feat: check PACKAGE_JSON_FALLBACK_MANAGER for default fallback manager

README.md

@@ -90,8 +90,11 @@ does not exist or does not have the `packageManager` property:
 PackageJson.new(fallback_manager: :yarn_classic)
 ```
 
-The default fallback is `:npm`; supported package managers are `:npm`,
-`:yarn_classic`, and `:pnpm`.
+If a fallback manager is not defined, then the value of the env variable
+`PACKAGE_JSON_FALLBACK_MANAGER` will be used if set, or otherwise falling back
+to `npm`.
+
+Supported package managers are `:npm`, `:yarn_classic`, and `:pnpm`.
 
 If the `package.json` does not exist, then the `packageManager` property will be
 included based on this value, but it will _not_ be updated if the file already

lib/package_json.rb

@@ -14,15 +14,19 @@ class NotImplementedError < Error; end
 
   attr_reader :manager, :path
 
-  def self.read(path_to_directory = Dir.pwd, fallback_manager: :npm)
+  def self.fetch_default_fallback_manager
+    ENV.fetch("PACKAGE_JSON_FALLBACK_MANAGER", "npm").to_sym
+  end
+
+  def self.read(path_to_directory = Dir.pwd, fallback_manager: PackageJson.fetch_default_fallback_manager)
     unless File.exist?("#{path_to_directory}/package.json")
       raise Error, "#{path_to_directory} does not contain a package.json"
     end
 
     new(path_to_directory, fallback_manager: fallback_manager)
   end
 
-  def initialize(path_to_directory = Dir.pwd, fallback_manager: :npm)
+  def initialize(path_to_directory = Dir.pwd, fallback_manager: PackageJson.fetch_default_fallback_manager)
     @path = path_to_directory
 
     ensure_package_json_exists(fallback_manager)

sig/package_json.rbs

@@ -10,6 +10,8 @@ class PackageJson
   attr_reader manager: Managers::Base
   attr_reader path: String
 
+  def self.fetch_default_fallback_manager: () -> Symbol
+
   def self.read: (?String path_to_directory, ?package_manager: (:npm | :yarn_classic | :pnpm)) -> PackageJson
 
   def initialize: (?String path_to_directory, ?package_manager: (:npm | :yarn_classic | :pnpm)) -> PackageJson