Add a cool jS project

I found a very cool JS project and want to embed this in my Enfold site.
I use a child theme and i have to add some .js and .css files.
.css files can be placed in custom.css but i can’t get the .js files work. I checked the forum but there is no straigth answer to the question on how to add a custom .js file.
Can you help me?
THX Freek