use terser rather than uglifier

Uglifier(harmony: true) fails; :uglifier does not suit for ES6

See: lautis/uglifier#185

Gemfile

@@ -6,6 +6,7 @@ gem 'rails',                          '~> 6.0'
 
 gem 'sass-rails'
 gem 'uglifier'
+gem 'terser'
 gem 'jquery-rails',                   '~> 4'
 gem 'jquery-ui-rails',                '~> 5'
 gem 'turbolinks',                     '~> 5'

Gemfile.lock

@@ -216,8 +216,11 @@ GEM
       kaminari-core (= 1.2.2)
     kaminari-core (1.2.2)
     libdatadog (2.0.0.1.0)
+    libdatadog (2.0.0.1.0-x86_64-linux)
     libddwaf (1.6.2.0.0)
       ffi (~> 1.0)
+    libddwaf (1.6.2.0.0-x86_64-linux)
+      ffi (~> 1.0)
     lograge (0.12.0)
       actionpack (>= 4)
       activesupport (>= 4)
@@ -279,6 +282,8 @@ GEM
     nokogiri (1.14.3)
       mini_portile2 (~> 2.8.0)
       racc (~> 1.4)
+    nokogiri (1.14.3-x86_64-linux)
+      racc (~> 1.4)
     optimist (3.0.1)
     parallel (1.23.0)
     parser (3.2.2.0)
@@ -451,6 +456,8 @@ GEM
       attr_extras (>= 6.2.4)
       diff-lcs
       patience_diff
+    terser (1.1.14)
+      execjs (>= 0.3.0, < 3)
     thor (1.2.1)
     tilt (2.1.0)
     timecop (0.9.6)
@@ -552,6 +559,7 @@ DEPENDENCIES
   spring (~> 2.1.0)
   spring-commands-rspec
   super_diff
+  terser
   timecop
   tinymce-rails
   turbolinks (~> 5)

config/environments/production.rb

@@ -26,7 +26,7 @@
   config.public_file_server.enabled = ENV['RAILS_SERVE_STATIC_FILES'].present?
 
   # Compress JavaScripts and CSS.
-  config.assets.js_compressor = Uglifier.new(harmony: true)
+  config.assets.js_compressor = :terser
   # config.assets.css_compressor = :sass
 
   # Do not fallback to assets pipeline if a precompiled asset is missed.