# @lit-labs/testing

A package containing utilities for testing Lit components.

> [!WARNING]
>
> This package is part of [Lit Labs](https://lit.dev/docs/libraries/labs/). It
> is published in order to get feedback on the design and may receive breaking
> changes or stop being supported.
>
> Please read our [Lit Labs documentation](https://lit.dev/docs/libraries/labs/)
> before using this library in production.
>
> Give feedback: https://github.com/lit/lit/discussions/3359

## Overview

This package contains utilities that will be useful for testing Lit elements,
especially for ensuring your components are compatible with server-side
rendering (SSR).

These are meant to be used with [Web Test Runner
(WTR)](https://modern-web.dev/docs/test-runner/overview/). A WTR plugin and
fixture functions are provided that allow testing of components being rendered
server-side and loaded to the browser document both with and without hydration.

## Usage

### Web Test Runner Plugin for Lit SSR

This plugin registers the `lit-ssr-render` server command utilized by the SSR
fixture functions.

Add the plugin to your config file.

```js
// web-test-runner.config.js
import {litSsrPlugin} from '@lit-labs/testing/web-test-runner-ssr-plugin.js';

export default {
  plugins: [litSsrPlugin()],
};
```

Signature

`litSsrPlugin(options: LitSsrPluginOptions = {}): TestRunnerPlugin<Payload>`

- `options: object` - Options object containing the following properties
  - `workerInitModules: string` - Modules to be loaded before the fixture modules. This allows registering e.g. Node.js ESM hooks or general setup. Accepts absolute paths, paths relative to the working directory, or external package modules.

### Fixtures

The package exports functions that will generate fixtures by passing the
provided Lit templates to the dev server to be rendered server-side, and
added to the browser document. The Web Test Runner plugin must be added to the
config file for this to work.

#### `ssrFixture`

Signature

`ssrFixture<T extends HTMLElement>(template, options): T`

- `template: TemplateResult` - Lit template to be rendered. It must contain a
  single top level element.
- `options: object` - Options object containing the following properties
  - `modules: string[]` - Relative paths to modules to be loaded before
    rendering. Usually will contain custom element definitions.
  - `base?: string` - Base location for resolving module paths provided. If not
    provided, the call site will be treated as the base effectively making it
    the same as providing `import.meta.url`.
  - `hydrate?: boolean` - Whether to hydrate the template after being loaded to
    the browser. Defaults to true if omitted.
- Returns the top level element rendered in the document.

Example

```js
// my-element.test.js
import {ssrFixture} from '@lit-labs/testing/fixtures.js';
import {html} from 'lit';
import {assert} from '@esm-bundle/chai';

suite('my-element', () => {
  test('is rendered server-side', async () => {
    const el = await ssrFixture(html`<my-element></my-element>`, {
      modules: ['./my-element.js'],
      base: 'http://localhost:8000/dist/components/',
      hydrate: false,
    });
    assert.equal(el.shadowRoot.querySelector('p').textContent, 'Hello, World!');
  });
});
```

#### `csrFixture`, `ssrNonHydratedFixture`, and `ssrHydratedFixture`

`csrFixture` renders the provided template client-side.
`ssrNonHydratedFixture` and `ssrHydratedFixture` are just `ssrFixture` with the
`hydrate` option pre-filled.

These are provided to have the same call signature so the same test code can be
repeated with different rendering methods.

Example

```js
// my-element.test.js
import {
  csrFixture,
  ssrNonHydratedFixture,
  ssrHydratedFixture,
} from '@lit-labs/testing/fixtures.js';
import {html} from 'lit';
import {assert} from '@esm-bundle/chai';

for (const fixture of [csrFixture, ssrNonHydratedFixture, ssrHydratedFixture]) {
  suite(`my-element rendered with ${fixture.name}`, () => {
    test('renders as expected', async () => {
      const el = await fixture(html`<my-element></my-element>`, {
        modules: ['./my-element.js'],
      });
      assert.equal(
        el.shadowRoot.querySelector('p').textContent,
        'Hello, World!'
      );
    });
  });
}
```

#### `cleanupFixtures`

Call this if you wish to clean up the created fixture from the document, for
instance, in a cleanup hook.

Example

```js
import {cleanupFixtures} from '@lit-labs/testing/fixtures.js';

teardown(() => {
  cleanupFixtures();
});

// or

afterEach(() => {
  cleanupFixtures();
});
```

### Notes

Any `lit` imports including those for custom element definitions, **must**
follow the fixture imports so that
`@lit-labs/ssr-client/lit-element-hydrate-support.js` is imported before it.

The fixture functions expect a Lit `TemplateResult` with a single top level
element.