generator: switch install prerequisite from yarn to npm and update specs

Author: ihabadham

lib/generators/react_on_rails/install_generator.rb

@@ -61,13 +61,13 @@ def invoke_generators
       # js(.coffee) are not checked by this method, but instead produce warning messages
       # and allow the build to continue
       def installation_prerequisites_met?
-        !(missing_node? || missing_yarn? || ReactOnRails::GitUtils.uncommitted_changes?(GeneratorMessages))
+        !(missing_node? || missing_npm? || ReactOnRails::GitUtils.uncommitted_changes?(GeneratorMessages))
       end
 
-      def missing_yarn?
-        return false unless ReactOnRails::Utils.running_on_windows? ? `where yarn`.blank? : `which yarn`.blank?
+      def missing_npm?
+        return false unless ReactOnRails::Utils.running_on_windows? ? `where npm`.blank? : `which npm`.blank?
 
-        error = "yarn is required. Please install it before continuing. https://yarnpkg.com/en/docs/install"
+        error = "npm is required. Please install it before continuing. https://docs.npmjs.com/downloading-and-installing-node-js-and-npm"
         GeneratorMessages.add_error(error)
         true
       end

spec/react_on_rails/generators/install_generator_spec.rb

@@ -78,8 +78,8 @@
 
     specify "when npm is exist" do
       stub_const("RUBY_PLATFORM", "linux")
-      allow(install_generator).to receive(:`).with("which yarn").and_return("/path/to/bin")
-      expect(install_generator.send(:missing_yarn?)).to be false
+      allow(install_generator).to receive(:`).with("which npm").and_return("/path/to/bin")
+      expect(install_generator.send(:missing_npm?)).to be false
     end
   end
 
@@ -94,8 +94,8 @@
 
     specify "when npm is missing" do
       stub_const("RUBY_PLATFORM", "linux")
-      allow(install_generator).to receive(:`).with("which yarn").and_return("")
-      expect(install_generator.send(:missing_yarn?)).to be true
+      allow(install_generator).to receive(:`).with("which npm").and_return("")
+      expect(install_generator.send(:missing_npm?)).to be true
     end
   end
 
@@ -110,8 +110,8 @@
 
     specify "when npm is exist" do
       stub_const("RUBY_PLATFORM", "mswin")
-      allow(install_generator).to receive(:`).with("where yarn").and_return("/path/to/bin")
-      expect(install_generator.send(:missing_yarn?)).to be false
+      allow(install_generator).to receive(:`).with("where npm").and_return("/path/to/bin")
+      expect(install_generator.send(:missing_npm?)).to be false
     end
   end
 
@@ -124,10 +124,10 @@
       expect(install_generator.send(:missing_node?)).to be true
     end
 
-    specify "when yarn is missing" do
+    specify "when npm is missing" do
       stub_const("RUBY_PLATFORM", "mswin")
-      allow(install_generator).to receive(:`).with("where yarn").and_return("")
-      expect(install_generator.send(:missing_yarn?)).to be true
+      allow(install_generator).to receive(:`).with("where npm").and_return("")
+      expect(install_generator.send(:missing_npm?)).to be true
     end
   end
 end