Finally, if you do not wish to grant others the right to use a private or unpublished package under any terms:
{ "license": "UNLICENSED" }
Consider also setting "private": true to prevent accidental publication.
Sorry if I miss something, but seem to me that license field must exist and, if desired, to prevent accidental publication, private field should by set to true.
According to https://docs.npmjs.com/files/package.json#license:
Sorry if I miss something, but seem to me that license field must exist and, if desired, to prevent accidental publication, private field should by set to true.