You can now modify the contents of the src/ folder if you want.
Build from source
Inside the project directory, conactjs/, run the following to install all dependencies.
npm install
To build .js files you can use in your project, run
npx parcel build
This will create a dist/ directory containing the .js files.
Manual tests
The tests inside the tests/ directory only work if you have previously built contactjs from source as described above.
These tests include the .js files from the dist/ directory, so your build will automatically be covered by those tests.