Add yarn as option for package.json dependency manager detection

app/models/blob_viewer/package_json.rb

@@ -7,11 +7,15 @@ module BlobViewer
     self.file_types = %i(package_json)
 
     def manager_name
-      'npm'
+      yarn? ? 'yarn' : 'npm'
+    end
+
+    def yarn?
+      json_data['engines'].present? && json_data['engines']['yarn'].present?
     end
 
     def manager_url
-      'https://www.npmjs.com/'
+      yarn? ? 'https://yarnpkg.com/' : 'https://www.npmjs.com/'
     end
 
     def package_name
@@ -38,7 +42,11 @@ module BlobViewer
     end
 
     def npm_url
-      "https://www.npmjs.com/package/#{package_name}"
+      if yarn?
+        "https://yarnpkg.com/package/#{package_name}"
+      else
+        "https://www.npmjs.com/package/#{package_name}"
+      end
     end
   end
 end

spec/models/blob_viewer/package_json_spec.rb

@@ -27,11 +27,55 @@ RSpec.describe BlobViewer::PackageJson do
     end
   end
 
-  describe '#package_url' do
-    it 'returns the package URL' do
-      expect(subject).to receive(:prepare!)
+  context 'yarn' do
+    let(:data) do
+      <<-SPEC.strip_heredoc
+        {
+          "name": "module-name",
+          "version": "10.3.1",
+          "engines": {
+            "yarn": "^2.4.0"
+          }
+        }
+      SPEC
+    end
+
+    let(:blob) { fake_blob(path: 'package.json', data: data) }
+
+    subject { described_class.new(blob) }
+
+    describe '#package_url' do
+      it 'returns the package URL', :aggregate_failures do
+        expect(subject).to receive(:prepare!)
+
+        expect(subject.package_url).to eq("https://yarnpkg.com/package/#{subject.package_name}")
+      end
+    end
 
-      expect(subject.package_url).to eq("https://www.npmjs.com/package/#{subject.package_name}")
+    describe '#manager_url' do
+      it 'returns the manager URL', :aggregate_failures do
+        expect(subject).to receive(:prepare!)
+
+        expect(subject.manager_url).to eq("https://yarnpkg.com/")
+      end
+    end
+  end
+
+  context 'npm' do
+    describe '#package_url' do
+      it 'returns the package URL', :aggregate_failures do
+        expect(subject).to receive(:prepare!)
+
+        expect(subject.package_url).to eq("https://www.npmjs.com/package/#{subject.package_name}")
+      end
+    end
+
+    describe '#manager_url' do
+      it 'returns the manager URL', :aggregate_failures do
+        expect(subject).to receive(:prepare!)
+
+        expect(subject.manager_url).to eq("https://www.npmjs.com/")
+      end
     end
   end