LitElement JavaScript starter

This project includes a sample component using LitElement with JavaScript.

This template is generated from the lit-starter-js package in the main Lit repo. Issues and PRs for this template should be filed in that repo.

About this release

This is a pre-release of Lit 3.0, the next major version of Lit.

Lit 3.0 has very few breaking changes from Lit 2.0:

• Drops support for IE11
• Published as ES2021
• Removes a couple of deprecated Lit 1.x APIs

Lit 3.0 should require no changes to upgrade from Lit 2.0 for the vast majority of users. Once the full release is published, most apps and libraries will be able to extend their npm version ranges to include both 2.x and 3.x, like "^2.7.0 || ^3.0.0".

Lit 2.x and 3.0 are interoperable: templates, base classes, directives, decorators, etc., from one version of Lit will work with those from another.

Please file any issues you find on our issue tracker.

Setup

Install dependencies:

npm i

Testing

This sample modern-web.dev's @web/test-runner for testing. See the modern-web.dev testing documentation for more information.

Tests can be run with the test script, which will run your tests against Lit's development mode (with more verbose errors) as well as against Lit's production mode:

npm test

For local testing during development, the test:dev:watch command will run your tests in Lit's development mode (with verbose errors) on every change to your source files:

npm test:watch

Alternatively the test:prod and test:prod:watch commands will run your tests in Lit's production mode.

Dev Server

This sample uses modern-web.dev's @web/dev-server for previewing the project without additional build steps. Web Dev Server handles resolving Node-style "bare" import specifiers, which aren't supported in browsers. It also automatically transpiles JavaScript and adds polyfills to support older browsers. See modern-web.dev's Web Dev Server documentation for more information.

To run the dev server and open the project in a new browser tab:

npm run serve

There is a development HTML file located at /dev/index.html that you can view at http://localhost:8000/dev/index.html. Note that this command will serve your code using Lit's development mode (with more verbose errors). To serve your code against Lit's production mode, use npm run serve:prod.

Editing

If you use VS Code, we highly recommend the lit-plugin extension, which enables some extremely useful features for lit-html templates:

• Syntax highlighting
• Type-checking
• Code completion
• Hover-over docs
• Jump to definition
• Linting
• Quick Fixes

The project is setup to recommend lit-plugin to VS Code users if they don't already have it installed.

Linting

Linting of JavaScript files is provided by ESLint. In addition, lit-analyzer is used to type-check and lint lit-html templates with the same engine and rules as lit-plugin.

The rules are mostly the recommended rules from each project, but some have been turned off to make LitElement usage easier. The recommended rules are pretty strict, so you may want to relax them by editing .eslintrc.json.

To lint the project run:

npm run lint

Formatting

Prettier is used for code formatting. It has been pre-configured according to the Lit's style. You can change this in .prettierrc.json.

Prettier has not been configured to run when committing files, but this can be added with Husky and pretty-quick. See the prettier.io site for instructions.

Static Site

This project includes a simple website generated with the eleventy static site generator and the templates and pages in /docs-src. The site is generated to /docs and intended to be checked in so that GitHub pages can serve the site from /docs on the master branch.

To enable the site go to the GitHub settings and change the GitHub Pages "Source" setting to "master branch /docs folder".

To build the site, run:

npm run docs

To serve the site locally, run:

npm run docs:serve

To watch the site files, and re-build automatically, run:

npm run docs:watch

The site will usually be served at http://localhost:8000.

Note: The project uses Rollup to bundle and minify the source code for the docs site and not to publish to NPM. For bundling and minification, check the Bundling and minification section.

Bundling and minification

As stated in the static site generation section, the bundling and minification setup in the Rollup configuration in this project is there specifically for the docs generation.

We recommend publishing components as unoptimized JavaScript modules and performing build-time optimizations at the application level. This gives build tools the best chance to deduplicate code, remove dead code, and so on.

Please check the Publishing best practices for information on publishing reusable Web Components, and Build for production for building application projects that include LitElement components, on the Lit site.

More information

See Get started on the Lit site for more information.