In this blog post, we'll use vitest, @testing-library/svelte, and svelte-htm to test Svelte components that seemed to be hard to test. Such as two-way bindings, name slots, Context API, ...etc.

As a Svelte advocate, the fantastic DX is one of the reasons I love Svelte. While component testing is something that I think we can still improve over time.

You can find the source code here: davipon/svelte-component-test-recipes

Follow me @davipon

Setup

Let's install libraries in your SvelteKit project.

# Minimal setup
npm install -D vitest c8 @testing-library/svelte jsdom
# Companion libraries for Testing Library
npm install -D @testing-library/jest-dom  @testing-library/dom @testing-library/user-event @types/testing-library__jest-dom
# Test harness libraries
npm install -D svelte-htm svelte-fragment-component patch-package

vite.config.ts

Vitest can read your root vite.config.(js|ts) to match with the plugins and setup as your Vite app (SvelteKit), and here is my setup:

// vite.config.ts
import { sveltekit } from '@sveltejs/kit/vite';
import type { UserConfig } from 'vite';
import { configDefaults, type UserConfig as VitestConfig } from 'vitest/config';

const config: UserConfig & { test: VitestConfig['test'] } = {
  plugins: [sveltekit()],
  define: {
    // Eliminate in-source test code
    'import.meta.vitest': 'undefined'
  },
  test: {
    // jest like globals
    globals: true,
    environment: 'jsdom',
    // in-source testing
    includeSource: ['src/**/*.{js,ts,svelte}'],
    // Add @testing-library/jest-dom matchers & mocks of SvelteKit modules
    setupFiles: ['./setupTest.ts'],
    // Exclude files in c8
    coverage: {
      exclude: ['setupTest.ts']
    },
    deps: {
      // Put Svelte component here, e.g., inline: [/svelte-multiselect/, /msw/]
      inline: []
    },
    // Exclude playwright tests folder
    exclude: [...configDefaults.exclude, 'tests']
  }
};

export default config;

setupTest.ts

You may notice that there is a setupTest.ts file. We can add @testing-library/jest-dom matchers & mocks of SvelteKit there:

// setupTest.ts
/* eslint-disable @typescript-eslint/no-empty-function */
import matchers from '@testing-library/jest-dom/matchers';
import { expect, vi } from 'vitest';
import type { Navigation, Page } from '@sveltejs/kit';
import { readable } from 'svelte/store';
import * as environment from '$app/environment';
import * as navigation from '$app/navigation';
import * as stores from '$app/stores';

// Add custom jest matchers
expect.extend(matchers);

// Mock SvelteKit runtime module $app/environment
vi.mock('$app/environment', (): typeof environment => ({
  browser: false,
  dev: true,
  prerendering: false
}));

// Mock SvelteKit runtime module $app/navigation
vi.mock('$app/navigation', (): typeof navigation => ({
  afterNavigate: () => {},
  beforeNavigate: () => {},
  disableScrollHandling: () => {},
  goto: () => Promise.resolve(),
  invalidate: () => Promise.resolve(),
  invalidateAll: () => Promise.resolve(),
  prefetch: () => Promise.resolve(),
  prefetchRoutes: () => Promise.resolve()
}));

// Mock SvelteKit runtime module $app/stores
vi.mock('$app/stores', (): typeof stores => {
  const getStores: typeof stores.getStores = () => {
    const navigating = readable<Navigation | null>(null);
    const page = readable<Page>({
      url: new URL('http://localhost'),
      params: {},
      routeId: null,
      status: 200,
      error: null,
      data: {}
    });
    const updated = { subscribe: readable(false).subscribe, check: () => false };

    return { navigating, page, updated };
  };

  const page: typeof stores.page = {
    subscribe(fn) {
      return getStores().page.subscribe(fn);
    }
  };
  const navigating: typeof stores.navigating = {
    subscribe(fn) {
      return getStores().navigating.subscribe(fn);
    }
  };
  const updated: typeof stores.updated = {
    subscribe(fn) {
      return getStores().updated.subscribe(fn);
    },
    check: () => false
  };

  return {
    getStores,
    navigating,
    page,
    updated
  };
});

The @testing-library/jest-dom library provides a set of custom jest matchers that you can use to extend vitest. These will make your tests more declarative and clear to read and maintain. You can also check Common mistakes with React Testing Library #Using the wrong-assertion.

