Comparison of the various ERB trim modes available

README.md

I found the ERB documentation about the various trim modes confusing.

I've written a number of tests that attempt to demonstrate the trim mode behaviour, and optional percentage sign behaviour.

I think I have a better idea of what's going on but I'm still not sure I could do a very good job of describing what's happening.

I've taken the names of the trim modes from the ERB source code (see comments around TrimScanner#initialize).

Gemfile

source 'https://rubygems.org'

gem 'minitest'

Gemfile.lock

GEM
  remote: https://rubygems.org/
  specs:
    minitest (5.6.1)

PLATFORMS
  ruby

DEPENDENCIES
  minitest

test_erb_trim_modes.rb

require 'minitest/autorun'
require 'erb'

class ErbTest < Minitest::Test
  def test_all_trim_modes_on_single_line_ruby_code_and_expressions
    erb_template = "<% if true %>
<%= hello_world_method %>
<% end %>"

    assert_equal "\nhello world\n", output_with_trim_mode(erb_template, nil)
    assert_equal "\nhello world\n", output_with_trim_mode(erb_template, '-')
    assert_equal "hello world", output_with_trim_mode(erb_template, '>')
    assert_equal "hello world", output_with_trim_mode(erb_template, '<>')
  end

  def test_all_trim_modes_on_multi_line_ruby_code_and_expressions
    erb_template = "<%
if true
%>
<%=
hello_world_method
%>
<%
end
%>"

    assert_equal "\nhello world\n", output_with_trim_mode(erb_template, nil)
    assert_equal "\nhello world\n", output_with_trim_mode(erb_template, '-')
    assert_equal "hello world", output_with_trim_mode(erb_template, '>')
    assert_equal "\nhello world\n", output_with_trim_mode(erb_template, '<>')
  end

  def test_explicit_trim_mode_on_ruby_expression
    erb_template = "<% if true %>
<%= hello_world_method -%>
<% end %>
"

    assert_equal "\nhello world\n", output_with_trim_mode(erb_template, '-')
  end

  def test_explicit_trim_mode_on_ruby_expression_and_if_ruby_statement
    erb_template = "<% if true -%>
<%= hello_world_method -%>
<% end %>
"

    assert_equal "hello world\n", output_with_trim_mode(erb_template, '-')
  end

  def test_explicit_trim_mode_on_ruby_expression_and_end_ruby_statement
    erb_template = "<% if true %>
<%= hello_world_method -%>
<% end -%>
"

    assert_equal "\nhello world", output_with_trim_mode(erb_template, '-')
  end

  def test_explicit_trim_mode_on_ruby_expression_and_if_and_end_ruby_statements
    erb_template = "<% if true -%>
<%= hello_world_method -%>
<% end -%>
"

    assert_equal 'hello world', output_with_trim_mode(erb_template, '-')
  end

  def test_combining_percentage_and_no_trim_mode
    erb_template = "% if true
<%= hello_world_method %>
% end"

    assert_equal "hello world\n", output_with_trim_mode(erb_template, '%')
  end

  def test_combining_percentage_and_trim_line_1
    erb_template = "% if true
<%= hello_world_method %>
% end"

    assert_equal 'hello world', output_with_trim_mode(erb_template, '%>')
  end

  def test_combining_percentage_and_trim_line_2
    erb_template = "% if true
<%= hello_world_method %>
% end"

    assert_equal 'hello world', output_with_trim_mode(erb_template, '%<>')
  end

  def test_combining_percentage_and_explicit_trim_line
    erb_template = "% if true
<%= hello_world_method -%>
% end"

    assert_equal 'hello world', output_with_trim_mode(erb_template, '%-')
  end

  private

  def hello_world_method
    'hello world'
  end

  def output_with_trim_mode(erb_template, trim_mode)
    safe_mode = nil
    ERB.new(erb_template, safe_mode, trim_mode).result(binding)
  end
end