Using Sprockets to compile javascript assets

By default (in Blacklight 7) the assets generator adds the following things to your application to enable Sprockets to build a javascript bundle:

Added to the Gemfile:

gem 'bootstrap', '~> 4.0'
gem 'popper_js'
gem 'twitter-typeahead-rails', '0.11.1.pre.corejavascript'

These requires are injected into app/assets/javascripts/application.js:

//= require jquery3
//= require popper
//= require twitter/typeahead
//= require bootstrap

If you are using Uglifier in production (rails default) and you encounter this error:

Uglifier::Error: Unexpected token: operator (>). To use ES6 syntax, harmony mode must be enabled with Uglifier.new(:harmony => true).

Then change this line in config/environments/production.rb:

  config.assets.js_compressor = :uglifier

to

  config.assets.js_compressor = Uglifier.new(harmony: true)