In config/environments/production.rb, add config.assets.uglifier = { …

…harmony: true }

Doing this due to error popping up on testing heroku deployment.
See here: AlexanderPavlenko/sprockets_uglifier_with_source_maps#9

config/environments/production.rb

@@ -24,6 +24,7 @@
   config.public_file_server.enabled = ENV['RAILS_SERVE_STATIC_FILES'].present?
 
   # Compress JavaScripts and CSS.
+  config.assets.uglifier = { harmony: true }
   config.assets.js_compressor = :uglifier
   # config.assets.css_compressor = :sass