SvelteKit runtime modules like $app/stores and $app/navigation are not set until SvelteKit's start function is called, which means you won't have them in a test environment because tests are isolated.

This error occurs if we test the component which uses $app modules without mocking them:

In the context of unit testing, any small gaps in functionality can be resolved by simply mocking that module.

Caveats - svelte-htm ⚠️

This repo use patch-package to work around the issue when you pass svelte-htm inline component in a render function: New component root property may throw errors #6584

Here are the steps:

1. Change the source code in the node_modules:

// /node_modules/svelte/internal/index.mjs
- root: options.target || parent_component.$$.root
+ root: options.target || parent_component?.$$.root

1. Run npx patch-package svelte
2. Add "postinstall": "patch-package" script in your package.json

Possible solution: https://github.com/testing-library/svelte-testing-library/issues/48#issuecomment-1240163835

---

OK! The setup is ready. Let's start with a simple component test.

Testing component props

Here's our svelte component:

$lib/props/DefaultProps.svelte

<script>
  export let answer = 'a mystery';
</script>

<p>The answer is {answer}</p>

It's a simple component with only one prop answer with a default value.

Let's see how to test the default value and pass a new one:

// $lib/props/DefaultProps.test.ts
import { render, screen } from '@testing-library/svelte';
import DefaultProps from './DefaultProps.svelte';

it("doesn't pass prop", () => {
  render(DefaultProps);
  expect(screen.queryByText('The answer is a mystery')).toBeInTheDocument();
});

it('set and update prop', async () => {
  // Pass your prop to the render function
  const { component } = render(DefaultProps, { answer: 'I dunno' });

  expect(screen.queryByText('The answer is I dunno')).toBeInTheDocument();

  // Update prop using Svelte's Client-side component API
  await component.$set({ answer: 'another mystery' });
  expect(screen.queryByText('The answer is another mystery')).toBeInTheDocument();
});

Get your component props type

If you're using TypeScript, a recent release of @testing-library/svelte had improved props typing for render function:

Sometimes you may want to predefine your props before passing. We can use Svelte's native utility type ComponentProps. ComponentProps takes in a Svelte component type and gives you a type corresponding to the component’s props.

// $lib/props/DefaultProps.test.ts
import type { ComponentProps } from 'svelte';
import { render, screen } from '@testing-library/svelte';
import DefaultProps from './DefaultProps.svelte';

it('Pass predefined prop to the component', () => {
  const prop: ComponentProps<DefaultProps> = { answer: 'TypeScript!' };

  render(DefaultProps, prop);

  expect(screen.getByText('The answer is TypeScript!'));
});

I highly recommend reading this excellent post from Andrew Lester: Typing Components in Svelte.

Testing component events

The component we're going to test has a button that'll dispatch a custom event message when you click on it. It's the component from svelte.dev/tutorials/component-events.

$lib/events/ComponentEvent.svelte

<script>
  import { createEventDispatcher } from 'svelte';

  const dispatch = createEventDispatcher();

  function sayHello() {
    dispatch('message', {
      text: 'Hello!'
    });
  }
</script>

<button on:click={sayHello}> Click to say hello </button>

To test component events, we use a combination of vitest utility function vi.fn and the Svelte client-side component API component.$on. We also use @testing-library/user-event instead of the built-in fireEvent to simulate user interactions.

user-event applies workarounds and mocks the UI layer to simulate user interactions like they would happen in the browser. Check Common mistakes with React Testing Library #Not using @testing-library/user-event.

