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Documentation Designing Your Site Template Engines

ERB and Beyond

Bridgetown’s implementation language, Ruby, has a rich history of providing ERB for templates and view layers across a wide variety of tools and frameworks. Other Ruby-based template languages such as Haml, Slim, and Serbea garner enthusiastic usage as well.

Bridgetown makes it easy to add both ERB-based and Serbea-based templates and components to any site. In additional, there are plugins you can easily install for Haml and Slim support. Under the hood, Bridgetown uses the Tilt gem to load and process these Ruby templates.

Interested in switching your entire site to use ERB or Serbea by default? It’s possible to do that with just a simple configuration change.

Table of Contents #

Usage #

For ERB, simply define a page/document with an .erb extension, rather than .html. You’ll still need to add front matter to the top of the file (or at the very least two lines of triple dashes ---) for the file to get processed. In the Ruby code you embed, you’ll be interacting with the underlying Ruby API for Bridgetown objects (aka Bridgetown::Page, Bridgetown::Site, etc.). Here’s an example:

title: I'm a page!

<h1><%= %></h1>

<p>Welcome to <%= %>!</p>

<footer>Authored by <%= %></footer>

Front matter is accessible via the data method on pages, posts, layouts, and other documents. Site config values are accessible via the site.config method, and loaded data files via as you would expect.

In addition to site, you can also access the site_drop object which will provide similar access to various data and config values similar to the site variable in Liquid.

If you need to escape an ERB tag (to use it in a code sample for example), use two percent signs:

Here's my **Markdown** file.

And my <%%= "ERB code sample" %>

You can easily loop through resources in a collection:

<% collections.posts.resources.each do |post| %>
  <li><a href="<%= post.relative_url %>"><%= %></a></li>
<% end %>

Or using the paginator, along with the link_to helper:

<% paginator.resources.each do |post| %>
  <li><%= link_to, post %></li>
<% end %>

Serbea #

Serbea is a “superset” of ERB which provides the same benefits as ERB but uses curly braces like Liquid {% %} or {{ }} and adds support for filters and render directives. Use the file extension .serb. Here’s an example of the above ERB code rewritten in Serbea:

{% collections.posts.resources.each do |post| %}
  <li><a href="{{ post.relative_url }}">{{ }}</a></li>
{% end %}


{% paginator.resources.each do |post| %}
  <li>{{ | link_to: post }}</li>
{% end %}

Notice this is using the Liquid-like filter syntax for link_to. You can use this kind of syntax with any helpers available in all Ruby templates, as well as methods on objects themselves. Examples:

{{ | markdownify }}

{{ | titleize }}

{{ | array_to_sentence_string: "or" }}

{{ | upcase }} <!-- in this case upcase is a method on the String object itself! -->

(Under the hood, a Ruby method’s first argument will be supplied with the value of the left-side of the pipe | operator, and subsequent arguments continue after that as you write the filter syntax.)

For Serbea code samples in Markdown, use the serb tag. And like ERB, you can escape using two percent signs:


And·my·{%%= "ERB·code·sample" %}

Serbea also provides a raw helper just like Liquid for escaping Serbea code:

Process me! {% do_something %}

Don't process me! {% raw %}{% do_something %}{% endraw %}

There’s a VS Code extension available for Serbea which includes syntax highlighting as well as commands to convert selected ERB syntax to Serbea, and even a Serbea + Markdown highlighter.

For details on HTML output safety, see below (Serbea and ERB differ slightly on how escaping is accomplished).

Dot Access Hashes #

Data hashes support standard hash key access, but most of the time you can use “dot access” instead for a more familar look. For example:

<%= %> (but <%=[:title] %> or <%=["title"] %> also work)

<%= %>

<%= %>

<% # You can freely mix hash access and dot access: %>

<%=[].github %>

Partials #

To include a partial in your ERB template, add a _partials folder to your source folder, and save a partial starting with _ in the filename. Then you can reference it using the <%= render "filename" %> helper (or use the partial alias if you’re more comfortable with that). For example, if we were to move the footer above into a partial:

<!-- src/_partials/_author_footer.erb -->
<footer>Authored by <%=[:authors].first[:name] %></footer>
title: I'm a page!

