Prepare for testing

Gemfile.lock

@@ -45,6 +45,8 @@ GEM
       parser (>= 3.1.1.0)
     rubocop-rake (0.6.0)
       rubocop (~> 1.0)
+    rubocop-rspec (2.11.1)
+      rubocop (~> 1.19)
     ruby-progressbar (1.11.0)
     unicode-display_width (2.1.0)
 
@@ -58,6 +60,7 @@ DEPENDENCIES
   rspec
   rubocop (~> 1.29)
   rubocop-rake (~> 0.6)
+  rubocop-rspec
 
 BUNDLED WITH
    2.1.4

format-staged.gemspec

@@ -20,9 +20,10 @@ Gem::Specification.new do |s|
   s.add_dependency 'colorize'
 
   s.add_development_dependency 'rake', '~> 13.0'
+  s.add_development_dependency 'rspec'
   s.add_development_dependency 'rubocop', '~> 1.29'
   s.add_development_dependency 'rubocop-rake', '~> 0.6'
-  s.add_development_dependency 'rspec'
+  s.add_development_dependency 'rubocop-rspec'
 
   s.metadata = {
     'rubygems_mfa_required' => 'true'

spec/git.rb

@@ -0,0 +1,72 @@
+# frozen_string_literal: true
+
+require 'tmpdir'
+require 'fileutils'
+require 'English'
+
+##
+# Test helpers for managing a git repository
+module Git
+  ##
+  # A git repository
+  class Repo
+    attr_reader :path
+
+    def initialize
+      @path = Dir.mktmpdir
+
+      git 'init'
+    end
+
+    def file_in_tree(name, content)
+      set_content name, content
+      stage name
+      commit "Add #{name}"
+    end
+
+    def set_content(name, content)
+      absolute = Pathname.new(path) + name
+
+      FileUtils.mkdir_p absolute.dirname
+      File.write absolute, content.end_with?("\n") ? content : "#{content}\n"
+    end
+
+    def stage(file)
+      git 'add', file
+    end
+
+    def commit(message)
+      git 'commit', '-m', message
+    end
+
+    def content(file)
+      git 'show', ":#{file}"
+    end
+
+    def cleanup
+      FileUtils.remove_entry path
+    end
+
+    def git(*cmd)
+      in_repo do
+        output = IO.popen(['git'] + cmd) do |io|
+          io.read.chomp
+        end
+
+        raise 'Failed to run git' unless $CHILD_STATUS.success?
+
+        output
+      end
+    end
+
+    def in_repo(&block)
+      Dir.chdir path, &block
+    end
+  end
+
+  def self.new_repo
+    repo = Repo.new
+    yield repo if block_given?
+    repo
+  end
+end

spec/test_spec.rb

@@ -0,0 +1,22 @@
+# frozen_string_literal: true
+
+require_relative 'git'
+
+require 'format_staged'
+
+describe FormatStaged do
+  def repo
+    @repo ||= Git.new_repo do |r|
+      r.file_in_tree 'index.js', <<~CONTENT
+        function foo () {
+          return 'foo'
+        }
+      CONTENT
+    end
+  end
+
+  after :each do
+    @repo&.cleanup
+    @repo = nil
+  end
+end