest-preview/transforms/css",
"^(?!.*\\.(js|jsx|mjs|cjs|ts|tsx|css|json)$)": "jest-preview/transforms/file",

(You might not need to configure CSS transform if you only use Svelte built-in CSS solution.)

If you use any global CSS, import in in your jest setup file. You can also opt-in to Automatic Mode by setting autoPreview to true so your failed test gets a preview automatically! 🤯

// setupTests.js
import { jestPreviewConfigure } from 'jest-preview';
import './app/styles/global.css'; // Import any global CSS

// Enable autoPreview so Jest Preview runs automatically
// whenever your test fails, without you having to do anything!
autoPreview: true,

That's it! Now you can use Jest Preview in your test. First, start Jest Preview Dashboard by


Just put debug() wherever you want to preview the UI?

import { render, fireEvent, screen } from '@testing-library/svelte';
import { debug } from 'jest-preview';
import App from '../../App.svelte';

describe('Counter Component', () => {
it('it changes count when button is clicked', async () => {
const button = screen.getByText(/Clicks:/);
expect(button.innerHTML).toBe('Clicks: 0');
expect(button.innerHTML).toBe('Clicks: 1');

+ debug();

Then run your test command (e.g: npm run test). The UI will be previewed at http://localhost:3336

You might as well add some NPM scripts for convenience:

"scripts": {
"dev": "next dev",
"build": "next build",
"start": "next start",
"lint": "next lint",
+ "test": "jest --watch",
+ "jest-preview": "jest-preview"
npm run jest-preview

npm run test

You may even install npm-run-all to simplify this further:

npm install --save-dev npm-run-all
"scripts": {
"dev": "next dev",
"build": "next build",
"start": "next start",
"lint": "next lint",
"test": "jest --watch",
"jest-preview": "jest-preview",
+ "test:preview": "npm-run-all -p test jest-preview"
npm run test:preview