// $lib/events/ComponentEvent.test.ts
import { render, screen } from '@testing-library/svelte';
import ComponentEvent from './ComponentEvent.svelte';
import userEvent from '@testing-library/user-event';

it('Test ComponentEvent component', async () => {
  const user = userEvent.setup();

  const { component } = render(ComponentEvent);

  // Mock function
  let text = '';
  const mock = vi.fn((event) => (text = event.detail.text));
  component.$on('message', mock);

  const button = screen.getByRole('button');
  await user.click(button);

  expect(mock).toHaveBeenCalled();
  expect(text).toBe('Hello!');
});

We first create a mock function and pass it to the component.$on, so we can monitor it whenever the component dispatch a message event.

Testing the bind: directive (two-way binding)

We use Keypad.svelte from svelte.dev/tutorial/component-bindings:

$lib/bindings/Keypad.svelte

<script lang="ts">
  import { createEventDispatcher } from 'svelte';

  export let value = '';

  const dispatch = createEventDispatcher();

  const select = (num: number) => () => (value += num);
  const clear = () => (value = '');
  const submit = () => dispatch('submit');
</script>

<div class="keypad">
  <button on:click={select(1)}>1</button>
  <button on:click={select(2)}>2</button>
  <button on:click={select(3)}>3</button>
  <button on:click={select(4)}>4</button>
  <button on:click={select(5)}>5</button>
  <button on:click={select(6)}>6</button>
  <button on:click={select(7)}>7</button>
  <button on:click={select(8)}>8</button>
  <button on:click={select(9)}>9</button>

  <button disabled={!value} on:click={clear}>clear</button>
  <button on:click={select(0)}>0</button>
  <button disabled={!value} on:click={submit}>submit</button>
</div>

<style>
  .keypad {
    display: grid;
    grid-template-columns: repeat(3, 5em);
    grid-template-rows: repeat(4, 3em);
    grid-gap: 0.5em;
  }

  button {
    margin: 0;
  }
</style>

There is no programmatic interface to test bind:, use:, slots, and Context API. Instead of creating a dummy svelte component (e.g. TestHarness.svelte) to test your component, we can use svelte-htm to simplify your testing code.

Assume we use Keypad.svelte in one of the +page.svelte:

<Keypad bind:value={} on:submit={} />

To test the two-way binding of value, we use a writeable store as property value:

// $lib/bindings/Keypad.test.ts
import { get, writable } from 'svelte/store';
import { render, screen } from '@testing-library/svelte';
import html from 'svelte-htm';
import Keypad from './Keypad.svelte';
import userEvent from '@testing-library/user-event';

describe('Test Keypad component', async () => {
  const user = userEvent.setup();

  it('Test two-way binding', async () => {
    // Create a writable store to enable reactivity
    const pin = writable('');
    const mock = vi.fn();

    render(html`<${Keypad} bind:value=${pin} on:submit=${mock} />`);

    // Get Keypad button "1"
    const button1 = screen.getByText('1');
    await user.click(button1);
    await user.click(button1);
    expect(get(pin)).toBe('11');

    const submitButton = screen.getByText('submit');
    await user.click(submitButton);
    expect(mock).toHaveBeenCalled();

    const clearButton = screen.getByText('clear');
    await user.click(clearButton);
    expect(get(pin)).toBe('');
  });
});

Testing the use: directive (Svelte Actions)

We use the example from svelte.dev/tutorial/actions:

// $lib/actions/clickOutside.ts
export function clickOutside(node: HTMLElement) {
  const handleClick = (event: Event) => {
    if (!node.contains(event.target as Node)) {
      node.dispatchEvent(new CustomEvent('outclick'));
    }
  };

  document.addEventListener('click', handleClick, true);

  return {
    destroy() {
      document.removeEventListener('click', handleClick, true);
    }
  };
}

Then use it in src/routes/actions/+page.svelte:

<script lang="ts">
  import { clickOutside } from '$lib/actions/clickOutside';

  let showModal = true;
</script>

<button on:click={() => (showModal = true)}>Show Modal</button>
{#if showModal}
  <div class="box" use:clickOutside on:outclick={() => (showModal = false)}>Click outside me!</div>
{/if}

