When creating HTML templates with Rails, my go-to templating language is the default: ERB. It’s an incredibly powerful yet simple way of creating dynamic documents and it’s built right in to Ruby. What’s not to like? Until recently, I had never used ERB templates outside of Rails. Here are a few things I learned:

First things first

Getting started with ERB is easy. Just initialize a new ERB object with a string template, and then call #result on the ERB object, like so:

template = "Hello, today is <%= Time.now.strftime('%A') %>"
ERB.new(template).result
# => "Hello, today is Sunday"

Templates are just strings

Chances are you don’t want to have to define an ERB template in the same file as you render the result. That was my main hangup: how was ERB going to get my template file? Then, I learned about the File library and something clicked: “I’m working with strings!” The example changes to something like this:

# template.erb

Hello, today is <%= Time.now.strftime('%A') %>

# renderer.rb

template = File.open('template.erb', 'rb', &:read)
ERB.new(template).result
# => "Hello, today is Sunday"

Binding

Now that we can read a template from a file, what about being able to call variables and methods from my template? In Rails, I would define my variables in the Controller and methods in Helpers, but I’m not using Rails. Then I learned about Binding, which is the way to pass the “execution context” of some code around. Execution context is just a fancy way of saying variables and methods. Any variables and methods defined in the class that you call #binding on, as well as any methods defined in any included module, get passed to ERB, ready to use.

# template.erb

Hello <%= name %>, today is <%= Time.now.strftime('%A') %>.

<%= benediction %>

# renderer.rb

class Renderer 
  attr_reader :template, :name

  def initialize
    @template = File.open('template.erb', 'rb', &:read)
    @name = "Jonathan"
  end

  def benediction
    Time.now.hour >= 12 ? "Have a wonderful afternoon!" : "Have a wonderful morning!"
  end

  def render
    ERB.new(template).result(binding)
  end
end

Renderer.new.render 
# => "Hello Jonathan, today is Sunday.\n\nHave a wonderful afternoon!"

The Renderer class above works, but it isn’t ideal. It doesn’t conform to the Single Responsibility Principal, putting both rendering the template and providing the execution context. Let’s refactor to fix this:

# view.rb
def View
  attr_reader :name

  def initialize
    @name = "Jonathan"
  end

  def benediction
    Time.now.hour >= 12 ? "Have a wonderful afternoon!" : "Have a wonderful morning!"
  end

  def get_binding
    binding
  end

  def build
    Renderer.new(self)
  end
end

# renderer.rb
class Renderer 
  attr_reader :template, :binding_klass

  def initialize(binding_klass)
    @template = File.open('template.erb', 'rb', &:read)
    @binding_klass = binding_klass
  end

  def render
    ERB.new(template).result(binding_klass.get_binding)
  end
end

Since the execution context that the template needs is now outside of Renderer, I have to pass in self to Renderer from View. In View, you’ll see that I also defined a method called get_binding which just returns the binding. Why couldn’t I just call binding directly on the instance of View I passed in? Let’s try it:

def render
  ERB.new(template).result(binding_klass.binding)
end

Renderer.new.render
# => NoMethodError: private method `binding' called

So, binding is private, but we can access it through a method like get_binding.

Encoding

Finally, let’s spice up this template with an emoji. Emoji, after all, are just Unicode code points. Lets see:

# template.erb

Hello <%= name %> 😬, today is <%= Time.now.strftime('%A') %>.

Renderer.new.render
# =>  incompatible character encodings: ASCII-8BIT and UTF-8

That’s not what I expected! I eventually figured out that I needed to change the way that I was opening my template file to include an encoding:

File.open("template-file-path", 'rb', encoding: 'utf-8', &:read)

Opening the template as a utf-8 file makes everything run perfectly.

Rendering ERB for Fun and Profit

Rendering ERB templates without Rails is a little daunting at first. Once you get the hang of it, it’s as powerful and simple as ERB itself.

If you want to read more about ERB without Rails, I highly recommend this article by Stuart Ellis.