I have a multi-branch pipeline for a private NPM package. 

Within my Jenkinsfile I have a defined a "Publish" stage within my pipeline script for automatically deploying after a successful CI build. This stage looks like:

    stage('Publish') {
      if (env.BRANCH_NAME == "master" || env.BRANCH_NAME == "develop" || env.BRANCH_NAME.startsWith("release/") || env.BRANCH_NAME.startsWith('hotfix/')) {
        nodejs(nodeJSInstallationName: 'NodeJS 6.x', configId: 'abcdefgh-1234-5678-9999-0987654321') {
          sh 'npm login'
          sh 'npm publish'
        }
      }
      else if (env.BRANCH_NAME.startsWith('feature/')) {
        echo "On A feature branch. Not publishing!"
      }
      else {
        echo "Skipping publish - This build is not ocurring on a publish-able branch!"
      }
    }

 When the script for npm login fires I get an error:
[Pipeline] sh
[workspace] Running shell script
+ npm adduser
Username: (jenkins) npm ERR! cb() never called!
I could put the credentials in the script, but man does that suck. I spent a good amount of time setting up the credentials repository in Jenkins so that slick developers wouldn't be able to hold me hostage!

How can I get authentication to work?

Other notes:

We're internally using Sonatype Nexus for our internal hosted NPM registry. I have two configuration files stored, one for accessing the proxy repository (combo of public npmjs and our hosted) and our hosted.