@stencil/vitest

First-class testing utilities for Stencil components, powered by Vitest.

Quick Start

1. Install

npm i --save-dev @stencil/vitest vitest

For browser testing, also install:

npm i -D @vitest/browser-playwright
# or
npm i -D @vitest/browser-webdriverio

2. Create `vitest.config.ts`

import { defineVitestConfig } from '@stencil/vitest/config';
import { playwright } from '@vitest/browser-playwright';

export default defineVitestConfig({
  stencilConfig: './stencil.config.ts',
  test: {
    projects: [
      // Unit tests - node environment for functions / logic
      {
        test: {
          name: 'unit',
          include: ['src/**/*.unit.{ts,tsx}'],
          environment: 'node',
        },
      },
      // Spec tests - via a node DOM of your choice
      {
        test: {
          name: 'spec',
          include: ['src/**/*.spec.{ts,tsx}'],
          environment: 'stencil',
          setupFiles: ['./vitest-setup.ts'],

          // Optional environment options

          // environmentOptions: {
          //   stencil: {
          //     domEnvironment: 'happy-dom' | 'jsdom' | 'mock-doc' (default)
          //                      ^^ Make sure to install relevant packages
          //   },
          // },
        },
      },
      // Browser tests
      {
        test: {
          name: 'browser',
          include: ['src/**/*.test.{ts,tsx}'],
          setupFiles: ['./vitest-setup.ts'],
          browser: {
            enabled: true,
            provider: playwright(),
            headless: true,
            instances: [{ browser: 'chromium' }],
          },
        },
      },
    ],
  },
});

refer to Vitest's documentation for all configuration options

3. Load your components

// vitest-setup.ts

// Load Stencil components.
// Adjust according to your build output of choice *
await import('./dist/test-components/test-components.esm.js');

export {};
// * Bear in mind, you may need `buildDist: true` (in your stencil.config)
// or `--prod` to use an output other than the browser lazy-loader

4. Write Tests

// src/components/my-button/my-button.spec.tsx

import { render, h, describe, it, expect } from '@stencil/vitest';

describe('my-button', () => {
  it('renders with text', async () => {
    const { root, waitForChanges } = await render(<my-button label="Click me" />);
    root.click();
    await waitForChanges();
    await expect(root).toEqualHtml(`
      <my-button class="hydrated">
        <mock:shadow-root>
          <button class="button button--secondary button--small" type="button">
            <slot></slot>
          </button>
        </mock:shadow-root>
        Small
      </my-button>
    `);
  });
});

5. Run tests

// package.json
{
  "scripts": {
    "test": "stencil-test",
    "test:watch": "stencil-test --watch",
    "test:e2e": "stencil-test --project browser",
    "test:spec": "stencil-test --project spec"
  }
}

API

Rendering

`render(VNode)`

Render a component for testing.

import { render, h } from '@stencil/vitest';

const { root, waitForChanges, setProps, unmount } = await render(<my-component name="World" />);

// Access the element
expect(root.textContent).toContain('World');

// Update props
root.name = 'Stencil';
await waitForChanges();
// or
await setProps({ name: 'Stencil' });

// Unmount component
unmount();

`waitForReady` Option

By default, render() waits for components to be fully hydrated before returning. It detects when Stencil applies the hydrated flag (class or attribute) to your component, respecting your stencil.config settings.

// Default behaviour - waits for hydration
const { root } = await render(<my-component />);

// Skip hydration wait (useful for testing loading states)
const { root } = await render(<my-component />, { waitForReady: false });

Available matchers:

// DOM assertions
expect(element).toHaveClass('active');
expect(element).toHaveClasses(['active', 'primary']); // Contains all / partial match
expect(element).toMatchClasses(['active']); // Exact match
expect(element).toHaveAttribute('aria-label', 'Close');
expect(element).toEqualAttribute('type', 'button');
expect(element).toEqualAttributes({ type: 'button', disabled: true });
expect(element).toHaveProperty('value', 'test');
expect(element).toHaveTextContent('Hello World'); // includes shadow DOM text
expect(element).toHaveLightTextContent('Hello World'); // light DOM only
expect(element).toEqualText('Exact text match'); // includes shadow DOM text
expect(element).toEqualLightText('Exact text match'); // light DOM only

// Shadow DOM
expect(element).toHaveShadowRoot();
await expect(element).toEqualHtml('<div>Expected HTML</div>');
await expect(element).toEqualLightHtml('<div>Light DOM only</div>');

Spying and Mocking

Spy on component methods, props, and lifecycle hooks to verify behaviour without modifying your component code.

