You don't need any 3rd plugin plugins to Quick Look source code/ASCII text  files. Invoking Quick Look by pressing the spacebar on JavaScript/CSS source code file (which are plain text ASCII encoded) will show you the plain source code text.

Here's what QuickLook shows when invoking Quick Look on a JavaScript file named `callback.js`:

[![enter image description here][1]][1]

Installing QLColorCode using Homebrew by running `brew cask install qlcolorcode` and invoking Quick Look on the same file immediately thereafter shows this color coded output:

[![enter image description here][2]][2]

Follow the installation instructions on the [project homepage](https://github.com/anthonygelibert/QLColorCode) to make sure your have correctly installed the plugin:

> This is a Quick Look plugin that renders source code with syntax highlighting, using the [Highlight library](http://www.andre-simon.de/).
>
> To install Highlight, [download the library manualy](http://www.andre-simon.de/zip/download.php), or use homebrew `brew install highlight`
>
> To install the plugin, just drag it to `~/Library/QuickLook`. You may need to create that folder if it doesn't already exist.
>
> Alternative, if you use [Homebrew-Cask](https://github.com/caskroom/homebrew-cask), install with `brew cask install qlcolorcode`.


  [1]: https://i.sstatic.net/Pdvw8.png
  [2]: https://i.sstatic.net/y4hxX.png