<h1><%= %></h1>

<p>Welcome to <%= %>!</p>

<%= render "author_footer" %>

You can also pass variables to partials using either a locals hash or as keyword arguments:

<%= render "some/partial", key: "value", another_key: 123 %>

<%= render "some/partial", locals: { key: "value", another_key: 123 } %>

As an alternative to passing the partial filename as the first argument, you can supply a template keyword argument instead. This makes it easier to pass all arguments via a separate hash:

<% options = { template: "mypartial", title: "Hello!" } %>
<%= partial **options %>

Partials also support capture blocks, which can then be referenced via the content local variable within the partial.

Rendering Ruby Components #

For better encapsulation and reuse of Ruby-based templates as part of a “design system” for your site, we encourage you to write Ruby components using either Bridgetown::Component or GitHub’s ViewComponent library. Check out the documentation and code examples here.

Liquid Filters, Tags, and Components #

Bridgetown includes access to some helpful Liquid filters as helpers within your ERB templates:

<!-- July 9th, 2020 -->
<%= date_to_string site.time, "ordinal" %>

These helpers are actually methods of the helper object which is an instance of Bridgetown::RubyTemplateView::Helpers.

A few Liquid tags are also available as helpers too, such as class_map and asset_path.

In addition to using Liquid helpers, you can also render Liquid components from within your ERB templates via the liquid_render helper.

  Rendering a component:
  <%= liquid_render "test_component", param: "Liquid FTW!" %>
<!-- src/_components/test_component.liquid -->
<p>{{ param }}</p>

Layouts #

You can add an .erb layout and use it in much the same way as a Liquid-based layout. You can freely mix’n’match ERB layouts with Liquid-based documents and Liquid-based layouts with ERB documents.


layout: default
somevalue: 123

<h1><%= %></h1>

<main>An ERB layout! <%= %> / somevalue: <%= %></main>

<%= yield %>


layout: testing

A standard Liquid page. {{ }}

If in your layout or a layout partial you need to output the paths to your frontend assets, you can do so with a asset_path helper just like with Liquid layouts:

<link rel="stylesheet" href="<%= asset_path :css %>" />
<script src="<%= asset_path :js %>" defer></script>

Markdown #

When authoring a document using ERB, you might find yourself wanting to embed some Markdown within the document content. That’s easy to do using a markdownify block:

<%= markdownify do %>
   ## I'm a header!

   * Yay!
   <%= "* Nifty!" %>
<% end %>

You can also pass in any string variable as a method argument:

<%= markdownify some_string_var %>

Alternatively, you can author a document with a .md extension and configure it via template_engine: erb to get processed through ERB. (Continue reading for additional information.)

Sometimes you may want to output a file that doesn’t end in .html. Perhaps you want to create a JSON index of a collection, or a special XML feed. If you have familiarity with other Ruby site generators or frameworks, you might instinctively reach for the solution where you use a double extension, say, posts.json.erb to indicate the final extension (json) and the template type (erb).

Bridgetown doesn’t support double extensions but rather provides a couple of alternative mechanisms to specify your template engine of choice. The first option is to set the file’s permalink using front matter. Here’s an example of posts.json.erb using a custom permalink:

permalink: /posts.json
    collections.posts.resources.each_with_index do |post, index|
      last_item = index == collections.posts.resources.length - 1
      "title": <%= jsonify %>,
      "url": "<%= absolute_url post.url %>"<%= "," unless last_item %>
  <% end %>

This ensures the final relative URL will be /posts.json. (Of course you can also set the permalink to anything you want, regardless of the filename itself.)

The second option is to switch template engines using front matter or site-wide configuration. That will allow you to write posts.json and have it use ERB automatically (instead of the default which is Liquid). Find out more about choosing template engines here.

The link_to and url_for helpers let you create anchor tags which will link to any source page/document/static file (or any relative/absolute URL you pass in).

To link to source content, pass in a path to file in your src folder that translates to a published URL. For example, if you have a blog post saved at src/_posts/

<%= link_to "Click me!", "_posts/" %>

<!-- output: -->
<a href="/blog/my-nifty-article">Click me!</a>

The link_to helper uses url_for, so you can use that to get the url directly:

<% article_url = url_for("_posts/") %>

Note that url_for is also aliased to link in order to provide compatibility with the link Liquid tag.