Setup requirement: Load your components in a beforeAll block (typically in your setup file). The spy system patches customElements.define, so components must be registered after the test framework initializes.
// vitest-setup.ts
- await import('./dist/test-components/test-components.esm.js');

+ import { beforeAll } from 'vitest';
+ beforeAll(async () => {
+   await import('./dist/test-components/test-components.esm.js');
+ });

Method Spying

Spy on methods while still calling the original implementation:

const { root, spies } = await render(<my-button>Click me</my-button>, {
  spyOn: {
    methods: ['handleClick'],
  },
});

// Trigger the method
root.shadowRoot?.querySelector('button')?.click();

// Assert the method was called
expect(spies?.methods.handleClick).toHaveBeenCalledTimes(1);
expect(spies?.methods.handleClick).toHaveBeenCalledWith(expect.objectContaining({ type: 'click' }));

// Reset call history
spies?.methods.handleClick.mockClear();

Method Mocking

Replace methods with pre-configured mocks:

// Create mock with desired return value *before* render
const fetchUserMock = vi.fn().mockResolvedValue({
  id: '123',
  name: 'Test User',
  email: 'test@example.com',
});

// Mock is applied before initialisation
const { root, spies, waitForChanges } = await render(<user-profile userId="123" />, {
  spyOn: {
    mocks: { fetchUserData: fetchUserMock },
  },
});
await waitForChanges();

expect(fetchUserMock).toHaveBeenCalledWith('123');
expect(root.shadowRoot?.querySelector('.name')?.textContent).toBe('Test User');

Access the original implementation to augment rather than fully replace:

const fetchMock = vi.fn();
const { spies } = await render(<my-component />, {
  spyOn: { mocks: { fetchData: fetchMock } },
});

// Wrap the original to add logging or modify behaviour
fetchMock.mockImplementation(async (...args) => {
  console.log('Fetching data with args:', args);
  const result = await spies?.mocks.fetchData.original?.(...args);
  console.log('Got result:', result);
  return result;
});

Prop Spying

Track when props are changed:

const { spies, setProps, waitForChanges } = await render(<my-button variant="primary">Click me</my-button>, {
  spyOn: {
    props: ['variant', 'disabled'],
  },
});

await setProps({ variant: 'danger' });
await waitForChanges();

expect(spies?.props.variant).toHaveBeenCalledWith('danger');
expect(spies?.props.variant).toHaveBeenCalledTimes(1);

Lifecycle Spying

Spy on lifecycle methods. Methods that don't exist on the component are auto-stubbed:

const { spies, setProps, waitForChanges } = await render(<my-button>Click me</my-button>, {
  spyOn: {
    lifecycle: ['componentWillLoad', 'componentDidLoad', 'componentWillRender', 'componentDidRender'],
  },
});

// Lifecycle methods are called during initial render
expect(spies?.lifecycle.componentWillLoad).toHaveBeenCalledTimes(1);
expect(spies?.lifecycle.componentDidRender).toHaveBeenCalledTimes(1);

// Trigger a re-render
await setProps({ variant: 'danger' });
await waitForChanges();

// Re-render lifecycle methods called again
expect(spies?.lifecycle.componentWillRender).toHaveBeenCalledTimes(2);
expect(spies?.lifecycle.componentDidRender).toHaveBeenCalledTimes(2);

