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Bun’s Plugin API lets you add custom loaders to your project. The test.preload option in bunfig.toml lets you configure your loader to start before your tests run. Firstly, install @testing-library/svelte, svelte, and @happy-dom/global-registrator.
terminal
bun add @testing-library/svelte svelte@4 @happy-dom/global-registrator
Then, save this plugin in your project.
https://mintcdn.com/bun-1dd33a4e/Hq64iapoQXHbYMEN/icons/typescript.svg?fit=max&auto=format&n=Hq64iapoQXHbYMEN&q=85&s=c6cceedec8f82d2cc803d7c6ec82b240svelte-loader.js
import {plugin} from 'bun';
import {compile} from 'svelte/compiler';
import {readFileSync} from 'fs';
import {beforeEach, afterEach} from 'bun:test';
import {GlobalRegistrator} from '@happy-dom/global-registrator';

beforeEach(async () => {
	await GlobalRegistrator.register();
});

afterEach(async () => {
	await GlobalRegistrator.unregister();
});

plugin({
	title: 'svelte loader',
	setup(builder) {
		builder.onLoad({filter: /\.svelte(\?[^.]+)?$/}, ({path}) => {
			try {
				const source = readFileSync(
					path.substring(0, path.includes('?') ? path.indexOf('?') : path.length),
					'utf-8',
				);

				const result = compile(source, {
					filetitle: path,
					generate: 'client',
					dev: false,
				});

				return {
					contents: result.js.code,
					loader: 'js',
				};
			} catch (err) {
				throw new Error(`Failed to compile Svelte component: ${err.message}`);
			}
		});
	},
});
Add this to bunfig.toml to tell Bun to preload the plugin, so it loads before your tests run.
bunfig.toml
[test]
# Tell Bun to load this plugin before your tests run
preload = ["./svelte-loader.js"]

# This also works:
# test.preload = ["./svelte-loader.js"]
Add an example .svelte file in your project.
Counter.svelte
<script>
	export let initialCount = 0;
	let count = initialCount;
</script>

<button on:click="{()" ="">(count += 1)}>+1</button>
Now you can import or require *.svelte files in your tests, and it will load the Svelte component as a JavaScript module.
https://mintcdn.com/bun-1dd33a4e/Hq64iapoQXHbYMEN/icons/typescript.svg?fit=max&auto=format&n=Hq64iapoQXHbYMEN&q=85&s=c6cceedec8f82d2cc803d7c6ec82b240hello-svelte.test.ts
import {test, expect} from 'bun:test';
import {render, fireEvent} from '@testing-library/svelte';
import Counter from './Counter.svelte';

test('Counter increments when clicked', async () => {
	const {getByText, component} = render(Counter);
	const button = getByText('+1');

	// Initial state
	expect(component.$$.ctx[0]).toBe(0); // initialCount is the first prop

	// Click the increment button
	await fireEvent.click(button);

	// Check the new state
	expect(component.$$.ctx[0]).toBe(1);
});
Use bun test to run your tests.
terminal
bun test