I finally got the Message Mixer project (JavaScript/Node.js) working in a webpage instead of in Node,
while keeping the original JavaScript files (meant for Node) unchanged.
I wrote code to handle the require function and to deal with how the project uses process for user input and output.
Here it is, as a Codecademy workspace: https://www.codecademy.com/workspaces/66e12544be0868cfb7f0a2bd
I included a few jsdoc comments too.