Resetting Spies

Reset all spies at once using resetAll(). This clears call histories AND resets mock implementations:

const fetchMock = vi.fn().mockReturnValue('mocked');
const { root, spies, setProps, waitForChanges } = await render(<my-button variant="primary">Click me</my-button>, {
  spyOn: {
    methods: ['handleClick'],
    mocks: { fetchData: fetchMock },
    props: ['variant'],
  },
});

// Trigger some calls
root.shadowRoot?.querySelector('button')?.click();
await setProps({ variant: 'danger' });

// Reset everything
spies?.resetAll();

// Call histories cleared
expect(spies?.methods.handleClick).toHaveBeenCalledTimes(0);
expect(spies?.props.variant).toHaveBeenCalledTimes(0);

// Mock implementations reset to default (returns undefined)
expect(fetchMock()).toBeUndefined();

Nested Components

When the root element is not a custom element, or when you have multiple custom elements, use getComponentSpies() to retrieve spies for specific elements:

import { render, getComponentSpies, h } from '@stencil/vitest';

// Root is a div, not a custom element
const { root } = await render(
  <div>
    <my-button>Click me</my-button>
  </div>,
  {
    spyOn: { methods: ['handleClick'] },
  },
);

// Query the nested custom element
const button = root.querySelector('my-button') as HTMLElement;

// Get spies for the nested element
const buttonSpies = getComponentSpies(button);
expect(buttonSpies?.methods.handleClick).toBeDefined();

// Multiple instances have independent spies
const { root: container } = await render(
  <div>
    <my-button class="a">A</my-button>
    <my-button class="b">B</my-button>
  </div>,
  { spyOn: { methods: ['handleClick'] } },
);

const spiesA = getComponentSpies(container.querySelector('.a') as HTMLElement);
const spiesB = getComponentSpies(container.querySelector('.b') as HTMLElement);

// Each has its own spy instance
container.querySelector('.a')?.shadowRoot?.querySelector('button')?.click();
expect(spiesA?.methods.handleClick).toHaveBeenCalledTimes(1);
expect(spiesB?.methods.handleClick).toHaveBeenCalledTimes(0);

Per-Component Configurations

When rendering multiple component types, use the components property for tag-specific spy configs:

import { render, getComponentSpies, h } from '@stencil/vitest';

const { root } = await render(
  <my-card cardTitle="Test">
    <my-button slot="footer">Click me</my-button>
  </my-card>,
  {
    spyOn: {
      lifecycle: ['componentDidLoad'], // base - applies to all
      components: {
        'my-card': { props: ['cardTitle'] },
        'my-button': { methods: ['handleClick'] },
      },
    },
  },
);

const cardSpies = getComponentSpies(root);
const buttonSpies = getComponentSpies(root.querySelector('my-button') as HTMLElement);

// Both get base lifecycle spy + their specific config
expect(cardSpies?.lifecycle.componentDidLoad).toHaveBeenCalled();
expect(cardSpies?.props.cardTitle).toBeDefined();

expect(buttonSpies?.lifecycle.componentDidLoad).toHaveBeenCalled();
expect(buttonSpies?.methods.handleClick).toBeDefined();

Event Testing

Test custom events emitted by your components:

const { root, spyOnEvent, waitForChanges } = await render(<my-button />);

// Spy on events
const clickSpy = spyOnEvent('buttonClick');
const changeSpy = spyOnEvent('valueChange');

// Trigger events
root.click();
await waitForChanges();

// Assert events were emitted
expect(clickSpy).toHaveReceivedEvent();
expect(clickSpy).toHaveReceivedEventTimes(1);
expect(clickSpy).toHaveReceivedEventDetail({ buttonId: 'my-button' });

// Access event data
expect(clickSpy.events).toHaveLength(1);
expect(clickSpy.firstEvent?.detail).toEqual({ buttonId: 'my-button' });
expect(clickSpy.lastEvent?.detail).toEqual({ buttonId: 'my-button' });

Snapshots

The package includes a custom snapshot serializer for Stencil components that properly handles shadow DOM:

import { render, h } from '@stencil/vitest';
...
const { root } = await render(<my-component />);
expect(root).toMatchSnapshot();

Snapshot output example:

<my-component>
  <mock:shadow-root>
    <button class="primary">
      <slot />
    </button>
  </mock:shadow-root>
  Click me
</my-component>

Screenshot Testing

Browser tests can include screenshot comparisons using Vitest's screenshot capabilities:

import { render, h } from '@stencil/vitest';
...
const { root } = await render(<my-button variant="primary">Primary Button</my-button>);
await expect(root).toMatchScreenshot();

Refer to Vitest's screenshot testing documentation for more details.

