Why does rails require JavaScript Runtime?

Solution 1:

Look into the Rails Asset Pipeline. It does fancy stuff with JavaScript (and CSS) files. Notably:

1. It converts CoffeeScript into JavaScript
2. It combines all javascript files into one
3. It minifies that file

The JavaScript runtime is used for minification.

Solution 2:

Railsguides says it has to do with compression:

You will need an ExecJS supported runtime in order to use uglifier. If you are using Mac OS X or Windows you have a JavaScript runtime installed in your operating system. Check the ExecJS documentation for information on all of the supported JavaScript runtimes.

http://guides.rubyonrails.org/asset_pipeline.html#javascript-compression

I thought I had seen Rails actually check the validity of both the stylesheets and the javascript in your app, but maybe it was just SASS prepreocessing.