Local variables are one of the things CoffeeScript doesn't compile to
modern JavaScript automatically: it uses `var` instead of `const` or
`let`.
Besides, using `$this = $(this)` is usually done to reference the
current object in another function where the current object is a
different one. Here we were using it with no clear purpose.
These methods were added in 2009 and are supported by 98.5% of the
browsers, including Internet Explorer 9, 10 and 11. We were already
using them in some places.
