initialize: add path parameter (#5)

This adds an optional path parameter to the initialize method which
can be used to load alternate version of the Handlebars runtime.

Author: gi

lib/handlebars/engine.rb

@@ -16,7 +16,10 @@ class Engine
     #
     # @param lazy [true, false] immediately loads and initializes the JavaScript
     #   environment.
-    def initialize(lazy: false)
+    # @param path [String, nil] the path to the version of Handlebars to load.
+    #   If `nil`, the contents of `Handlebars::Source.bundled_path` is loaded.
+    def initialize(lazy: false, path: nil)
+      @path = path
       init! unless lazy
     end
 
@@ -204,7 +207,7 @@ def init!
       return if @init
 
       @context = MiniRacer::Context.new
-      @context.load(::Handlebars::Source.bundled_path)
+      @context.load(@path || ::Handlebars::Source.bundled_path)
       @context.load(File.absolute_path("engine/init.js", __dir__))
 
       @init = true

spec/handlebars/engine_spec.rb

@@ -1,5 +1,7 @@
 # frozen_string_literal: true
 
+require "tempfile"
+
 RSpec.describe Handlebars::Engine do
   let(:engine) { described_class.new(**engine_options) }
   let(:engine_context) { engine.instance_variable_get(:@context) }
@@ -41,6 +43,36 @@
         expect(engine_context).to be nil
       end
     end
+
+    context "when `path` is defined" do
+      let(:file) { Tempfile.open }
+
+      before do
+        engine_options[:path] = file.path
+        file.write <<~HANDLEBARS
+          var Handlebars = {
+            compile: () => "compile",
+            precompile: () => "precompile",
+            template: () => "template",
+            registerPartial: () => "registerPartial",
+            unregisterPartial: () => "unregisterPartial",
+            registerHelper: () => "registerHelper",
+            unregisterHelper: () => "unregisterHelper",
+            partials: {},
+            VERSION: "VERSION",
+          };
+        HANDLEBARS
+        file.rewind
+      end
+
+      after do
+        file.close
+      end
+
+      it "loads the file contents" do
+        expect(engine_context.eval("VERSION")).to eq("VERSION")
+      end
+    end
   end
 
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