To test svelte actions, we test the function but not the component which uses the function. Be careful that we need to use use:action=${/** yourActionFunction *} in svelte-htm's tagged templates:

import { render, screen } from '@testing-library/svelte';
import html from 'svelte-htm';
import userEvent from '@testing-library/user-event';
import { clickOutside } from './clickOutside';

it('Test clickOutside svelte action', async () => {
  const user = userEvent.setup();
  const mock = vi.fn();

  render(html`
    <button>Outside the button</button>
    <button
      use:action=${clickOutside /** or (node) => yourAction(node, params) */}
      on:outclick=${mock}
    >
      Click outside me!
    </button>
  `);

  const button = screen.getByText('Outside the button');
  await user.click(button);
  expect(mock).toHaveBeenCalled();
});

Testing slots

We have four examples from svelte.dev/slots:

• Slot fallbacks
• Named slots
• Optional slots
• Slot props

Slot fallbacks

$lib/slot-fallbacks/Box.svelte

<div class="box">
  <slot>
    <em>no content was provided</em>
  </slot>
</div>

It's easy to test using svelte-htm:

// $lib/slot-fallbacks/Box.test.ts
import { render, screen } from '@testing-library/svelte';
import html from 'svelte-htm';
import Box from './Box.svelte';

describe('Test slot fallbacks', () => {
  it('Put some elements', () => {
    render(html`<${Box}><h2>Hello!</h2></${Box}>`);
    expect(screen.getByText('Hello!')).toBeInTheDocument();
  });

  it('Test slot fallback', () => {
    render(html`<${Box} />`);
    expect(screen.getByText('no content was provided')).toBeInTheDocument();
  });
});

Named slots

$lib/named-slots/ContactCard.svelte

<article class="contact-card">
  <h2>
    <slot name="name">
      <span class="missing">Unknown name</span>
    </slot>
  </h2>

  <div class="address">
    <slot name="address">
      <span class="missing">Unknown address</span>
    </slot>
  </div>

  <div class="email">
    <slot name="email">
      <span class="missing">Unknown email</span>
    </slot>
  </div>
</article>

// $lib/named-slots/ContactCard.test.ts
import { render, screen } from '@testing-library/svelte';
import html from 'svelte-htm';
import ContactCard from './ContactCard.svelte';

describe('Test name slots', () => {
  it('Only put slot "name"', () => {
    render(html`
      <${ContactCard}>
        <span slot="name"> P. Sherman </span>
      </${ContactCard}>
    `);

    // Fallbacks
    expect(screen.getByText('Unknown address')).toBeInTheDocument();
    expect(screen.getByText('Unknown email')).toBeInTheDocument();
  });
});

Optional slots ($$slot)

Check the implementation here: svelte.dev/tutorial/optional-slots or src/lib/optional-slots/*

There are two components, and both accept slots:

• Comment.svelte: Accepts any content in the slot
• Project.svelte: Check if named slot comments exists

$lib/optional-slots/Project.svelte

<script lang="ts">
  export let title: string;
  export let tasksCompleted = 0;
  export let totalTasks = 0;
</script>

<article class:has-discussion={$$slots.comments}>
  <div>
    <h2>{title}</h2>
    <p>{tasksCompleted}/{totalTasks} tasks completed</p>
  </div>
  {#if $$slots.comments}
    <div class="discussion">
      <h3>Comments</h3>
      <slot name="comments" />
    </div>
  {/if}
</article>

We can test the optional slot by checking the class name has-discussion:

import { render, screen } from '@testing-library/svelte';
import html from 'svelte-htm';
import Project from './Project.svelte';
import Comment from './Comment.svelte';

describe('Test optional slots', () => {
  it('Put Comment component in Project', () => {
  render(html`
    <${Project} title="Add TypeScript support" tasksCompleted="{25}" totalTasks="{57}">
      <div slot="comments">
        <${Comment} name="Ecma Script" postedAt=${new Date('2020-08-17T14:12:23')}>
          <p>Those interface tests are now passing.</p>
        </${Comment}>
      </div>
    </${Project}>
  `);

  const article = screen.getAllByRole('article')[0];

  expect(article).toHaveClass('has-discussion');
  });

  it('No slot in Project component', () => {
  render(html`
    <${Project} title="Update documentation" tasksCompleted="{18}" totalTasks="{21}" />
  `);

  const article = screen.getAllByRole('article')[0];

  expect(article).not.toHaveClass('has-discussion');
  expect(screen.queryByText('Comments')).not.toBeInTheDocument();
  });
});

