@lit-labs/testing

A package containing utilities for testing Lit components.

Status

@lit-labs/testing is part of the Lit Labs set of packages - it is published in order to get feedback on the design and not ready for production. Breaking changes are likely to happen frequently.

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

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.

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

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

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

// 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

// 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

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.