Convert files to ES6

[elchroy]

Modify library and files to ES6 standard

The following updates were added.

• Change normal functions to arrow functions.
• Use template literals for string interpolation
• Use let for some variables.

[msfeldstein]

This looks fine but what is the motivation for moving to ES6? My understanding is it's useful for larger projects, but for simple things like this, the compatibility issues might still be more problems than its worth