@lit-labs/eleventy-plugin-lit

A plugin for Eleventy that pre-renders Lit web components at build time, with optional hydration.

Contents

• Status
• Setup
  • Install
  • Register plugin
  • Configure component modules
  • Enable experimental VM modules
  • Watch mode
• Usage
  • Component compatibility
  • Passing data to components
• Declarative Shadow DOM
  • Polyfill
• Hydration
• Bootup
  • Example bootup strategy
• Roadmap
• Issues and comments
• Contributing

Status

🚧 @lit-labs/eleventy-plugin-lit 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. 🚧

Setup

Install

npm i @lit-labs/eleventy-plugin-lit

Register plugin

Edit your .eleventy.js config file to register the Lit plugin:

const litPlugin = require('@lit-labs/eleventy-plugin-lit');

module.exports = function (eleventyConfig) {
  eleventyConfig.addPlugin(litPlugin, {
    mode: 'worker',
    componentModules: [
      'js/demo-greeter.js',
      'js/other-component.js',
    ],
  });
};

Configure mode

Use the mode setting to tell the plugin which mode to use for rendering. The plugin currently supports either 'worker' or 'vm'.

'worker' mode (default) utilizes worker threads to render components in isolation.

'vm' mode utilizes vm.Module for context isolation and therefore eleventy must be executed with the --experimental-vm-modules Node flag enabled. This flag is available in Node versions 12.16.0 and above.

NODE_OPTIONS=--experimental-vm-modules eleventy

Configure component modules

🚧 Note: Support for specifying component modules in Eleventy front matter is on the roadmap. Follow #2494 for progress and discussion. 🚧

Use the componentModules setting to tell the plugin where to find the definitions of your components.

Pass an array of paths to .js files containing Lit component definitions. Paths are interpreted relative to to the directory from which the eleventy command is executed.

Each .js file should be a JavaScript module (ESM) that imports lit with a bare module specifier and defines a component with customElements.define.

Note that in 'worker' mode, Node determines the module system accordingly, and as such care must be taken to ensure Node reads them as ESM files while still reading the eleventy config file as CommonJS.

Some options are:

1. Add {"type": "module"} to your base package.json, make sure the eleventy config file ends with the .cjs extension, and supply it as a command line argument to eleventy.

   eleventy --config=.eleventy.cjs

2. Put all component .js files in a subdirectory with a nested package.json with {"type": "module"}.

Watch mode

Use addWatchTarget to tell Eleventy to watch for changes in your JavaScript directory:

eleventyConfig.addWatchTarget('js/');

Usage

Whenever you use a custom element in your Eleventy Markdown and HTML files, @lit-labs/eleventy-plugin-lit will automatically render its template and styles directly into your HTML.

For example, given a markdown file hello.md:

# Greetings

<demo-greeter name="World"></demo-greeter>

And a component definition js/demo-greeter.js:

import {LitElement, html, css} from 'lit';

class DemoGreeter extends LitElement {
  static styles = css`
    b { color: red; }
  `;

  static properties = {
    name: {},
  };

  render() {
    return html`Hello <b>${this.name}</b>!`;
  }
}
customElements.define('demo-greeter', DemoGreeter);

Then the Eleventy will produce greeting/index.html:

<h1>Greetings</h1>

<demo-greeter name="World">
  <template shadowroot="open">
    <style>
      b { color: red; }
    </style>
  </template>
  Hello <b>World</b>!
</demo-greeter>

The <template shadowroot="open"> element above is an HTML standard called declarative shadow DOM. See the Declarative Shadow DOM section below for more details.

Component compatibility

🚧 Note: Expanding this section with full details on component compatibility is on the roadmap. Follow #2494 for progress and discussion. 🚧

There are currently a number of restrictions that determine whether a component will be compatible with Lit pre-rendering, because not all of the component lifecycle methods are currently invoked, and the DOM APIs that can be used in certain lifecycle methods are restricted.

The Lit team is working on finalizing and documenting the SSR lifecycle and restrictions, follow #2494 for more details.

Passing data to components

🚧 Note: Support for passing data as properties is on the roadmap. Follow #2494 for progress and discussion. 🚧

Data can be passed to your components by setting attributes (see the name attribute in the example above), or passing child elements.

Declarative Shadow DOM

Lit SSR depends on Declarative Shadow DOM, a browser feature that allows Shadow DOM to be created and attached directly from HTML, without the use of JavaScript.

Polyfill

As of February 2022, Chrome and Edge have native support for Declarative Shadow DOM, but Firefox and Safari don't yet.

Therefore, unless you are developing for a very constrained environment, you must use the Declarative Shadow DOM Polyfill to emulate this feature in browsers that don't yet support it.

Install the polyfill from NPM:

npm i @webcomponents/template-shadowroot

For usage, see the example bootup strategy which demonstrates a recommended method for efficiently loading the polyfill alongside Lit hydration support.

⏱️ The Declarative Shadow DOM polyfill must be applied after all pre-rendered HTML has been parsed, because it is a one-shot operation. You can guarantee this timing by importing the polyfill from a type=module script, or by placing it at the end of your <body> tag.

Note that even if you do not require hydration, you will still need to polyfill Declarative Shadow DOM, otherwise your pre-rendered components will never be displayed in some browsers.

Hydration

Hydration is the process where statically pre-rendered components are upgraded to their JavaScript implementations, becoming responsive and interactive.

Lit components can automatically hydrate themselves when they detect that a Shadow Root has already been attached, as long as Lit's experimental hydrate support module has been installed by importing @lit-labs/ssr-client/lit-element-hydrate-support.js.

