What should I put in the license field of package.json if my code is only for use by the company I work for?
UNLICENSED is now a valid license option in npm. It means that no license has been applied to the code.
This is not to be confused with the license that was confusingly called "The Unlicense".
You can set the license to no license like this:
"license": "UNLICENSED",
In addition, if you want to prevent the package from being published on npmjs.org, then adding private to package.json will help:
"private": true