You can pass additional keyword arguments to link_to which will be translated to HTML attributes:

<%= link_to "Join our livestream!", "_events/", class: "event", data_expire: "2020-11-08" %>

<!-- output: -->
<a href="/events/livestream" class="event" data-expire="2020-11-08">Join our livestream!</a>

In order to simplify more complex lists of HTML attributes you may also pass a hash as the value of one of the keyword arguments. This will convert all pairs in the hash into HTML attributes and prepend each key in the hash with the keyword argument:

<%= link_to "Join our livestream!", "_events/", data: { controller: "testable", action: "testable#test" } %>

<!-- output: -->
<a href="/events/livestream" data-controller="testable" data-action="testable#test">Join our livestream!</a>

link_to uses html_attributes under the hood to handle this converstion.

You can also pass relative or aboslute URLs to link_to and they’ll just pass-through to the anchor tag without change:

<%= link_to "Visit Bridgetown", "" %>

Finally, if you pass a Ruby object (i.e., it responds to url), it will work as you’d expect:

<%= link_to "My last page", collections.pages.resources.last %>

<!-- output: -->
<a href="/this/is/my-last-page">My last page</a>

Slotted Content #

You can contain portions of content in a template file (whether for pages, layouts, or another resources) within “slots”. These content slots can then be rendered higher up the rendering pipeline. For example, a resource can define a slot, and its layout can render it. Or a layout itself can define a slot and its parent layout can render it. You can render slots within partials as well.

Bridgetown’s Ruby components also has its own slotting mechanism.

Here’s an example of using slots in ERB templates to relocate page-specific styles up to the HTML <head>.

In your src/_partials/head.erb file, append the following:

<%= slotted :html_head %>

Then on one of your ERB pages, try adding something like:

<% slot :html_head do %>
    h1 {
      color: navy;
<% end %>

You’ll then be able to verify that the new style tag only gets rendered out in <head> for the particular page where the slot is provided.

Slotted content will automatically adhere to the format of the context where slot is called. In other words, if you’re in a Markdown file, the slotted content will also be converted from Markdown to HTML. (Additional converter plugins will need to opt-in to support this feature.) To disable this functionality, pass transform: false.

The slotted helper can also provide default content should the slot not already be defined:

<%= slotted :aside do %>
  <p>This only displays if there's no "aside" slot defined.</p>
<% end %>

Multiple captures using the same slot name will be cumulative. The above aside slot could be appended to by calling slot :aside multiple times. If you wish to change this behavior, you can pass replace: true as a keyword argument to slot to clear any previous slot content. Use with extreme caution!

For more control over slot content, you can use the pre_render and post_render hooks. Builders can register hooks to transform slots in specific ways based on their name or context:

class Builders::BeamMeUpSlotty < SiteBuilder
  def build
    hook :slots, :pre_render do |slot|
      slot.content.upcase! if == "upcase_me"

Within the hook, you can call slot.context to access the definition context for that slot (a resource, a layout, etc.).

Both slot and slotted accept an argument instead of a block for content. So you could call <% slot :slotname, "Here's some content" %> rather than supplying a block, or even pass in something like front matter data!

Don’t let the naming fool you…Bridgetown’s slotted content feature is not related to the concept of slots in custom elements and shadow DOM (aka web components). But there are some surface-level similarities. Many view-related frameworks provide some notion of slots (perhaps called something else like content or layout blocks), as it’s helpful to be able to render named “child” content within “parent” views. If you’re looking for information on using actual HTML slots, check out our new Declarative Shadow DOM documentation.

Other HTML Helpers #

html_attributes #

Available as part of the bundled Streamlined gem.

html_attributes allows you to pass a hash and have it converted to a string of HTML attributes:

<p <%= html_attributes({ class: "my-class", id: "some-id" }) %>>Hello, World!</p>

<!-- output: -->
<p class="my-class" id="some-id">Hello, World!</p>

html_attributes also allows for any value of the passed hash to itself be a hash. This will result in individual attributes being created from each pair in the hash. When doing this, the key the hash was paired with will be prepended to each attribute name:

<button <%= html_attributes({ data: { controller: "clickable", action: "click->clickable#test" } }) %>>Click Me!</button>

<!-- output: -->
<button data-controller="clickable" data-action="click->clickable#test">Click Me!</button>

capture #

If you need to capture a part of your template and store it in a variable for later use, you can use the capture helper.

