feat: support PackageJson~read

README.md

@@ -56,13 +56,22 @@ end
 package_json.manager.run("lint", ["--fix"])
 ```
 
-The `PackageJson` class represents a `package.json` on disk within a directory,
-creating the file if it does not already exist.
-
-Because it is expected that the `package.json` might be changed by external
+The `PackageJson` class represents a `package.json` on disk within a directory;
+because it is expected that the `package.json` might be changed by external
 sources such as package managers, `PackageJson` reads and writes to and from the
 `package.json` as needed rather than representing it in memory.
 
+You can initialize a `PackageJson` with either `new` or `read` depending on if
+you want to ensure the `package.json` exists or throw if it doesn't:
+
+```ruby
+# this will create the `package.json` if it does not exist
+PackageJson.new(:npm, "path/to/directory")
+
+# this will error if the `package.json` does not exist
+PackageJson.read("path/to/directory", :npm)
+```
+
 A `PackageJson` also comes with a `manager` that can be used to manage
 dependencies and run scripts. The manager will be inferred by the
 [`packageManager`](https://nodejs.org/api/packages.html#packagemanager) property

lib/package_json.rb

@@ -14,6 +14,14 @@ class NotImplementedError < Error; end
 
   attr_reader :manager, :path
 
+  def self.read(path_to_directory = Dir.pwd, fallback_manager = :npm)
+    unless File.exist?("#{path_to_directory}/package.json")
+      raise Error, "#{path_to_directory} does not contain a package.json"
+    end
+
+    new(fallback_manager, path_to_directory)
+  end
+
   def initialize(fallback_manager = :npm, path_to_directory = Dir.pwd)
     @path = path_to_directory
 

spec/package_json_spec.rb

@@ -9,6 +9,118 @@
     expect(PackageJson::VERSION).not_to be_nil
   end
 
+  describe ".read" do
+    context "when the package.json does not exist" do
+      it "raises an error" do
+        expect { described_class.read }.to raise_error(
+          PackageJson::Error, "#{Dir.pwd} does not contain a package.json"
+        )
+      end
+    end
+
+    context "when the package.json already exists with the packageManager property" do
+      it "does not error" do
+        with_package_json_file({ "version" => "1.0.0" }) do
+          expect { described_class.new }.not_to raise_error
+        end
+      end
+
+      it "uses the packageManager property" do
+        with_package_json_file({ "version" => "1.0.0", "packageManager" => "pnpm" }) do
+          package_json = described_class.new
+
+          expect(package_json.manager).to be_a PackageJson::Managers::PnpmLike
+        end
+      end
+
+      it "ignores the fallback manager" do
+        with_package_json_file({ "version" => "1.0.0", "packageManager" => "pnpm" }) do
+          package_json = described_class.new(:yarn_classic)
+
+          expect(package_json.manager).to be_a PackageJson::Managers::PnpmLike
+        end
+      end
+
+      it "supports having a version specified" do
+        with_package_json_file({ "version" => "1.0.0", "packageManager" => "[email protected]" }) do
+          package_json = described_class.new
+
+          expect(package_json.manager).to be_a PackageJson::Managers::PnpmLike
+        end
+      end
+
+      it "requires a major version for yarn" do
+        with_package_json_file({ "version" => "1.0.0", "packageManager" => "yarn" }) do
+          expect { described_class.new }.to raise_error(PackageJson::Error, "a major version must be present for Yarn")
+        end
+      end
+
+      it "only supports yarn v1" do
+        with_package_json_file({ "version" => "1.0.0", "packageManager" => "yarn@2" }) do
+          expect { described_class.new }.to raise_error(PackageJson::Error, "only Yarn classic is supported")
+        end
+      end
+
+      it "supports a full version being specified for yarn" do
+        with_package_json_file({ "version" => "1.0.0", "packageManager" => "[email protected]" }) do
+          package_json = described_class.new
+
+          expect(package_json.manager).to be_a PackageJson::Managers::YarnClassicLike
+        end
+      end
+
+      it "does not change the packageManager property" do
+        with_package_json_file({ "version" => "1.0.0", "packageManager" => "pnpm" }) do
+          described_class.new(:yarn_classic)
+
+          expect_package_json_with_content({ "version" => "1.0.0", "packageManager" => "pnpm" })
+        end
+      end
+
+      it "raises an error if the package manager is not supported" do
+        with_package_json_file({ "version" => "1.0.0", "packageManager" => "unknown" }) do
+          expect { described_class.new }.to raise_error(
+            PackageJson::Error,
+            'unsupported package manager "unknown"'
+          )
+        end
+      end
+    end
+
+    context "when the package.json already exists without the packageManager property" do
+      it "does not error" do
+        with_package_json_file({ "version" => "1.0.0" }) do
+          expect { described_class.new }.not_to raise_error
+        end
+      end
+
+      it "uses the fallback manager" do
+        with_package_json_file({ "version" => "1.0.0" }) do
+          package_json = described_class.new(:yarn_classic)
+
+          expect(package_json.manager).to be_a PackageJson::Managers::YarnClassicLike
+        end
+      end
+
+      it "does not add the packageManager property" do
+        with_package_json_file({ "version" => "1.0.0" }) do
+          described_class.new(:yarn_classic)
+
+          expect_package_json_with_content({ "version" => "1.0.0" })
+        end
+      end
+
+      it "raises an error if the fallback manager is not supported" do
+        with_package_json_file({ "version" => "1.0.0" }) do
+          expect { described_class.new(:unknown) }.to raise_error(
+            PackageJson::Error,
+            'unsupported package manager "unknown"'
+          )
+        end
+      end
+    end
+  end
+
   describe ".new" do
     context "when the package.json does not exist" do
       it "does not error" do