When you say 'contributing' the I'm guessing that you are going to work with JSCAD V2. Please follow the notes in Early Adoption of V2.
The JSCAD V2 API is different, so you'll also need to see the documentation. You can generate the documentation as well by running..
npm run docs
There will be a full set of documentation in the 'out' directory, i.e. HTML files. Just open 'index.html' with any browser.