import, require, and test Svelte components with bun test

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.

svelte-loader.ts

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.ts"]

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

---

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.

hello-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