[解決済み] gem 'uglifier'をロードしようとしたときにエラーが発生しました。(Bundler::GemRequireError)。

質問

なぜこのエラーが発生するのでしょうか？

shibly@mybox:~/blog$ rails server
/home/shibly/.gem/ruby/2.2.4/gems/bundler-1.11.2/lib/bundler/runtime.rb:80:in `rescue in block (2 levels) in require': There was an error while trying to load the gem 'uglifier'. (Bundler::GemRequireError)
    from /home/shibly/.gem/ruby/2.2.4/gems/bundler-1.11.2/lib/bundler/runtime.rb:76:in `block (2 levels) in require'
    from /home/shibly/.gem/ruby/2.2.4/gems/bundler-1.11.2/lib/bundler/runtime.rb:72:in `each'
    from /home/shibly/.gem/ruby/2.2.4/gems/bundler-1.11.2/lib/bundler/runtime.rb:72:in `block in require'
    from /home/shibly/.gem/ruby/2.2.4/gems/bundler-1.11.2/lib/bundler/runtime.rb:61:in `each'
    from /home/shibly/.gem/ruby/2.2.4/gems/bundler-1.11.2/lib/bundler/runtime.rb:61:in `require'
    from /home/shibly/.gem/ruby/2.2.4/gems/bundler-1.11.2/lib/bundler.rb:99:in `require'
    from /home/shibly/blog/config/application.rb:7:in `<top (required)>'
    from /home/shibly/.gem/ruby/2.2.4/gems/railties-4.2.5/lib/rails/commands/commands_tasks.rb:78:in `require'
    from /home/shibly/.gem/ruby/2.2.4/gems/railties-4.2.5/lib/rails/commands/commands_tasks.rb:78:in `block in server'
    from /home/shibly/.gem/ruby/2.2.4/gems/railties-4.2.5/lib/rails/commands/commands_tasks.rb:75:in `tap'
    from /home/shibly/.gem/ruby/2.2.4/gems/railties-4.2.5/lib/rails/commands/commands_tasks.rb:75:in `server'
    from /home/shibly/.gem/ruby/2.2.4/gems/railties-4.2.5/lib/rails/commands/commands_tasks.rb:39:in `run_command!'
    from /home/shibly/.gem/ruby/2.2.4/gems/railties-4.2.5/lib/rails/commands.rb:17:in `<top (required)>'
    from bin/rails:9:in `require'
    from bin/rails:9:in `<main>

どのように解決するのですか？

UbuntuにNodeJSをインストールするには、以下のコマンドを実行する必要があります。

sudo apt-get install nodejs

または、OSXでは次のように実行します。

brew install nodejs

UglifierはJSラッパーなので、JSランタイムの実行またはJSインタプリタが必要です。私ならNodeJSをインストールすることを選択します。