Add unit tests for to_html_lang filter to ensure correctness and Liquid integration

Author: liberaldev

test/test_plugin_html_lang.rb

@@ -0,0 +1,39 @@
+# frozen_string_literal: true
+
+require "helper"
+require "liquid"
+require_relative "../_plugins/html_lang"
+
+describe Jekyll::HtmlLangFilter do
+  include Jekyll::HtmlLangFilter
+
+  describe "#to_html_lang" do
+    it "returns the same string if no underscore is present" do
+      _(to_html_lang("en")).must_equal "en"
+      _(to_html_lang("fr")).must_equal "fr"
+    end
+
+    it "converts Jekyll locale with underscore to BCP 47 format" do
+      _(to_html_lang("zh_cn")).must_equal "zh-CN"
+      _(to_html_lang("zh_tw")).must_equal "zh-TW"
+      _(to_html_lang("pt_br")).must_equal "pt-BR"
+    end
+
+    it "returns the input as is if it is not a string" do
+      _(to_html_lang(nil)).must_be_nil
+      _(to_html_lang(123)).must_equal 123
+    end
+  end
+
+  describe "integration with Liquid" do
+    it "is registered as a liquid filter" do
+      template = Liquid::Template.parse("{{ 'zh_cn' | to_html_lang }}")
+      _(template.render).must_equal "zh-CN"
+    end
+
+    it "works correctly in a liquid template for simple lang" do
+      template = Liquid::Template.parse("{{ 'en' | to_html_lang }}")
+      _(template.render).must_equal "en"
+    end
+  end
+end