<% test_capturing = capture do %>
  This is how <%= "#{"cap"}turing" %> works!
<% end %>

<%= test_capturing.reverse %>

One interesting use case for capturing is you could assign the captured text to a layout data variable. Using memoization, you could calculate an expensive bit of template once and then reuse it either in that layout or in a partial.


<% # add this code to a layout: %>
<%[:save_this_for_later] ||= capture do
  puts "saving this into the layout!"
%>An <%= "expensive " + "routine" %> to be saved<% end %>

Some text...

<%= partial "use_the_saved_variable" %>
<% # src/_partials/_use_the_saved_variable.erb %>
Print this: <%=[:save_this_for_later] %>

Because of the use of the ||= operator, you’ll only see “saving this into the layout!” print to the console once when the site builds even if you use the layout on thousands of pages!

Custom Helpers #

If you’d like to add your own custom template helpers, you can use the helper DSL within builder plugins. Read this documentation to learn more.

Alternatively, you could open up the Helpers class and define additional methods:

# plugins/site_builder.rb

Bridgetown::RubyTemplateView::Helpers.class_eval do
  def uppercase_string(input)
<%= uppercase_string "i'm a string" %>

<!-- output: -->

As a best practice, it would be best to define your helpers as methods of a dedicated Module which could then be used for both Liquid filters and ERB helpers simultaneously. Here’s how you might go about that in your plugin:

# plugins/filters.rb

module MyFilters
  def lowercase_string(input)

Liquid::Template.register_filter MyFilters
Bridgetown::RubyTemplateView::Helpers.include MyFilters

Usage is pretty straightforward:

<%= lowercase_string "WAY DOWN LOW" %>
{{ "WAY DOWN LOW" | lowercase_string }}

Escaping and HTML Safety #

The ERB template engine uses a safe output buffer—the same one used in Rails.

That means that you’ll sometimes find that if you simply output a front matter variable or some other string value that contains HTML tags and entities, the string will be “escaped” so that the actual angle brackets and so forth are displayed in the website content (rather than being interpreted as valid HTML tags).

Often that’s the right call for security purposes to avoid XSS attacks or to bypass potential markup errors. However, to explicitly mark a string as safe, you can use the html_safe method. Bridgetown provides the raw or safe helpers as well. You can also use a double-equals sign to bypass escaping entirely.

<%= some_value.html_safe %>
<!-- or -->
<%= raw some_value %>
<!-- or -->
<%= safe some_value %>
<-- or -->
<%== some_value %>

Note that using html_safe directly requires the value to be a string already. If you use the raw/safe helpers, it will first perform to_s automatically. Also bear in mind that <%= yield %> or <%= content %> or rendering components/partials won’t perform escaping on the rendered template output. (This is for obvious reasons—otherwise you’d get a visual mess of escaped HTML tags.)

If you find a particular use case where escaping occurs (or doesn’t occur) in an unexpected manner, please file a bug report in the Bridgetown GitHub repo.

When Using Serbea #

Serbea only escapes values by default when using the double-braces syntax {{ }}. When using {%= %}, escaping does not occur by default.

str = "<p>Escape me!</p>"

{{ str }} <!-- output: &lt;p&gt;Escape me!&lt;/p&gt; -->
{%= str %} <!-- output: <p>Escape me!</p> -->

To explicitly escape a value when using percent signs, use the escape or h helper. To explicitly mark a value as safe when using double-braces, use the safe or raw filter:

str = "<p>Escape me!</p>"

{{ str | safe }} <!-- output: <p>Escape me!</p> -->
{%= escape(str) %} <!-- output: &lt;p&gt;Escape me!&lt;/p&gt; -->

Haml and Slim #

Bridgetown comes with ERB support out-of-the-box, but you can easily add support for either Haml or Slim by installing our officially supported plugins.

All you’d need to do is run bundle add bridgetown-haml (or bridgetown-slim) and add init :"bridgetown-haml" or init :"bridgetown-slim" to config/initializers.rb, and then you can immediately start using .haml or .slim pages, layouts, partials, and components in your Bridgetown site.

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