@lit-labs/testing

A package containing utilities for testing Lit components.

> [!WARNING]
>
> This package is part of Lit 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
> 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). 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

$3

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

- 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.

$3

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(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, {
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, {
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();
});
`

$3

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.