Utils

`serializeHtml(element, options?)`

Serializes an HTML element to a string, including shadow DOM content. Useful for debugging or creating custom assertions.

import { serializeHtml } from '@stencil/vitest';

const html = serializeHtml(element, {
  serializeShadowRoot: true, // Include shadow DOM (default: true)
  pretty: true, // Prettify output (default: true)
  excludeStyles: true, // Exclude <style> tags (default: true)
});

`prettifyHtml(html)`

Formats HTML string with indentation for readability.

import { prettifyHtml } from '@stencil/vitest';

const formatted = prettifyHtml('<div><span>Hello</span></div>');
// Returns:
// <div>
//   <span>
//     Hello
//   </span>
// </div>

`waitForStable(elementOrSelector, timeout?)`

Waits for an element to be rendered and visible in the DOM. Only works in real browser environments (not jsdom/happy-dom).

Accepts either an Element or a CSS selector string. When a selector is provided, it polls until the element appears in the DOM.

import { render, waitForStable, h } from '@stencil/vitest';

// Wait for a rendered element to be stable / visible
const { root } = await render(<my-component />);
await waitForStable(root);

// Wait for an element using a selector (useful when element isn't in DOM yet)
await waitForStable('my-component .inner-element');

// Custom timeout (default: 5000ms)
await waitForStable('my-component', 10000);

Note: In non-browser environments, waitForStable logs a warning and returns immediately.

`waitForExist(selector, timeout?)`

Waits for an element matching the selector to exist in the DOM. Unlike waitForStable, this works in both real browsers and mock DOM environments (jsdom/happy-dom).

Returns the element if found, or null if timeout is reached.

import { waitForExist } from '@stencil/vitest';

// Wait for an element to appear in the DOM
const element = await waitForExist('my-component .lazy-loaded');

// Custom timeout (default: 5000ms)
const element = await waitForExist('#dynamic-content', 10000);

CLI

The stencil-test CLI wraps both Stencil builds with Vitest testing.

Add to package.json

{
  "scripts": {
    "test": "stencil-test",
    "test:watch": "stencil-test --watch"
  }
}

Usage

# Build once, test once
stencil-test

# Watch mode (rebuilds on component changes, interactive Vitest)
stencil-test --watch

# Watch mode with dev server
stencil-test --watch --serve

# Production build before testing
stencil-test --prod

# Pass arguments to Vitest
stencil-test --watch --coverage

# Test specific files
stencil-test button.spec.ts

# Test specific project
stencil-test --project browser

CLI Options

The stencil-test CLI supports most of Stencil's CLI options and all of Vitest CLI options

For full Stencil CLI options, see Stencil CLI docs.
For full Vitest CLI options, see Vitest CLI docs.

Global Variables

The stencil-test CLI exposes global variables that can be accessed in your tests to check which CLI flags were used:

Global	Type	Description
`__STENCIL_PROD__`	`boolean`	`true` when `--prod` flag is passed
`__STENCIL_SERVE__`	`boolean`	`true` when `--serve` flag is passed
`__STENCIL_PORT__`	`string`	Port number when `--port` is specified

if (__STENCIL_PROD__) {
  console.log('Running tests against production build');
}

if (__STENCIL_SERVE__) {
  const baseUrl = `http://localhost:${__STENCIL_PORT__ || '3333'}`;
}

TypeScript Support

Add to your tsconfig.json for type definitions:

{
  "compilerOptions": {
    "types": ["@stencil/vitest/globals"]
  }
}

License

MIT

Contributing

See CONTRIBUTING.md for development setup and guidelines.

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