use terser instead of uglifier

Gemfile

@@ -21,6 +21,7 @@ gem "sassc-rails"
 gem "slimmer"
 gem "sprockets-rails"
 gem "uglifier"
+gem "terser"
 
 group :test do
   gem "cucumber-rails", require: false

Gemfile.lock

@@ -801,6 +801,7 @@ DEPENDENCIES
   simplecov-rcov
   slimmer
   sprockets-rails
+  terser
   timecop
   uglifier
   unparser

config/environments/production.rb

@@ -25,7 +25,7 @@
   config.public_file_server.enabled = ENV["RAILS_SERVE_STATIC_FILES"].present?
 
   # Compress JavaScript.
-  config.assets.js_compressor = :uglifier
+  config.assets.js_compressor = :terser
 
   # Compress CSS using a preprocessor.
   # config.assets.css_compressor = :sass

config/initializers/sprockets.rb

@@ -0,0 +1,17 @@
+# This adds terser as a recognised compressor
+# to Sprockets. Without this patch, Sprockets
+# will not be able to run terser.
+#
+# Code originates from:
+# https://stackoverflow.com/a/70086366
+
+require "terser"
+
+module Sprockets
+  class Environment < Base
+    def js_compressor=(compressor)
+      register_compressor "application/javascript", :terser, Terser::Compressor
+      super
+    end
+  end
+end