Slot props

$lib/slot-props/Hoverable.svelte

<script lang="ts">
  let hovering: boolean;

  function enter() {
    hovering = true;
  }

  function leave() {
    hovering = false;
  }
</script>

<div on:mouseenter={enter} on:mouseleave={leave}>
  <slot {hovering} />
</div>

To test the slot prop, we can create a writable store and subscribe to the let:hovering, and use user.hover to simulate hovering, which is not viable using the native fireEvent:

import { get, writable } from 'svelte/store';
import { render, screen } from '@testing-library/svelte';
import html from 'svelte-htm';
import Hoverable from './Hoverable.svelte';
import userEvent from '@testing-library/user-event';

it('Test slot props', async () => {
  const user = userEvent.setup();
  const hovering = writable(false);

  render(html`
    <${Hoverable} let:hovering=${hovering}>
      <div data-testid="hover" class:active=${hovering}>
          <p>Hover over me!</p>
      </div>
    </${Hoverable}>
  `);

  const element = screen.getByText('Hover over me!');
  await user.hover(element);

  expect(get(hovering)).toBeTruthy();
  expect(screen.getByTestId('hover')).toHaveClass('active');
});

Testing the Context API

Here is the component that uses getContext:

$lib/contexts/ContextComponent.svelte

<script>
  import { getContext } from 'svelte';

  const userDetails = getContext('user-details');
</script>

<div>
  <div>
    <strong>User Name:</strong>
    {userDetails.username}
  </div>

  <div>
    <strong>User Login Status:</strong>
    {userDetails.islogin}
  </div>
</div>

We can create a parent component that does setContext to test the Context API. We use svelte-fragment-component, which provides some useful component lifecycle properties and a context property. In combination with svelte-htm the example could be written as:

// $lib/contexts/ContextComponent.test.ts
import { render, screen } from '@testing-library/svelte';
import html from 'svelte-htm';
import Fragment from 'svelte-fragment-component';
import ContextComponent from './ContextComponent.svelte';

it('Test Context API', () => {
  const userDetails = { username: 'abc@example.com', islogin: 'yes' };

  render(html`
    <${Fragment} context=${{ 'user-details': userDetails }}>
      <${ContextComponent}/>
    </$>
  `);

  expect(screen.getByText('abc@example.com')).toBeInTheDocument();
  expect(screen.getByText('yes')).toBeInTheDocument();
});

Testing components that use SvelteKit runtime modules ($app/*)

Sometimes your component needs to interact with SvelteKit modules like $app/stores or $app/navigation:

<script lang="ts">
  import { page } from '$app/stores';
  import logo from './svelte-logo.svg';
</script>

<header>
  <nav>
    <svg viewBox="0 0 2 3" aria-hidden="true">
      <path d="M0,0 L1,2 C1.5,3 1.5,3 2,3 L2,0 Z" />
    </svg>
    <ul>
      <li class:active={$page.url.pathname === '/'}>
        <a data-sveltekit-prefetch href="/">Home</a>
      </li>
      <li class:active={$page.url.pathname === '/about'}>
        <a data-sveltekit-prefetch href="/about">About</a>
      </li>
    </ul>
  </nav>
</header>

And a test might look like this:

import { render, screen } from '@testing-library/svelte';
import Header from './Header.svelte';

it('Render About page', () => {
  render(Header);

  const home = screen.getByText('Home');
  expect(home).toBeInTheDocument();

  const about = screen.getByText('About');
  expect(about).toBeInTheDocument();
});

To pass our test, we need to mock the SvelteKit runtime modules. (Please check the setupTest.ts section.)

