Run commands via yarn when not exist node_modules

Currently, commands(`webpack` and `webpacker-dev-server`) are need to exist
under the `node_modules/.bin` directory.

But when Yarn Plug'n'Play using, `node_modules/.bin` is not created.
Ref: https://github.com/yarnpkg/rfcs/blob/master/accepted/0000-plug-an-play.md

In this case, need to execute the command via `yarn`.
So if `node_modules` does not exist, executes the command via `yarn`.

Author: y-yagi

lib/webpacker/dev_server_runner.rb

@@ -45,15 +45,22 @@ def detect_port!
 
       def execute_cmd
         env = Webpacker::Compiler.env
-        cmd = [
-          "#{@node_modules_bin_path}/webpack-dev-server",
-          "--config", @webpack_config
-        ]
+
+        cmd = if node_modules_bin_exist?
+          ["#{@node_modules_bin_path}/webpack-dev-server"]
+        else
+          ["yarn", "webpack-dev-server"]
+        end
+        cmd += ["--config", @webpack_config]
         cmd += ["--progress", "--color"] if @pretty
 
         Dir.chdir(@app_path) do
-          exec env, *cmd
+          Kernel.exec env, *cmd
         end
       end
+
+      def node_modules_bin_exist?
+        File.exist?("#{@node_modules_bin_path}/webpack-dev-server")
+      end
   end
 end

lib/webpacker/webpack_runner.rb

@@ -5,11 +5,22 @@ module Webpacker
   class WebpackRunner < Webpacker::Runner
     def run
       env = Webpacker::Compiler.env
-      cmd = [ "#{@node_modules_bin_path}/webpack", "--config", @webpack_config ] + @argv
+
+      cmd = if node_modules_bin_exist?
+        ["#{@node_modules_bin_path}/webpack"]
+      else
+        ["yarn", "webpack"]
+      end
+      cmd += ["--config", @webpack_config] + @argv
 
       Dir.chdir(@app_path) do
-        exec env, *cmd
+        Kernel.exec env, *cmd
       end
     end
+
+    private
+      def node_modules_bin_exist?
+        File.exist?("#{@node_modules_bin_path}/webpack")
+      end
   end
 end

test/dev_server_runner_test.rb

@@ -0,0 +1,52 @@
+require "test_helper"
+require "webpacker/dev_server_runner"
+
+class DevServerRunnerTest < Webpacker::Test
+  def setup
+    @original_node_env, ENV["NODE_ENV"] = ENV["NODE_ENV"], "development"
+    @original_rails_env, ENV["RAILS_ENV"] = ENV["RAILS_ENV"], "development"
+  end
+
+  def teardown
+    ENV["NODE_ENV"] = @original_node_env
+    ENV["RAILS_ENV"] = @original_rails_env
+  end
+
+  def test_run_cmd_via_node_modules
+    app_path = File.expand_path(".", Dir.pwd)
+    cmd = ["#{app_path}/node_modules/.bin/webpack-dev-server", "--config", "#{test_app_path}/config/webpack/development.js"]
+
+    verify_command(cmd, use_node_modules: true)
+  end
+
+  def test_run_cmd_via_yarn
+    cmd = ["yarn", "webpack-dev-server", "--config", "#{test_app_path}/config/webpack/development.js"]
+
+    verify_command(cmd, use_node_modules: false)
+  end
+
+  private
+    def test_app_path
+      File.expand_path("test_app", __dir__)
+    end
+
+    def verify_command(cmd, use_node_modules: true)
+      cwd = Dir.pwd
+      Dir.chdir(test_app_path)
+
+      klass = Webpacker::DevServerRunner
+      instance = klass.new([])
+      mock = Minitest::Mock.new
+      mock.expect(:call, nil, [{}, *cmd])
+
+      klass.stub(:new, instance) do
+        instance.stub(:node_modules_bin_exist?, use_node_modules) do
+          Kernel.stub(:exec, mock) { klass.run([]) }
+        end
+      end
+
+      mock.verify
+    ensure
+      Dir.chdir(cwd)
+    end
+end

test/test_app/config/webpack/development.js

@@ -0,0 +1,52 @@
+require "test_helper"
+require "webpacker/webpack_runner"
+
+class WebpackRunnerTest < Webpacker::Test
+  def setup
+    @original_node_env, ENV["NODE_ENV"] = ENV["NODE_ENV"], "development"
+    @original_rails_env, ENV["RAILS_ENV"] = ENV["RAILS_ENV"], "development"
+  end
+
+  def teardown
+    ENV["NODE_ENV"] = @original_node_env
+    ENV["RAILS_ENV"] = @original_rails_env
+  end
+
+  def test_run_cmd_via_node_modules
+    app_path = File.expand_path(".", Dir.pwd)
+    cmd = ["#{app_path}/node_modules/.bin/webpack", "--config", "#{test_app_path}/config/webpack/development.js"]
+
+    verify_command(cmd, use_node_modules: true)
+  end
+
+  def test_run_cmd_via_yarn
+    cmd = ["yarn", "webpack", "--config", "#{test_app_path}/config/webpack/development.js"]
+
+    verify_command(cmd, use_node_modules: false)
+  end
+
+  private
+    def test_app_path
+      File.expand_path("test_app", __dir__)
+    end
+
+    def verify_command(cmd, use_node_modules: true)
+      cwd = Dir.pwd
+      Dir.chdir(test_app_path)
+
+      klass = Webpacker::WebpackRunner
+      instance = klass.new([])
+      mock = Minitest::Mock.new
+      mock.expect(:call, nil, [{}, *cmd])
+
+      klass.stub(:new, instance) do
+        instance.stub(:node_modules_bin_exist?, use_node_modules) do
+          Kernel.stub(:exec, mock) { klass.run([]) }
+        end
+      end
+
+      mock.verify
+    ensure
+      Dir.chdir(cwd)
+    end
+end