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Islands Architecture

The term Islands Architecture was coined a few years ago by frontend architect Katie Sylor-Miller and further popularized by Preact creator Jason Miller. It describes a way of architecting website frontends around independent component trees, all rendered server-side initially as HTML but then “hydrated” on the frontend independently of one another.

Starting in Bridgetown 1.3, we’re bringing islands architecture to you with a seamless integration between our view components and our esbuild frontend bundling system. And for even more flexibility, you can even orient your Roda routes around “islands” for a truly modular, full-stack approach to web development.

This is an early step forward for the framework, so your feedback is crucial as we increasingly align our best practices with the latest improvements across the industry.

Table of Contents #

Creating Your First Island #

If you’ve just created a new Bridgetown project, you’re ready to take it to the next level. If you’re upgrading from Bridgetown 1.2 or previous, you’ll need to be using esbuild (not Webpack) and must update your base esbuild configuration. It’s a snap! Just run:

bin/bridgetown esbuild update

See the esbuild setup documentation for further upgrade instructions.

You may also need to update your esbuild.config.js file so it includes the following configuration option:

  globOptions: {
    excludeFilter: /\.(dsd|lit)\.css$/

Next, in the src/_islands folder (please create one if it’s not already present), add your first island “entrypoint” which we’ll call breezy.js:

class BreezyDay extends HTMLElement {
  static {
    customElements.define("breezy-day", this)
  connectedCallback() {
    this.textContent = "Welcome to your first island."

This JavaScript file, and anything else you import through it, is contained to a single island, meaning it won’t be bundled in with your main JavaScript bundle (aka frontend/javascript/index.js). Great! But how do you load an island?

With <is-land>, of course!

Installing the Island #

Bridgetown comes with a [bundled configuration][/docs/bundled-configurations] to set up the <is-land> web component, a tiny package which lets you lazy-load islands just as they become visible on-screen or are interacted with (aka clicked).

Run this command to add it to your main JavaScript bundle:

bin/bridgetown configure is-land

Now let’s add the island to a page. Pick one of the pages on your site (it needs to be ERB for the purposes of this example) and add this bit of code:

<is-land on:visible import="<%= asset_path 'islands/breezy.js' %>">
    Approaching the island. . .

Let’s break this down:

  1. The <is-land> tag gets set up to import the new breezy.js island you’ve created once it’s visible to the reader.
  2. At first the <breezy-day> element will be undefined, but once the island loads, it will be upgraded/hydrated.
  3. Once that moment occurs, the “Approaching…” text will get replaced with “Welcome…”. That’s just a simple example—more likely you’d want to write your island JavaScript to set up event handlers, or display more up-to-the-second data pulled from an API, etc.

Rendering Island Components in Ruby #

Generally we wouldn’t recommend directly adding your island markup inside of <is-land> on any given page, but rather render a component tree of one or more components you define. For this purpose, we’ve included src/_islands as a folder which can load Ruby components. And by using folder-based namespaces, you can easily keep island-specific components separated.

Let’s move <breezy-day> into its own Ruby component. We’ll create a new src/_islands/breezy_day folder and add the namespaced Element class to element.rb:

class BreezyDay::Element < Bridgetown::Component
  attr_reader :fallback

  def initialize(fallback: "Approaching the island. . .")
    @fallback = fallback

And then define our element.erb template:

  <%= fallback %>

Now in our original page template code, we can simply render the component and that’s that.

<is-land on:visible import="<%= asset_path 'islands/breezy.js' %>">
  <%= render %>

You can of course render shared components from src/_components from your island component as well, along with other island-specific components you may define.

Scoped Styling via Declarative Shadow DOM (DSD) #

For many (most?) islands, you’ll want to provide styles as part of your markup. We strongly recommend using shadow DOM inside your web components, and the native DSD support) in Bridgetown 1.3+ makes this a breeze! (Pardon the pun!)

Let’s add some styling to our <breezy-day> component via DSD. Edit your element.erb as so:

  <%= dsd do %>
    <%= dsd_style %>
  <% end %>
  <%= fallback %>

And add a stylesheet as element.dsd.css:

:host {
  display: block;
  color: LightSeaGreen;
  font-weight: bold;

Now when you view the page, your island element will appear with the styled color and bold text.

What’s great about this approach is (a) only this element is affected by your stylesheet and nothing else on your page, and (b) these styles aren’t loaded until the page containing your island is viewed, keeping your main CSS bundle size low. For more general information on how to use and style your HTML templates and components using shadow DOM, check out our documentation on DSD.

The <is-land> web component automatically polyfills DSD, which is an added benefit of using it. Otherwise, the Turbo bundled configuration also includes a site-wide polyfill for DSD. As of the time of this writing, only Firefox (and some older versions of Safari) do not offer built-in DSD support.

Sidecar CSS files processed through the dsd_style helper do not get run through PostCSS—aka they must be 100% “vanilla” CSS. Don’t be surprised if you try using a feature that’s uniquely enabled by your PostCSS config and it’s not available within the DSD template.