Testing data fetching components using msw

Sometimes you may want to load some data when a component is mounted. We can use msw - Mock Service Worker. It will let you mock REST and GraphQL network requests, no more stubbing of fetch, axios, ...etc.

Setup msw

First, install msw: npm install -D msw

Create a folder src/mocks and three files in it:

// src/mocks/handlers.ts
import { rest } from 'msw'

// Define handlers that catch the corresponding requests and return the mock data.
// Will add handler later
export const handlers = []

// src/mocks/server.ts
import { setupServer } from 'msw/node';
import { handlers } from './handlers';
// This configures a Service Worker with the given request handlers.
export const server = setupServer(...handlers);

// src/mocks/setup.ts
import { server } from './server';

beforeAll(() => server.listen({ onUnhandledRequest: 'error' }));
afterAll(() => server.close());
afterEach(() => server.resetHandlers());

We also need to update our vite.config.ts:

- setupFiles: ['setupTest.ts'],
+ setupFiles: ['setupTest.ts', 'src/mocks/setup.ts'],
coverage: {
- exclude: ['setupTest.ts']
+ exclude: ['setupTest.ts', 'src/mocks']
},
deps: {
- inline: []
+ inline: [/msw/]
}

Mock REST API

Here is a component that will fetch posts from an external endpoint:

$lib/data-fetching/ExternalFetch.svelte

<script lang="ts">
  import axios from 'axios';

  interface Post {
    userId: number;
    id: number;
    title: string;
    body: string;
  }

  async function getRandomPost(): Promise<Post[]> {
    const { status, data } = await axios(`https://jsonplaceholder.typicode.com/posts`);

    if (status === 200) {
      return data;
    } else {
      throw new Error(data);
    }
  }
</script>

{#await getRandomPost()}
  <p>...waiting</p>
{:then posts}
  {#each posts as { id, userId, title, body }}
    <p>Post ID: {id}</p>
    <p>User ID: {userId}</p>
    <p>The title is {title}</p>
    <p>{body}</p>
  {/each}
{:catch error}
  <p style="color: red">{error.message}</p>
{/await}

I use axios here because there is an issue of msw: Unable to intercept requests on Node 18 #1388

To intercept the request during testing, we'll update src/mocks/handlers.ts like this:

// src/mocks/handlers.ts
import { rest } from 'msw';

// Mock Data
const posts = [
  {
    userId: 1,
    id: 1,
    title: 'first post title',
    body: 'first post body'
  },
  {
    userId: 2,
    id: 5,
    title: 'second post title',
    body: 'second post body'
  },
  {
    userId: 3,
    id: 6,
    title: 'third post title',
    body: 'third post body'
  }
];

// Define handlers that catch the corresponding requests and return the mock data.
export const handlers = [
  rest.get('https://jsonplaceholder.typicode.com/posts', (req, res, ctx) => {
    return res(ctx.status(200), ctx.json(posts))
  })
];

Here is the test file:

import { render, screen } from '@testing-library/svelte';
import ExternalFetch from './ExternalFetch.svelte';

it('render ExternalFetch', async () => {
  render(ExternalFetch);

  // Initial component state
  expect(screen.getByText('...waiting')).toBeInTheDocument();
  // Use await screen.find* instead of the await tick() or act()
  expect(await screen.findByText('first post body')).toBeInTheDocument();
});

Please check Common mistakes with React Testing Library #Using waitFor to wait for elements that can be queried with find* for the reason of using await screen.find*

Credits

• Unit Testing Svelte Components by Svelte Society
• [Archived] Testing and Debugging Svelte - svelte-society/recipes-mvp by swyx
• Official documents of Svelte Testing Library
• svelte-htm (& core implementations) by @sastan
• Vitest for unit testing #5285 by @benmccann

Resources

• Common mistakes with React Testing Library by Kent C. Dodds
• Design Patterns for Building Reusable Svelte Components by Eric Liu
• Typing Components in Svelte by Andrew Lester
• davipon/svelte-add-vitest - Svelte adder for Vitest
• Test Svelte Component Using Vitest & Playwright