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.

bun add @testing-library/svelte svelte@4 @happy-dom/global-registrator

Then, save this plugin in your project.

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.

[test]
# Tell Bun to load this plugin before your tests run
preload = ["./svelte-loader.ts"]

# This also works:
# test.preload = ["./svelte-loader.ts"]

---

Add an example .svelte file in your project.

<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.

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.

bun test