di-app.js

let createGetUserName = require('./di-lib.js')
const getUsername = createGetUserName(database)
getUserName(1).then(name => console.log('the name of user 1 is', name))

di-lib.js

// notice: no require statement!

function createGetUserName(database) { 
  return function getUserName(id) {
    return db.table('users').load(id)
      .then(user => user.name)
  }
}

module.exports = createGetUserName