ReactJS: adding scripts to project Unexpected token '<'

I am trying to integrate a javascript project into my reactjs one. The plain js project augments images in 3D space using matrix3d css. https://github.com/zoltan-dulac/Matrix-Construction-Set I want this project to act separately in a modal inside my reactjs app.
There are lots of js files and I found that it's a headache to add simple script tags to a reactjs project, so I've started with a blank js file. Adding the empty file to the project raises the following error in the browser, files with content also raise the same error.

Uncaught SyntaxError: Unexpected token '<'