npm ERR! code UNABLE_TO_VERIFY_LEAF_SIGNATURE
npm ERR! errno UNABLE_TO_VERIFY_LEAF_SIGNATURE
npm ERR! request to https://registry.npmjs.org/cordova-app-hello-world/-/cordova-app-hello-world-3.12.0.tgz failed, reason: unable to verify the first certificate
Something is messing with the SSL certificates on your machine or internet connection.