Use terser for javascript compression to get ES6 support

Gemfile

@@ -12,8 +12,8 @@ gem "pg"
 # Use SCSS for stylesheets
 gem "sassc-rails"
 
-# Use Uglifier as compressor for JavaScript assets
-gem "uglifier", ">= 1.3.0"
+# Use Terser as compressor for JavaScript assets
+gem "terser"
 
 # Use jquery as the JavaScript library
 gem "jquery-rails"

Gemfile.lock

@@ -468,12 +468,12 @@ GEM
       sprockets (>= 3.0.0)
     strong_migrations (0.7.7)
       activerecord (>= 5)
+    terser (1.1.5)
+      execjs (>= 0.3.0, < 3)
     thor (1.1.0)
     tilt (2.0.10)
     tzinfo (2.0.4)
       concurrent-ruby (~> 1.0)
-    uglifier (4.2.0)
-      execjs (>= 0.3.0, < 3)
     unicode-display_width (2.0.0)
     validates_email_format_of (1.6.3)
       i18n
@@ -572,7 +572,7 @@ DEPENDENCIES
   simplecov
   simplecov-lcov
   strong_migrations
-  uglifier (>= 1.3.0)
+  terser
   validates_email_format_of (>= 1.5.1)
   vendorer
   webmock

config/environments/production.rb

@@ -25,7 +25,7 @@ Rails.application.configure do
   config.public_file_server.enabled = ENV["RAILS_SERVE_STATIC_FILES"].present?
 
   # Compress JavaScripts and CSS.
-  config.assets.js_compressor = :uglifier
+  config.assets.js_compressor = Terser.new
   # config.assets.css_compressor = :sass
 
   # Do not fallback to assets pipeline if a precompiled asset is missed.