⏱️ The Lit hydration support module must be loaded before Lit or any components that depend on Lit are imported, because it modifies the initial startup behavior of the lit-element.js module and the LitElement class.

Bootup

It is important to preserve some constraints when designing a boot-up strategy for pages that use pre-rendered Lit components. In particular:

• The Declarative Shadow DOM polyfill must wait until all HTML has been parsed.
• Lit and Lit component definition modules must wait until the experimental Lit hydration support module has loaded.
• Lit component definition modules must wait until the Declarative Shadow DOM polyfill to have been invoked (if it was needed for the browser).

In the following diagram, each -> edge represents a timing sequence constraint:

parse    load       install lit
 HTML  polyfill   hydration support
  |       |        |
  v       v        v
 run polyfill    load lit
          |        |
          v        v
        load component
         definitions

Example bootup strategy

🚧 Note: The pattern described here will only work in modern browsers such as Firefox, Chrome, Edge, and Safari. IE11 is also supported, but will require a different pattern that is not yet documented here. Documenting this pattern is on the roadmap. Follow #2494 for progress and discussion. 🚧

The following demonstrates an example strategy for booting up a page that contains pre-rendered Lit components with Eleventy.

The Lit team is investigating ways to simplify this bootup strategy and help you generate it. Follow #2487 and #2490 for progress.

Typically in Eleventy your content is written in Markdown files which delegate the outer HTML shell to a layout. For example hello.md could delegate to the default.html layout like this:

---
layout: default.html
---

# Greetings

<demo-greeter name="World"></demo-greeter>

The file _includes/default.html would then contain the following:

<!DOCTYPE html>
<html>
  <head>
    <!-- As an optimization, immediately begin fetching the JavaScript modules
        that we know for sure we'll eventually need. It's important we don't
        execute them yet, though. -->
    <link
      rel="modulepreload"
      href="/node_modules/@lit-labs/ssr-client/lit-element-hydrate-support.js"
    />
    <link rel="modulepreload" href="/_js/component1.js" />
    <link rel="modulepreload" href="/_js/component2.js" />

    <!-- On browsers that don't yet support native declarative shadow DOM, a
         paint can occur after some or all pre-rendered HTML has been parsed,
         but before the declarative shadow DOM polyfill has taken effect. This
         paint is undesirable because it won't include any component shadow DOM.
         To prevent layout shifts that can result from this render, we use a
         "dsd-pending" attribute to ensure we only paint after we know
         shadow DOM is active. -->
    <style>
      body[dsd-pending] {
        display: none;
      }
    </style>
  </head>

  <body dsd-pending>
    <script>
      if (HTMLTemplateElement.prototype.hasOwnProperty('shadowRoot')) {
        // This browser has native declarative shadow DOM support, so we can
        // allow painting immediately.
        document.body.removeAttribute('dsd-pending');
      }
    </script>

    <!-- Pre-rendered Lit components will be generated here. -->
    {{ content }}

    <!-- At this point, browsers with native shadow DOM support will already
         be able to paint the initial fully styled state your components,
         without executing a single line of JavaScript! However, the components
         aren't interactive yet -- that's what hydration is for. -->

    <!-- Use a type=module script so that we can use dynamic module imports.
         Note this pattern will not work in IE11. -->
    <script type="module">
      (async () => {
        // Start fetching the Lit hydration support module (note the absence
        // of "await" -- we don't want to block yet).
        const litHydrateSupportInstalled = import(
          '/node_modules/@lit-labs/ssr-client/lit-element-hydrate-support.js'
        );

        // Check if we require the declarative shadow DOM polyfill. As of
        // February 2022, Chrome and Edge have native support, but Firefox
        // and Safari don't yet.
        if (!HTMLTemplateElement.prototype.hasOwnProperty('shadowRoot')) {
          // Fetch the declarative shadow DOM polyfill.
          const {hydrateShadowRoots} = await import(
            '/node_modules/@webcomponents/template-shadowroot/template-shadowroot.js'
          );

          // Apply the polyfill. This is a one-shot operation, so it is important
          // it happens after all HTML has been parsed.
          hydrateShadowRoots(document.body);

          // At this point, browsers without native declarative shadow DOM
          // support can paint the initial state of your components!
          document.body.removeAttribute('dsd-pending');
        }

        // The Lit hydration support module must be installed before we can
        // load any component definitions. Wait until it's ready.
        await litHydrateSupportInstalled;

        // Load component definitions. As each component definition loads, your
        // pre-rendered components will come to life and become interactive.
        //
        // You may also prefer to bundle your components into fewer JS modules.
        // See https://lit.dev/docs/tools/production/#building-with-rollup for
        // more details.
        import('/_js/component1.js');
        import('/_js/component2.js');
      })();
    </script>
  </body>
</html>

Roadmap

The following features and fixes are on the roadmap for this plugin. See the linked issues for more details, and feel free to comment on the issues if you have any thoughts or questions.

• [#2494] Document restrictions on SSR compatible components.

• [#2483] Allow specifying component definition modules in front matter instead of the componentModules setting.

• [#2485] Provide a mechanism for passing Eleventy data to components as properties, instead of attributes.

• [#2486] Patterns and documentation for supporting IE11.

• [#2487] Provide a mechanism for automatically generating and inserting an appropriate hydration configuration.

• [#2490] Simplify and optimize the polyfill + hydration bootup strategy.

Issues and comments

If you find any bugs in this package, please file an issue. If you have any questions or comments, start a discussion.

Contributing

Please see CONTRIBUTING.md.

Testing environment variables:

• SHOW_TEST_OUTPUT: Set to show all stdout and stderr from spawned eleventy invocations in test cases.