Phlex appends strings

When I evaluate code libraries, I like to open it up and get a sense of what it does at its very core. At the heart of Phlex is a string that Phlex appends HTML to for rendering in the browser. That string is named buffer, which is defaulted to +"".

# frozen_string_literal: true

class Phlex::SGML
  # ...
  def call(buffer = +"", context: {}, fragments: nil, &)
    state = Phlex::SGML::State.new(
        user_context: context,
        output_buffer: buffer,
        fragments: fragments&.to_set,
    )

    internal_call(parent: nil, state:, &)

    state.output_buffer << state.buffer
  end
  # ...
end

What is +""?

I never bothered to understand what +"" meant in Ruby until I looked under the hood of Phlex. You know that annoying # frozen_string_literal: true you see at the top of Ruby files? That tells Ruby that you can’t modify the string in place with operators like "fizz".upcase! or "fizz".concat("buzz").